AR-15 Upper Quick Fixes: Match Bolt Carriers

If you want an accurate rifle, don’t leave a lot of front/back bolt play (keep it .003″ but no more than .005″). Factory rifles run .012″ to .015″ play, which is OK if you need to leave room for dirt and grime in a military or field application. However, that amount of play is not ideal for a high-accuracy AR build. A lot of front/back bolt play allows rounds to be hammered into the chamber and actually re-formed in a non-consistent way, as they are loaded into the chamber.

The bolt affects accuracy in an AR-15 more than the carrier group. To get the most accuracy, the bolt and barrel have to be machined so that the headspacing is optimal when the round is chambered and the bolt locked. That is why if one orders a match-grade barrel for an AR-15 either the barrel comes with a bolt, the barrel manufacturer requires you to send in your bolt (prior to machining the barrel you’ve ordered), or the manufacturer requires you to send dimensions from your bolt.

The best accuracy usually comes from the bearing surface of the bullet nearly touching the rifling. Having the bullet jump any significant distance to the rifling tends to negatively affect accuracy. This is true in any rifle, not just the AR-15.

In addition to the above, you will want to keep the chamber, barrel extension, and carrier assembly clean to help insure consistent bolt lock ups critical to accuracy. You will also want to occasionally apply lube directly to bolt rings during shooting sessions.

Carrier key staking is also mentioned in the article. Young Manufacturing does not stake their carrier keys. “There has been a lot of talk about the pros and cons of staking the gas key on the carrier. Here is our opinion and why Young Manufacturing will not stake keys. We have been making carriers since 1991. The US Mil Spec. assembly drawing requires the carrier key to be staked. Contrary to some popular opinions staking does not SEAL the gas key. Staking keeps the screws from backing out Period. If you do not properly torque the screws to 56 inch pounds you will be staking a screw that is loose or one that is over torqued and prone to breakage.” Not sure how I fell about that, for me, a staked key is the only way to go. Something just does not feel right about it not being staked.

REFS:
http://bulletin.accurateshooter.com/2015/05/what-makes-an-ar-accurate-whitley-offers-answers/

Staking the gas key on the AR-15 and M-16 carrier.

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KeyMod, M-LOK, or Picatinny? Surveying the Future of Rail Accessories

There are three major rail types Picatinny, KeyMod and MLOK.

Picatinny are MIL spec MIL-STD-1913 and on are on many military firearms, so I dont think they will be going any place for a very long time. There are also a very large number of accessories using this standard that also will not change any time soon. There are a couple downsides to the Picatinny rail; first is they can get heavy add unnecessary weight to your rifle and bulk it up in a bad way, especially when placed on the fore end of your rifle. They also have sharp edges that can get snagged on or damage other gear as well as un-gloved hands.

The KeyMod system gets its name from the shape of the slots, which look like old-fashioned keyholes. You put the lug through the big circular opening and then slide the attachment forward, tightening the Allan wrench in the narrow part of the slot until your attachment is securely fastened. It also has an auto aligning feature when you tighten things down. The design is completely open source, so anyone who’s interested in manufacturing products using the KeyMod system can do so without having to pay royalties.  On the down side KeyMod did not do too good in the USSOCOM testing referenced below. Specifically the stress testing, this might sway me and others to look closer at our next rail, M-LOK. Especially if you are not heavily invested in one or the other.

The M-LOK system by Magpul uses slots in place of the keyholes. The attachment lugs on M-Lok accessories are t-shaped and bi-directional so they can be placed at the front or rear of the slots. While M-LOK is free licensed, it is not open source, and thus manufacturers must acquire a license from Magpul before making products using the M-LOK standard. Magpul claims this gives them more control in assuring that all M-LOK products are made to specifications ensuring compatibility. Program participation is open to any interested manufacturer. This might not matter with Magpul’s dominance in the accessories market.

I dont think Picatinny will go anyplace soon, especially for mounting scopes. It has already adapted to smaller sections that can be mounted on either KeyMod or M-LOK. M-LOK vs. KeyMod is where the real fight will be, and with the Magpul monster leading the charge for M-LOK we soon might see the end of KeyMod.

REF: https://www.pewpewtactical.com/keymod-vs-m-lok/

http://soldiersystems.net/2017/05/05/details-on-the-ussocom-sponsored-keymod-vs-m-lok-test-conducted-at-nswc-crane/

https://en.wikipedia.org/wiki/M-LOK#Licensing

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‘Ed’s Red’ Bore Cleaner

Just in case the they make it hard to find, I’ll put it here:

CONTENTS: Ed’s Red Bore Cleaner

1 part Dexron II, IIe or III ATF, GM Spec. D-20265 or later.

1 part Kerosene – deodorized, K1

1 part Aliphatic Mineral Spirits, Fed. Spec. TT-T-2981F, CAS
#64741-49-9, or may substitute “Stoddard Solvent”, CAS #8052-41-3, or
equivalent, (aka “Varsol”)

1 part Acetone, CAS #67-64-1.

(Optional up to 1 lb. of Lanolin, Anhydrous, USP per gallon, OK to
substitute Lanolin, Modified, Topical Lubricant, from the drug store)

                  MIXING INSTRUCTIONS FOR “ER” BORE CLEANER:

Mix outdoors, in good ventilation. Use a clean 1 gallon metal,
chemical-resistant, heavy gage PET or PVC plastic container. NFPA
approved plastic gasoline storage containers are also OK. Do NOT use
HDPE, which is permeable, because the acetone will eventually evaporate.
The acetone in ER will also attack HDPE, causing the container to
collapse, making a heck of a mess!

Add the ATF first. Use the empty container to measure the other
components, so that it is thoroughly rinsed. If you incorporate the
lanolin into the mixture, melt this carefully in a double boiler, taking
precautions against fire. Pour the melted lanolin it into a larger
container, rinsing the lanolin container with the bore cleaner mix, and
stirring until it is all dissolved.

I recommend diverting a small quantity, up to 4 ozs. per quart of the
50-50 ATF/kerosene mix for optional use as an “ER-compatible” gun oil.
This can be done without impairing the effectiveness of the remaining
mix.

This is a direct copy for this great source: http://handloads.com/articles/default.asp?id=9

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AR-15 Buy vs. Build

There are three options for acquiring and AR-15, it will be up to you to decide which method best suites you. Buy, build, and join the 80% club. In a nutshell buying from a reputable manufacture is the easiest, there are great manufactures that make great weapons. Building will let you get more involved in your rifle, and 80%er will give you the highest level of building most of us can get. If you are only going to own one AR style weapon, buying might be your best option. If you are hands on and will be owning more than one you will want to look at building.

Here are a few larger manufactures if you are looking to buy, this is in no way a complete list.

Colt: From their web site, “Colt’s rifles are the only rifles available to sportsmen, hunters and other shooters that are manufactured in the Colt factory and based on the same military standards and specifications as the United States issue Colt M16 rifle and M4 carbine. Colt customers want the best, and none of Colt’s competitors can match the quality, reliability, accuracy and performance built into every Colt rifle.” To me that statement means if you want what has been battle proven choose Colt. Just remember, battle proven might not be what you are looking for.

DPMS: Provides a very wide range of calibers for the AR’s to include .22, .223, 5.56 NATO, 308, 7.62, 204 Ruger, 243 WIN, 260 REM, 300 AAC Blackout, 338 Federal, 6.5 Creedmore, and 6.8 SPC II. That is a very wide range for one manufacture.

Bushmaster, SIG, Rock River Arms, Bravo Company Manufacturing, and the relatively new to the AR world Springfield Armory are all great choices for manufactured AR’s.

Here are a few pros and cons of both buying and building, we will start with buying:

Pros:

  • Easy: Buying a manufactures rifle that meets your needs is the easiest, quickest way to get into modern sporting rifles. You will be able to find a rifle you want and get it on the range without having to take the time to piece their firearm together.
  • Less Worry: Buying an AR rather than building gives you peace of mind that as long as you follow instructions when using your new firearm, it’s going to work as advertised.
  • Fit and Finish: When building, you have the option of picking components from different manufacturers. However, that assumed flexibility may not always come through, as some components have varying tolerances and dimensions, and may not fit or work properly with all parts. While AR systems generally share interchangeable pieces, you may wind up with parts that just don’t work together. Buying a complete rifle ensures the components fit together properly and work as advertised, taking out the guesswork and potential of making costly mistakes for prospective buyers.

 

Cons:

  • Plain: Buying off the shelf will give you a functioning, ready-to-shoot rifle, but depending on what you choose, you may feel underwhelmed at the features it offers. You can always upgrade parts as you go to give you the custom gun you want, but that costs more money and time, and could potentially wind up being more expensive than if you had built from scratch in the first place. Some manufactures may have proprietary parts that might not be upgradeable.

 

  • Cost: Manufacturers offer endless options in the AR market offering what seems to be an overwhelming amount of options. Building allows users to construct similarly capable weapons where they have complete control over which components and brands they opt for, allowing them to also choose where and what to spend extra on to get the desired result in a time frame that works for you and your wallet.

And now building:

Pros:

  • Make it yours: Building your rifle gives you the features you’re looking for, and you’ll likely be stuck with fewer unnecessary extra parts when you’re done.
  • Experience: One of the best ways to familiarize yourself with your AR is to disassemble and reassemble, learning the intricacies and minutiae of the system. Building an AR gives owners an education in how the rifles are constructed and how they operate, which can come in handy when it comes time to replace parts or help a fellow AR owner in need with quick fixes.
  • Ease of Supplies: The parts, from the barrel to the buffer tube, fire control group to the upper receiver itself, can be shipped directly to you. The only part that requires a FFL is the lower receiver.

Cons:

  • Wallet Creep: It can happen if you’re not clear on what you want, keep opting for add-ons, premium components, or changing your mind mid-build.
  • Tools: As assembling ARs requires some specialized tools to properly get your rifle together, something you’ll need to factor in when pricing your setup. Depending on what you have already you might still need an armorer’s wrench, a set of roll pin punches, a level, a vise and receiver vise blocks, a torque wrench, screwdrivers and more.

For the most adventurous builder there is the 80% lower. An 80% receiver comes partially completed with the trigger/hammer (fire control) recess unmilled, and the selector, trigger pin, and hammer pin holes often need to be drilled out. An 80% lower receiver can be purchased online, shipped, and received by the purchaser without a firearms dealer or FFL. In addition to the above tools you will need access to a drill, preferably a drill press, a router, and a good jig. Once you are done milling it out, it is just like a regular build.

REF:

Vendor Web Sites

https://www.nrablog.com/articles/2016/5/america-s-rifle-to-build-or-buy/

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The Relationship of Barrel Extension Diameter To Accuracy In The AR-15, Part 1

Slop in fitting the barrel extension to the upper receiver can rob your rifle of its potential accuracy. AR-15 barrels don’t screw directly into the receiver. They screw into a barrel extension that slips into the receiver. Since it is a slip fit there is a lot of play. Anyone who has ever re-barreled or watched videos of folks assembling an AR-15 upper will note that up until screwing the barrel nut in place the barrel will wiggle around. Trying to correct the issue by over torqueing can damage the receiver and affect the barrel harmonics. You should apply the recommended torque and try the corrective actions we will discuss below.

This can be fixed a couple ways. You can use green Loctite, the #620 is designed to withstand high temperatures like one would expect near the chamber. The 620 was designed for slip fits where gaps are large. It is thicker than the red Loctite and seems to fill gaps between the barrel extension and the corresponding inside surface of the receiver quite well.

You can purchase some shim stock, start with very thin 0.001″ stainless shim stock. Then with shop scissors cut a shim about ¾” wide and 2 ¼” long. The barrel extension has about one inch of bearing length, not including the flange and cutting the shim stock a little on the narrow side gives you leeway to keep it centered and not extending out from either the front or back. Wrap the shim around the extension and attempt a trial fitting, first without Loctite, to see if the barrel will go into the receiver with the shim stock in place. If I still have a lot of slop I cut another replacement piece of stock a little longer. If the fit is too tight to go in place you can gradually trim the length until everything goes tightly in place. Then as above apply green Loctite #620 after making sure everything is clean and degreased.

The downside to both these methods is removal, Loctite is a semi-permanent method. You can try to remove the barrel with heat, a block of wood and a hammer but risk severely damaging your receiver, barrel or both.

The article that this paper is based on is from American Gunsmith Magazine, dated March 2013. Almost exactly 5 years ago and there is much controversy on this subject. As with all things AR, it breaks down to personal preference. Do you want a rifle to go out and plink with, or be super accurate at over 500 yards? If the later, you might want to consider shimming it up, if not, standard build methods will be just fine for you.

REFS:

Various firearms forums

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Iron Sights and Optics

Now a days there are many different types of iron sights for your AR 15 rifle but for this paper we will focus on two, the A1 and A2 sights. The A1 type’s rear sight only supports windage (right-left) adjustments, and the A2’s rear sight has both windage and elevation adjustments. Because it is less complex, the A1 sight system is more rugged and less likely to be knocked out of zero but in theory has less granularity in adjustments.

To set mechanical zero run the rear sight in one direction, left or right, until it stops. Count the number of clicks in one direction, then again going the other direction until it stops. Divide by two. Then count that number of clicks in toward center. Record this. For the A2 turn the elevation knob to the 8/3 position and then 1 more click clockwise. Adjust the front sight until it is level with the sight post. You are now at mechanical zero.

Fire groups of three shots at the zero target from a distance of 25 yards. Find the center of your groupings and measure from there to the central vertical and horizontal lines in order to determine how far you need to adjust the sights. Lines on zero targets are in 1” increments from the center of the bullseye. To move your groupings to the left, turn the windage knob on the rear sight to the left and to the right to move your grouping to the right on the target. Looking from the top of the sight, the front sight post will need to be turned clockwise to raise groupings and counter-clockwise to lower them as they appear on the target. Repeat until you can consistently hit center mass.

Now that your Iron sights are at setup, let’s briefly discuss how to choose a scopes. First you need to decide what you want the scope for, Home defense, plinking, competition, or long range? With those questions in mind, consider the below:

 

1. A more expensive scope is usually made better.

2. A larger objective lens, the glass facing your target, can gather more light.

3. A larger scope allows more light transmission and usually enables more adjustments.

4. There’s really no reason to buy a scope that can outrange the ammo you’re shooting.

5. Lower magnification means wider field of view, which means it’s easier to acquire your target and keep on top of the situation around it.

Red dots can enhance you scope or be used alone. When used alone they allow you to remain focused on the target with both eyes opened. These sights allow you to point and shoot by placing the “dot” on the target and pulling the trigger. Red dot sights offer maximum available light transmission and wide fields of view. They are considered the fastest sights for target acquisition and also offer unlimited eye relief.

Let’s talk about the most common mount for your red dot and / or scope, Picatinny rails. The Picatinny rails are military standard MIL-STD-1913 (AR), which was adopted on February 3, 1995. They are the most common rails on modern AR-15 rifles. The recoil grooves are consistently spaced allowing manufactures to provide consistent and secure mounting hardware.

REFS:

https://info.stagarms.com/blog/bid/378263/Zeroing-the-Iron-Sights-on-Your-AR-15-Rifle

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AR-15 Barrel Extensions Retention

Correct barrel installation is very important. An incorrectly installed barrel can cause serious injury or death. We are going to discuss the correct way to get your barrel installed. First thing we need to do is gather up some tools and supplies:

Armorers wrench, torque wrench, punch, gas tube alignment rod, receiver clamp and / or padded vise, and grease. A grease like AeroShell 33MS or MCARBO designed for aircraft and graphite free is what you will need. Without the proper grease, your upper and barrel could eventually suffer from galvanic corrosion, galling, crack and come loose.

1. Grease the barrel extension prior to installing into the receiver.

2. Align the index pin with the recess in the upper receiver.

3. Push the barrel and upper together so the index pin fits in the slot. It is OK to use a rubber mallet to install the barrel to the upper if things are too tight.

4. Apply grease to the threads of the upper receiver and hand tighten the barrel nut assembly on to the upper.

5. With your torque wrench and armorers tool tighten and loosen the nut 3 times to seat the threads. Use the torque wrench to tighten only! Use your armors wrench to loosen. The goal is to align the space between the teeth at 12 o’clock.

6. Torque from a minimum of 30 ft lbs to a maximum of 80 ft lbs. Your goal is to align a gap between the teeth to 12 o clock of the upper to permit the gas tube to slide through. Once torqued, the gap in the delta ring, weld spring, and C clamp will need to be positioned to 12 o’clock as well.

a.) If it won’t line up, the easiest, and safest method is to order a barrel nut shim. If you are going to be doing several of these, might be worth having a few on hand. These are thin shims that will be installed in the barrel nut to help align the next gap / tooth. Follow the instructions included with the kit and it should fix any alignment issues.

7. Slide the gas tube into the upper receiver.

8. Push the front of the gas tube into the front sight / gas block base. There will be two holes on the end that goes to the sight post / gas block, one hole for the gas, another for the roll pin. Align the holes and use a punch to install the roll pin.

9. Drop on your hand guards and you are done.

REFS:

https://youtu.be/ey3cIB5u7q4

https://youtu.be/h_tELLisZMw

http://www.thenewrifleman.com/ar15-barrel-installation-guide/

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AR-15 Magazine Checks

Magazines work by using a spring to push a fresh round up in front of a bolt or slide. That cartridge is then pushed into the chamber either manually or by a compressed spring. The magazine under the bolt is exerting varying upward pressure on it and that those varying amounts of pressure change the downrange POI. Magazines have varying pressures, single stack magazines have a straight up, lighter pressure while double stack magazines will have a higher pressure as well as applying pressure from either side depending on round placement. Other factors in the magazine that can influence the bolt carrier group are springs, feed lips, and followers.

There are several ways to try and work with this. When competitive shooting never use you magazine as a rest. This will apply excessive and inconsistent pressure on the bolt. Lots of NRA sponsored events don’t allow this anyway, train as you compete. Use a reliable lead sled or rest. Try to use the same one if possible, if you cannot, sand bags or you range bag will work as long as the magazine is off the ground. Clean and maintain you magazines as shown below.

These instructions are for the military style metal, G.I. issue mags. The principal is the same for all magazines:

1.) Pry the tab at the rear of the magazine down so that the detents can clear the spine.

2.) Grab the tab with your needle nose pliers and pull the tab, sliding the floor plate from the bottom. Be careful the tab is under spring tension.

3.) Remove the spring and follower from the bottom of the magazine body.

4.) With the spring and follower out, wipe down the inside of the magazine by pulling a clean microfiber cloth through the hollow tube of the magazine body a couple of times.

5.) Wipe down the spring and follower to remove any remaining dust and sand. Depending on your environment, apply a light coat of lube if desired.

6.) Ensure that there are no dents and everything is in the shape it is supposed to be.

Save any serviceable parts and reassembly in the opposite order. 5 → 1

Match the magazine to the firearm. This can be accomplished by labeling your magazines. There are all kinds of ways to do this. Simplest is to use a white marker and a method that you can use to match the magazine to the weapon. Some manufactures have “dimples” on the magazine you can color in a pattern to label them

Using the above tips should help you improve your POI.

REFS:
U.S. Army TM 9-1005-319-10

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AR-15 Tools, Maintenance and Repair

As either a hobbyist or professional there are some basic tools you will need to build and maintain your AR 15 rifle. You don’t need to start off with the most expensive tools out there, several of my first tools came from Harbor Freight, and it is what I could afford at the time. As you move on and figure out what you use most, you can get some better tools in needed. Sometimes you need to make do with what you have, be careful not to scratch or damage the weapon you are working on. Here is a basic list:

Ball-peen hammer
Nylon/brass hammer
Screwdrivers:
Flat Tip
Phillips
Hex
Torx
Needle-nose pliers
Bench mat, I have been using $5 yoga mats from 5 and below.
Bench vise
Padded vise jaws
Set of roll pin holders and punches in numbers 1, 2, 3, & 4
Barrel nut tool or combination armorer’s wrench
Buttstock tool (collapsible stock only)

I will call this Basic advanced. I know people will disagree, but if you are only going to do one or two rifles you can get the job done without them;

Lower receiver vise block
Upper receiver vise block set
Universal bench block
Handguard removal tool

The course material does a great job listing the more specific tools required to build your AR 15:

Barrel-Specific Tools:

Torque wrench (½” drive, 30 ft./lb. to 150ft./lb.)
Taper starter punch (3/32”)
Flat punch (⅛” to peen the swivel rivet)
Staking punch
Sight adjustment tool (specific to your type of sight, A1 or A2)
Gas tube alignment pin
Snap ring pliers
Breaker bar
Strap wrench

Upper Receiver Specific Tools:

Headspace gauge set – Field, Go, and No-Go
Rear sight elevation spring tool (for assembling the A2 sight)
Sight adjustment tool (for your type of sight)
Ejector removal tool

Lower Receiver Specific Tools:

.151 diameter punch (for locating trigger parts & installation)
Pivot pin detent installation tool
Bolt catch pin punch
AR-15 hammer trigger jig
Hammer trigger drop block (for adjusting the hammer & trigger)

Using the basic set of tools listed above will help you clean your rifle. First thing you need to do is to clear the weapon, ensure the bolt carrier group is forward and the safety is on. Push the front and rear take down pins out and separate the upper from the lower receiver. Then disassemble the upper receiver pull the charging handle back and remove the bolt carrier group. The charging handle will drop down as you continue to pull and can be removed from the receiver. On the left rear side of the bolt carrier, you’ll see what looks like a cotter pin, this is the firing-pin retaining pin. Pull the pin straight out and the firing pin will drop out of the bolt. Turn and remove the cam pin the bolt will now slide out. Make sure the gas ring gaps are not lined up and are in good shape. Next remove the extractor pin, the extractor will then come out. Inspect the extractor assembly spring and rubber piece to ensure good working order. Clean everything up with a small cleaning brush and solvent then wipe down with CLP. Re-assemble the bolt by putting the extractor and spring assembly back in the bolt and inserting the extractor pin, return the bolt to the carrier and insert the cam pin and rotate, insert the firing pin and the retaining pin. Run a couple swabs with solvent followed by CLP down the barrel. Insert the bolt group and charging handle to the upper receiver

After putting it all back together you should do a functions check:

1. Place the selector switch on fire and squeeze and hold down the trigger. You should hear the hammer striking the firing pin.
2. While still holding the trigger, pull the charging handle fully to the rear and let go to reset the hammer.
3. Release the trigger. You should hear a mechanical click. This is the disconnector releasing the hammer onto the trigger.
4. Place the selector switch on safe and try to pull the trigger. The hammer should not fall. If it does there is a problem with your selector switch and/or trigger.
Now that we have described the tools needed to build and maintain their AR-15-style rifle, listed instructions for cleaning your rifle and talked about the functions check. Let’s talk about various ways to store your rifle. Locked up and out of the wrong hands is the preferred way to store your firearms. Locking gun cabinets start at around $100 and can go as high as you wallet will allow. You can get wall to wall carpet, lights, and dehumidifiers. My thought on this is play big or stay home, I only want to make this purchase once so go as big as I can. I also want to protect my investments, firearms, from fire and theft. A locking gun cabinet might not do the job but a good quality safe can handle upwards of 1400 degrees Fahrenheit and any crow bar a thief can throw at it. You can safeguard your other valuables as well, not just your firearms.

REF:
U.S. Army TM 9-1005-319-10

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AR-15 Direct Impingement and Piston-Driven

The original, Eugene Stoner AR 15 firearms are what is called direct impingement. When the bullet is fired it goes screaming down the barrel, as it passed the gas block it directs some of the gas down the gas tube into the gas key on the bolt carrier pushing it back and cycling the weapon. Excess gas is vented to the upper and lower receivers. For the sake of this paper that is the simple description

Now piston operation works pretty much the same up to the gas block. When it gets to the gas block most of the gas is vented into a piston and pushes a rod back that pushes the bolt carrier back cycling the weapon. The excess gas escapes at the piston by the gas block. Again, very simple terms.

1. Piston-driven guns run much cleaner. The gas is vented at the gas block versus venting into the upper and lower receivers.
2. On average, piston-driven guns cost more. There are some very expensive gas impingement ARs and some inexpensive piston-driven AR rifles. However, if you want to purchase the least expensive AR possible, it will be a gas impingement gun.
3. Gas impingement guns are more suppressor-friendly, especially those with an adjustable gas block that allows you to control the amount of gas directed back through the gas tube. It should be noted that with both systems, your rifle will fire dirtier with a suppressor.
4. Piston-driven guns run cooler. The gas that enters the upper and lower receivers is hot, making what is touches hot. On a piston-driven rifle the hot gas evacuates at the gas block, further away from your hands.
The one item that has a lot of controversy is the accuracy, many say that the direct impingement is more accurate. I am going to leave that one alone until I can check myself. Both piston-driven and gas impingement guns are very reliable. If I had to pick between a gas impingement and a piston-driven AR, I think I would try the piston driven, simply because I have not tried it yet and the research sounds like it might be a great option except for three things: parts for gas impingement ARs are easier to find, much more plentiful and less expensive. Still think I’ll do it.
If you own a gas impingement AR and want to try a piston-driven AR, just purchase a piston-driven upper receiver. Since the gas impingement and piston systems work independently of the lower receiver, you can alternate between both on the same lower receiver.

REF:

6 Facts About AR-15 Gas Impingement Vs. Piston

Build an AR-15: Direct Impingement or Piston Operation?

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