Tip: Importance of Gas Piston to M1A Accuracy.
From: Gus Fisher
Folks, you may not believe this one, but I assure you it's true. You can have an excellent quality NM barrel, sights, glass job, et all, and your pet rifle may not shoot that well. I have built quite a few "improved" rifles with standard G.I. barrels that shoot almost as well or better than other people's match rifles.
Here's why. We found you have to try different gas pistons in your rifle. The tolerance for this part is a range of about five ten/thousandth's, yep that"s .0001" increments. We separate them and try different sizes in each rifle till we find the one that shoots best. You actually see groups shrink or swell, depending on which size is in it.
Now for the bad news, we never came up with what size will automatically work in a gas cylinder. We measured hundreds of cylinders and pistons combinations in rifles that shot well. We never found a correlation between gas cylinder and piston size. Guess there are too many other variables that come into play.
I mention this because I've had people ask me about the pistons that Brookfield sells with a special coating. The idea is great, but you'd have to have a bunch of sizes in hand to see which one works best in your rifle. I have seen customers' faces light up after I told them about this. "That's the reason my rifle won't shoot any more!", is the normal reply. They put the original piston in their rifle and it shoots well again.
Wow! Thank you Fred. I never thought about trying different pistons for accuracy. This may explain why some rack grade rifles shoot so well compared to some tricked out $$$ grade rifles shooting just so so. I always try to get my rifle to shoot as close to the barrels potential as possible. I've never though about changing pistons in different cylinders. Is this something you experimented with or just stumbled on?
Jack, its something the Marine Corps Rifle Team armorers, of whom Gus Fisher was one, discovered. Of course, they had access to all the parts they wanted, so they could afford to tinker all they wanted until they found the magic combination of parts that made each rifle a tack driver. For us mear mortals, its more of a luck of the draw. I suppose you could go to a gun show with a micrometer and find five gas pistons in .0001" increments, try them all out in your rifle to find out which one works best, and then sell the other four at the next gunshow.
Josh, considering the above information. I would try both pistons off sand bags and do an honest comparison. The piston that shoots the better groups use for now. The other will still make a good spare. Since the tightness of fit doesn't seem to be a factor in anything except function, no correlation between how tight or lose for best accuracy, more or less luck of the draw. I doubt either piston will give you function trouble unless one is really off, you still should have 2 pistons that work well in your cylinder. I would be interested in the results of a few test groups with both pistons. If one is way better than another, it may work better in someone else's rifle. You could sell one if you wanted or just hang on to it in case you get another rifle or gas assembly in the future. This topic is news to me, so I'm all ears.
Josh, Tea, Fred and Gus are all absolutly correct! Your piston will fit in your gas cylinder or it won't depending on the distance from the beginning of the camming grove on your op rod and where the end of the op rod contacts the face of your piston. This will vary with reciever dimensions/headspace locations. Too much play and the hammering effect of your piston against the op rod will generate vibrations called "harmonics". Even just a few thousanths difference between "hammering" and constant pressure resulting in "pushing" will dramatically change your point of impact, especially in rapid fire. Two things that will also help alleviate this are the Buffer Technologies little plastic whodis that fits over your op rod spring and a stainless barrel. Good luck bubba!
Here you go Jack alot good info to be had. I have tried about 7 gas piston's in my armscrop after over cleaning the one that came in it with scotch bright stuid move on my part. None were the same just afew .0001 thousandth's different in the seven I had to try out finally found one that shoot the best. The first one was worse than the one I over cleaned that's when I found Gus's post on Lane's CSP tip' I don't think size matter's the smaller one worked best for me http://web.archive.org/web/20030705071825/http://www.jouster.com/lanestips/
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