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Suppressing and Shooting Subsonic, Can I make the gun cycle!


Rekraps
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Well, this has been an experience. Background is that I own 9 M1A's and 1 Select Fire SA M1A (factory). I'm not messing with the select fire gun, but did want to suppress and run subsonic loads through a M1A and see if could do so and have the gun cycle. Here is the story, the process, and the results:

 

The process included two phases:

 

  1. Supressing the gun, and
  2. Tuning it for subsonic loads

 

First of all, the M1A/M14 platform is not designed to be suppressor friendly, which I discovered right away. I'll skip the details of removing the flash hider as most of you are familiar with the process. Once done, I installed a Fulton Armory Flash Suppressor Thread Adaptor, installed the crown nut and front sight post. I decided to run the GemTech "One", .30 cal suppressor so as to have a dedicated matched round unit. After mating the GemTech quick detach adaptor/flash hider to the Fulton Armory Adaptor, it was time to gage the whether the Adaptor unit (collectively the Fulton and GemTech adaptor) were concentric. Using a bore gage, I did so. The rule is that the rod cannot touch the outer edge of any surface. It's important to note here, that of the 6 M1A's I modified, only three passed the test. It appears that the OEM flash suppressor threading may not be fully true for all units. Once done I installed a Shutter Adjustable Gas Plug to moderate recoil.

 

IF the unit passes the concentric test, great. If not, try remounting the adaptors again. If it still fails, then you are SOL for that gun.

 

Next I mounted the suppressor and conducted the concentric test again. Same as before. If it passes, lovely. If not, then it just does not.

 

Shot the gun using NATO supers with the can, mildly loud, but tolerable. Certainly not "shoot off the back deck" quiet. Then I switched to Sellier & Bellot 200 grain factory subsonic loads, with specs saying 1050 FPS. Wonderful! Easily shot with no hearing protection and super consistent. I zeroed at 75 feet, (using a bench rest) with a strike 1/5" above point of aim (looking to hit anything out to 100 yards). Shot groups were very tight, all inside a 1/2 inch diameter circle. The gun was, as you would expect, a bolt action with me having to cycle the action after each shot.

 

At this point in the project, I was very happy. Super pleasant gun to shoot and the S&B ammo met my expectations. And I could dedicate this gun to subs, so why not!

 

Now on the part 2. Could I make the gun cycle.

 

The issue is gas pressure and can it be made sufficient to work the action, but not push the bullet above the speed of sound. Spoiler alert! Nope, and here is why.

 

The following analysis uses a few math/volume estimates that appear to be correct based on the results. With a 200 grain bullet moving at 1050 fps, and a full NATO round of 147 grains moving at 2850 fps, quick math tells me that the pressure in the bore, for the subsonic round right about the time the bullet exits will be roughly 51% that of a full power round. Assuming that the designers over-engineered the gas system so that there was plenty of gas pressure to cycle the operating rod and overcome the operating rod spring compression, and enough energy to overcome the frictional resistance of the spring compression on the rounds in the magazine, I used a 15% pressure margin.

 

So my target pressure ratio to full load should be in the neighborhood of 85% of normal. Using a bit of math, I was able to determine that the volume of the barrel and gas cylinder (to include the gas plug) was 760.38 units and 4.56 units respectively, or 764.94 units in total. Since gas pressure and volume are proportional, raising gas pressure is as simple as reducing volume. Here is where the problems started.

 

The barrel accounts for 99% of the volume and it cannot be reduced (otherwise the bullet cannot be shot). No amount of reduction in volume of the gas cylinder will achieve the desired target of 85% of full load pressure. I even had the OEM gas plug threaded (3/8 x 24) and milled a threaded rod to exactly fit it AND to extend into the gas piston cavity. As expected, the gun shot but did not cycle.

 

So what next? Only thing left is the recoil spring. Well, experimentation (and a waste of a $20 spring) revealed that any OEM spring length less than about 80% of the original reduces the spring tension beyond what is necessary to impart the required inertia to feed and lock the bolt into place with a loaded magazine. Just for the heck of it, I used a shortened (~80%) spring length and shot the gun using the 200 grain subsonic load... no go. Not enough pressure to cycle it even with the reduced spring length.

 

So, after several days of grinding, measuring, calculating and so on, I have come to the conclusion that as far as my amateur gunsmith talents go, the M1A platform will always be a "bolt action" subsonic shooter. Plenty of fun to shoot like that!

 

Gun Details:

M1A Scout Squad

2.75 Burris forward mount Scout Scope

GemTech "One", .30 dedicated can

Fulton armory flash hider adaptor

GemTech quick mount .30 adaptor

Shuster Adjustable Gas Plug (not used when trying to modify gun)

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What about fire control work?Theres a lot of work sometimes making a supressor host run.Hammerand spring should be lightened.Anything thta the bolt carrier has to overcome during a reset.Barrel ports get opened up gas blocks to if need be.I bet it could be made to run however provly require a dedicated set if parts.
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Thanks for the tips. No doubt it could be done, but I have neither the tools, expertise nor dedication to the project to continue.

 

Which begs the question, does anyone out there have one configured to shoot subs reliably semi-auto? I have an engineering degree (Aerospace/Civil) so I keep going back to the gas pressure vs coefficient of friction of the many parts, AND the force/inertia necessary to overcome the recoil spring tension / magazine spring pressure on the rounds to be loaded and still be able to lock the bolt into battery. All, without moving the bullet into supersonic range. The S&B 200 grain bullet is about as heavy as one would want, and its moving ~ 1050 FPS... on the edge. This tells me the gas pressure is about at its upper limit, so all we have left is reducing friction and tension/compression on the parts. I already addressed the recoil spring, big no-go there as it takes quite a bit of force to strip a new round out of the magazine and lock it into battery. Seems like the mass of the sum of the parts is simply too much.

 

Now given unlimited resources, I suppose we could forge a magnesium operating rod and aluminum bolt/gas piston. To this we could mate a magazine (20 round) that only loads 5 rounds (to reduce magazine spring compression). Maybe this would reduce the overall mass of the parts sufficient to allow the subsonic gas pressure to do its job.

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  • 5 months later...

So guys, I have to tell you. The S&B 200 grain Subs out of a M1A with Gemtech One (.30 dedicated can) are super quiet. Shooting off my deck the rounds are spot on to about 100 yards then hit the dirt shortly thereafter. Using a Scout Squad 18 in barrel, and it appears to be that the barrel length and can volume are just perfect for this round. About the same noise as a .22 sub out of a Ruger 10/22 takedown with can.

If you have a M1A that  you can dedicate to suppressed, do it! 

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On 2/21/2023 at 8:36 PM, johnsonlmg41 said:

How did you know you wouldn't get an end cap/baffle strike?   Or did you just take the gamble? 

The M1A (M14) are, as you know, not designed for suppressor use. The Flash hider and corresponding end barrel threads are not always concentric. So.... to your point, I use an alignment rod to gage concentricity when mounting the suppressor. My success rate on these projects is about 50%, meaning that of the nine M1A's I own, only four were manufactured with the threads concentric enough to allow for the can alignment. 

This is why I have dedicated M1A's for suppressed.  Here's a photo of the complete rig. Very nice shooting gun.

 

Edited by Rekraps
free up space
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I think you missed my point?  Subsonic rounds don't stabilize well in a number of guns/ barrel twists, etc., so things can go "sideways" with the wrong combo.  I have 4 guns that are set up for this can, but the can I have for it is one of 4 and the mfr. quit doing them years ago.  It's super quiet and I don't want to risk it since I only have the one and the mounting/construction is rather unique.

DSC00407.JPG

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13 hours ago, johnsonlmg41 said:

I think you missed my point?  Subsonic rounds don't stabilize well in a number of guns/ barrel twists, etc., so things can go "sideways" with the wrong combo.  I have 4 guns that are set up for this can, but the can I have for it is one of 4 and the mfr. quit doing them years ago.  It's super quiet and I don't want to risk it since I only have the one and the mounting/construction is rather unique.

DSC00407.JPG

Perhaps I did! I have found that the S&B 200 grain subs perform perfectly for my rig. Zero is at 100 yards and the target shows no signs of bullet instability. It may be luck, but apparently the twist rate is suited for this round.

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On 2/22/2023 at 7:29 PM, johnsonlmg41 said:

I think you missed my point?  Subsonic rounds don't stabilize well in a number of guns/ barrel twists, etc., so things can go "sideways" with the wrong combo.  I have 4 guns that are set up for this can, but the can I have for it is one of 4 and the mfr. quit doing them years ago.  It's super quiet and I don't want to risk it since I only have the one and the mounting/construction is rather unique.

DSC00407.JPG

BTW, I have hunted all my life, and that wound does not look typical for a sub round.  Did you use supers through the can?

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Yes, I used federal match, but that isn't the wound, it's just been field dressed.  The actual wound is not visible, entry under the rifle, and I don't believe there was an exit on this one?

Stability of subs is always an issue so that's why I asked.  I don't recall the twist on these barrels, but I think it's 11.25.  They are set up for match ammo since it's better than 1/2 MOA with the federal ammo.  It's a 5R cut rifled barrel on these guns of which I think three transferables exist, and one postie and 30 some semi's. 

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On 2/26/2023 at 9:18 PM, johnsonlmg41 said:

Yes, I used federal match, but that isn't the wound, it's just been field dressed.  The actual wound is not visible, entry under the rifle, and I don't believe there was an exit on this one?

Stability of subs is always an issue so that's why I asked.  I don't recall the twist on these barrels, but I think it's 11.25.  They are set up for match ammo since it's better than 1/2 MOA with the federal ammo.  It's a 5R cut rifled barrel on these guns of which I think three transferables exist, and one postie and 30 some semi's. 

None of which, of course, I understand. But it sounds great! I was figuring that wound was from a cannon! Ha.

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On 2/26/2023 at 9:18 PM, johnsonlmg41 said:

Yes, I used federal match, but that isn't the wound, it's just been field dressed.  The actual wound is not visible, entry under the rifle, and I don't believe there was an exit on this one?

Stability of subs is always an issue so that's why I asked.  I don't recall the twist on these barrels, but I think it's 11.25.  They are set up for match ammo since it's better than 1/2 MOA with the federal ammo.  It's a 5R cut rifled barrel on these guns of which I think three transferables exist, and one postie and 30 some semi's. 

Speaking of which, I bought a really nice M1A Scout Squad from a guy here in GA, and when we met to transfer the gun he handed me a can of 600 rounds of LC LR ammo! Really nice sniper stuff. Very accurate.

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  • 1 month later...

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