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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: Supressing the gun, and 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)