AR-15 mods for subsonic loads

[omnimedic]

I have worked up a subsonic .223 load that is "Hollywood quiet", but as I expected, it won't cycle my gun. So, combining knowledge of firearms and college physics classes, here's what I'm thinking:

The purpose of the buffer is to absorb some of the momentum of the BCG coming back and the purpose of the spring is to slam the BCG closed to chamber another round. Without going into a dissertation on physics, the reduced recoil from a reduced load means less weight is needed in the buffer to counter the force of the BCG coming back... so theoretically, installing a lighter buffer to match the lower force of a reduced load should allow it to cycle, right? Has anyone actually done it?

[Theohazard]

Someone has probably done it, but subsonic .223 is not at all popular for a good reason; it's VERY hard to get it to cycle the action and even then it's got TERRIBLE ballistics compared to regular .223. At this point, most people would prefer a .22 LR.

I'm perfectly happy shooting supersonic loads with my .223 AR-15 suppressed; it's still quieter than an un-suppressed .22 rifle. And when I want to plink with a "Hollywood-quiet" rifle, I'll use a suppressed .22. And when I want an almost "Hollywood-quiet" subsonic rifle round that still packs a punch and will cycle the action in an AR-15, I'll use 300 Blackout.

[Mobuck]

Sounds like an excellent way to beat up your rifle or maybe eat some parts. If you want that level of quiet, get a 22 or go with the 300AAC.

[Jimro]

In theory yes, you are correct. A lighter buffer will reduce the mass of the BCG and Buffer that needs to be moved by gas pressure.

What I don't know is whether or not your load has enough gas pressure to cycle the action even with a reduced buffer mass.

Why not use a 22lr adapter and shoot subsonic that way?

Jimro

[omnimedic]

Thanks to everyone on the unsolicited opinions of subsonic .223. I am aware of 300 BLK and 22LR conversion... In fact, I have a dedicated 22LR upper.... that's not the point. Most of it is putting some of my education into some semblance of practical use along with the challenge of "just to see if I can". I want to experiment, learn, and try things that may lead to me to something more useful or practical for the future.

So, apparently I'm trying something that nobody else on here has tried yet....? Does anyone want to see results of my experiments when I'm done, or does everyone just write the whole idea off as completely stupid?

[Palmetto-Pride]

One thing you must get used to on gun forums is getting "unsolicited" opinions on more that your exact question or questions.......most people on here have good intentions. I would like to see your results!

[MarkCO]

It is possible. The load you have developed is part of the equation. Heavier bullets, fast powders with low density are the prescription for the load to generate the gas volume and dwell time.

Pistol length gas system with a 16" barrel, low mass carrier, empty buffer and either a well used or clipped coil buffer spring. Take down the mass, but don't take too much out of the spring.

Carrier mods, by machining them lighter to about 5 or 6 ounces, reducing the size of the gas vents in the carrier and making sure your gas key was sealed (not just staked) are some of the mods that help. A gasblock that does not leak is also a huge benefit.

If your upper is done right, you can get away with a dedicated carrier and buffer. Swap those out for subs and put the stock buffer and carrier back in for SS.

[bcarver]

I see you post count as 10. Don't be so sensitive.
A piston upper will eliminate the seal problem.
Yes a lighter buffer will help. go with the heavyist bullet you can at the highest velocity while still subsonic.
What you are looking for is enough gas without the velocity.
It is a hard balance to find in the .223 but not in larger calibers.

[Charles S]

Does anyone want to see results of my experiments when I'm done, or does everyone just write the whole idea off as completely stupid?

I would love to hear about the results of your experiment.

I have just resigned myself to hand cycling the action for my 223 when using subsonic ammo.

[omnimedic]

My comment about unsolicited advice was mostly tongue in cheek. I'm new to this forum but not forums in general and I expect unsolicited advice on any forum I post on, firearms or otherwise. I realize this is the Internet, where everyone has an opinion, they will share it whether asked or not, and sarcasm is lost.

Thanks to Mark and Jim for the additional info. I'll post results here when I get them.

[Boomer58cal]

You need 100g-ish bullet and a barrel with about a 1-7 twist. I don't know if they'll fit in the mag though.

It has been done but it's not easy.

Boomer

[Theohazard]

Originally Posted by omnimedic

Thanks to everyone on the unsolicited opinions of subsonic .223. I am aware of 300 BLK and 22LR conversion... In fact, I have a dedicated 22LR upper.... that's not the point.

The thing is, on these forums there's a huge range of knowledge and experience. We've got brand-new shooters, gun gurus, and everyone in between. I didn't know which angle you were coming from and if you were aware of the reasons that subsonic .223 is not common or popular. Now I know. But most of the time when people here ask about subsonic .223 they are mostly unaware, and that's where I was coming from in my response.

Most of it is putting some of my education into some semblance of practical use along with the challenge of "just to see if I can". I want to experiment, learn, and try things that may lead to me to something more useful or practical for the future.

That's cool. I wish you luck and I wish I could offer something that hasn't already been mentioned.

[Charles S]

The thing is, on these forums there's a huge range of knowledge and experience. We've got brand-new shooters, gun gurus, and everyone in between.

That is an accurate assessment. I thought about stating that with my reply, but think you stated it more eloquently than I had envisioned.

I guess I am saying. Keep in mind, there are probably people who are replying who have never even held a 223 can.

[semi_problomatic]

You may have to change buffer and spring as I doubt just 1 would work. Maybe a low mass BCG if you're not running a light one already. Or you could try an adjustable gas system. Kies was like $32. Usually you have to turn down your gas for suppressed, but theoretically with a system like kies you could also open it up for the reverse effect. I don't think a noveske would work since they have only 3 positions last time I checked. Open, suppressed, and closed. If changing over your gas system worked, it'd probably be a lot cheaper than replacing your internals and swapping between sub and supersonic would be a couple turns of an allen screw instead of swapping the guts of your rifle.

[Rbahri5206]

Can't you drill a slightly larger hole in the barrel to let more gas in the gas tube?

[semi_problomatic]

Depends. There are .0750 and .0625 gas block journals. Hole in the barrel is supposed to be .0625. There are guys who have opened the hole up as much as .078, but if the rifle fires fine now with regular rounds it could lead to over gassing the system when he stops his experiment; unless he used an adjustable system.

While over gassing is not usually catastrophic in itself, it does lead to increased fouling and faster parts wear.

Could just try a weaker spring first, as the buffer and bolt have to overcome the spring before moving. Ideally, the balance between spring, buffer, bolt and gas will be such that the bolt has enough energy to lock back while not having so much to bottom out the buffer.

[Rbahri5206]

Semi_problomatic not trying to argue but rather asking a follow up question, do you still get the over gassing if it's a sub sonic load like the op stated. He cannot get the rifle to cycle so by drilling the barrel larger and allowing more gas threw is this still gonna wear the parts out if he only uses the sub sonic load? What's your opinion.

[semi_problomatic]

".... it could lead to over gassing the system when he stops his experiment; unless he used an adjustable system. "

So no, while shooting subsonics it could work just fine. I just wouldn't want to drill into my barrel for an experiment when I could change a part.

Also, if he bored out the gas hole in the barrel and then used a suppressor with regular rounds it could severely over-gas the system.

[Rbahri5206]

Great point, so unless its a dedicated rifle to that round adjustable is the way to go

[semi_problomatic]

If it works. It very well could take a $8 spring and 20 seconds to change it out...

[fsted2a]

I have seen ARs that will cycle subsonic ammo, but they have all had silencers with gas blocks set to maximum. My idea would involve a skelotonized carrier, adjustable gas block, and ssnipping the buffer spring a bit at a time each round until it started cycling. Just my .02.l

[Sharkbite]

So, apparently I'm trying something that nobody else on here has tried yet....?

Uhhh NO...you are trying something LOTS of people have tried, and found impractical.

The 223 AR is just not a good subsonic platform. Not as anything more then a straight pull bolt action. I get it and i load 223 subs, but i dont expect em to cycle the gun.

Even if you did make your AR cycle with subs, it would not tolorate a full power round. Talk about “over gassed”, and that light a spring and buffer would prob be dangerous should a standard velocity round get in the gun.

SO many other (better) options.

[MarkCO]

Uhhh NO...you are trying something LOTS of people have tried, and found impractical.

BINGO

[Theohazard]

Zombie thread! The OP hasn’t been on TFL since 2015, so I doubt he’ll see your reply, Sharkbite! Don’t you hate it when someone replies to an old thread and bumps it up to the top of the thread page, and then you respond to the thread not realizing it’s super-old and the OP is long gone?

Welcome to TFL, fsted2a. You’ve pulled the classic move of joining TFL and using your first post to revive a long-dead thread, but we’re still glad to have you here!

[Mobuck]

I run a light weight bolt carrier on one of my 300AAC carbine gassed carbines when shooting sub-sonic. When switching from sub to super, all I need is to swap the carrier only.