Scratch piston AR

[Pnwbrrt]

Looking for ideas on a piston build. I was originally planning on just using a superlative kit but then my mind started wandering Into the possibilities. I've been exploring the idea of building a modified mr556 system but haven't quite figured out how yet. I'd like to maintain as much " milspec" compatibility as possible. I'm pretty sure I could modify a standard BCG into an mr556 style BCG (or just use one) but I don't feel like paying hk money. The only hang up I have on getting an mr556 system to work is the barrel/gas block. On ideas on over coming that obstacle? Any alternative piston build ideas? I like the idea of a long stroke system, but I'm not married to it.

Read More

 

[It-That]

The issue with modifying an AR bolt is two fold.

1, the gas key position is too far forward, so the pressure on it from a piston conversion kit puts rotational force on it. It means the tail end of the bolt carrier is pushed down in compression very hard and the front is pulled up into the top of the receiver. The reason this doesn't happen as it's designed is because the gas is routed through the carrier and pushes the bolt forward away from the carrier at the initial moment of force against the barrel in a straight line. The HK isn't just a piston kit, it moves a lot of stuff around inside to solve this imbalance.
2, the unlocking action is more than just moving the bolt carrier backwards to unlock the bolt from the extension with the cam pin. As is, the cam pin hole in the bolt is pretty much the weakest and most common metal fatigue cracking area. Piston kits just pile more force on that area, which is typically divided into other areas when the pressure is routed through the gas key and pneumatic pressure pushes the carrier and bolt apart from each other for that initial unlocking.

So that is just two big flaws in piston conversions. Not to mention spare parts would be dependent on you probably having to buy two so you have something to repair it in the future. They're all unique designs and proprietary parts and the business viability means the company might not be around when you eventually need a replacement part (this has happened to me for what was at the time, the hottest most popular kit.)

BUT, do not let any of this dissuade you from what you want. I have an alternative for you that is military production, has infinite replacement parts availability, and NOT HK. it comes from our wonderful allies in Taiwan, and it is called the T91 wolf upper. Pins right into your regular american AR15 lower. Here is a link for example.

Wolf A1 AR15 Upper T91 SALE - AtlanticFirearms.com

Wolf Performance Arms Offers Shooters the finest and most reliable AR-15 piston driven barreled upper produced in the world.

atlanticfirearms.com

 

[Member 34319]

You should reach out to @drewp or look through his threads. He has done some cut and weld projects that blended a few guns together.

 

[Pnwbrrt]

Thanks for your response!
Yes, I I see what you're saying about rotational force. the first modification that would be made of course, would be the gas key. I do believe I could over-come the need for venting in the bcg entirely. But then I would want to figure out some sort of guide system for the bolt as well. I have a few ideas for that.

I like the mr556 system, and it's very well proven that's is sort of why I was thinking about building around that concept.

I share the same concern as you regarding proprietary parts for proprietary systems and needing replacement parts down the line. That is sort of how I was lead down this road of a homemade system. There are a variety of piston systems out there, but none of them have all the features I want in a piston AR and all of them use to much proprietary parts to be able to easily upgrade Into what I want it to become.

I am familiar with the T91 upper, but I run into the same complaints with that as many others.

I may just need to purchase a couple of piston uppers to better conceptualize my ideas.

 

[Pnwbrrt]

You should reach out to @drewp or look through his threads. He has done some cut and weld projects that blended a few guns together.

I went to @drewp profile and looked at posts but didnt see anything of the cut and weld variety.

 

[drewp]

I do alot of cutting, riveting and welding just usually for myself....

 

[Pnwbrrt]

The issue with modifying an AR bolt is two fold.

1, the gas key position is too far forward, so the pressure on it from a piston conversion kit puts rotational force on it. It means the tail end of the bolt carrier is pushed down in compression very hard and the front is pulled up into the top of the receiver. The reason this doesn't happen as it's designed is because the gas is routed through the carrier and pushes the bolt forward away from the carrier at the initial moment of force against the barrel in a straight line. The HK isn't just a piston kit, it moves a lot of stuff around inside to solve this imbalance.
2, the unlocking action is more than just moving the bolt carrier backwards to unlock the bolt from the extension with the cam pin. As is, the cam pin hole in the bolt is pretty much the weakest and most common metal fatigue cracking area. Piston kits just pile more force on that area, which is typically divided into other areas when the pressure is routed through the gas key and pneumatic pressure pushes the carrier and bolt apart from each other for that initial unlocking.

So that is just two big flaws in piston conversions. Not to mention spare parts would be dependent on you probably having to buy two so you have something to repair it in the future. They're all unique designs and proprietary parts and the business viability means the company might not be around when you eventually need a replacement part (this has happened to me for what was at the time, the hottest most popular kit.)

BUT, do not let any of this dissuade you from what you want. I have an alternative for you that is military production, has infinite replacement parts availability, and NOT HK. it comes from our wonderful allies in Taiwan, and it is called the T91 wolf upper. Pins right into your regular american AR15 lower. Here is a link for example.

Wolf A1 AR15 Upper T91 SALE - AtlanticFirearms.com

Wolf Performance Arms Offers Shooters the finest and most reliable AR-15 piston driven barreled upper produced in the world.

atlanticfirearms.com

I actually did a little more digging into the T91 system and I think that if I decide to take a factory upper and modify it into what I want it to become that they may very well be the easiest option and by default the most dependable. Thanks for bringing that up.

 

[drewp]

I'd also suggest a pws long stroke piston upper. ar's aren't like building ak's or something...they are very well designed and have a "standard" so cutting them up doesn't make much sense. yes you can build a homemade gas system and yes you can chop up bolt carriers to do cool things but not many people can do that stuff

 

[It-That]

Mark serbu has an awesome YouTube channel I'd watch if you're curious about machining and lathe turning your own gun. Worlds of experience and even his AR alternatives fall flat.

 

[Pnwbrrt]

I'd also suggest a pws long stroke piston upper. ar's aren't like building ak's or something...they are very well designed and have a "standard" so cutting them up doesn't make much sense. yes you can build a homemade gas system and yes you can chop up bolt carriers to do cool things but not many people can do that stuff

I hear you.

The PWS rifles are phenomenal and I love them for what they are but I cannot make them what I want to make because of the proprietary barrel mounting.

I revisited my previously brief research into the T91 system after @It-That mentioned it earlier and I've designed a concept based off that system that I think is going to work. For right around $2k (realistically probably $4k after getting passed any potential issues and evolving it further after creation) I'd have exactly what I envision with some minor compromises (no forward assist, no adjustable gas block and no side folding capability)

G4 left hand side charging upper
custom barrel from X-caliber
full T91 operating system but with their .750 block, T91 anti-tilt h2 buffer, POF roller cam pin.
Centurion arms C4 midlength cut 2 piece free float quad rail.

I'm going back and forth with dead foot arms regarding the viability of their MCS but I don't think it's going to work out.

I think it's a good Segway into a project which will either evolve into something wonderful or a pile of parts and fragments of failures in a plastic bin on my closet floor.

 

[Pnwbrrt]

Mark serbu has an awesome YouTube channel I'd watch if you're curious about machining and lathe turning your own gun. Worlds of experience and even his AR alternatives fall flat.

I checked him out. Thanks for the recommendation, sweet channel. I suppose me saying from "scratch" probably was misleading as for now I was more looking to concoct a Frankengun based of the concepts off multiple systems.

 

[Pnwbrrt]

ar's aren't like building ak's or something...they are very well designed and have a "standard" so cutting them up doesn't make much sense. yes you can build a homemade gas system and yes you can chop up bolt carriers to do cool things but not many people can do that stuff

Everything I've cut up, milled out and slapped together while half drunk and ill prepared has turned out pretty good so far. It may have taken 5 attempts to perfect tolerances and 10 range session to diagnose issues but I have yet to lose a limb or an eye, and that I call a win.

 

[It-That]

Do you build AKs?
I'd really like to see this build, keep us posted. If you want to go into the serbu rabbit hole some more, I found him from coverage of Kentucky ballistics blowing one his guns up with bootleg slap rounds. He had to reach into his punctured neck, fish his artery out and pinch it shut with his thumb all the way to the hospital after that. Then serbu gave him more guns to test loads with to figure out that the cartridges were bootlegged reloads and basically save his brand.

I live designers' work and unconventional home builds.

 

[BattleReadyBunny]

MR556 and HK416, Sig 516, and Carcal 816 (all made by the same engineer), have a taller gas piston system than what would be on most retro fit AR15 gas piston system. If you look at this, the rails are taller than a standard AR15 to make room, due to this. They have upper receivers. If you could get the barrel nut and handguard from a HK416 or buy a geiselle/MI etc aftermarket rail and place it on your standard AR15 upper. You'll have a step. Much like if someone was building a DPMS 308 AR and got a DPMS high rail and DPMS low receiver.

You have a few options. Brownells sells HK416 repro kits.

BRN-4 PISTON UPPER RECEIVER KITS

BRN-4 PISTON UPPER RECEIVER KITS https://www.brownells.com/gun-parts/rifle-parts/rifle-receivers-parts/brn-4-piston-upper-receiver-kits/

www.brownells.com

As mentioned, the Wolf T1 is another piston option

Cheaper option to the HK416 is the Carcal 816. Components are made by PWS, like the Brownells stuff. Look into the history of this, it quite interesting and I own one. The Car816 features a simplified mr556/hk416/sig516 gas piston system, but still similar in design. And it features the extractor pin in the chamber area much like it's predecessor the hk416.

If you want to check out these uppers we have 16" and 11.5" in stock currently at limitless

I know this isn't a piece together route