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Discussion Starter · #1 ·
I know there's threads on micrometers but I thought I'd try to be very specific with my question. My gas piston is showing pitting and I'd like to squeeze a tad bit more accuracy out of the rifle, so I upgraded to a NM w/groove. Accuracy went down. USN4

I found Gus' thread on gas pistons and have read it 3-4x. http://m14forum.com/gus-fisher/109897-m14-gas-pistons-maintenance-their-effects-accuracy.html

I've been trying to figure out how to approach acquiring and testing different sized pistons to find the right one for my rifle. I figure if I put together a set of pistons then one of you other poor addicted souls will have to buy them from me (minus the one that worked best for me) GI1

I had originally thought the cost of a micrometer capable of accurately measuring the pistons would be prohibitive for only this purpose but a Midway add caught my eye. Would this RCBS Vernier Ball Micrometer be appropriate for the purpose of sizing the pistons as Gus discussed? http://www.midwayusa.com/product/544254/rcbs-vernier-ball-micrometer-1?cm_vc=ProductFinding

Are there better options in that $30-$60 price range for this purpose or am I likely making a mistake by considering that price range.
 

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If you are not shooting NM Loads you do not want the grooved piston

If you are not shooting NM Loads you do not want the grooved piston.
 

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I figure if I put together a set of pistons then one of you other poor addicted souls will have to buy them from me (minus the one that worked best for me) GI1
I think you're on the right track, but if you read Gus again for the 5th time you will realize that the micrometer has nothing to do with it. Get as many USGI pistons as you can afford, test fire them, keep the best one, and sell the rest to us addicts. Chances are that a piston that didn't shoot the best groups will be perfect in somene else's rifle.
 

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Discussion Starter · #4 ·
I think you're on the right track, but if you read Gus again for the 5th time you will realize that the micrometer has nothing to do with it. Get as many USGI pistons as you can afford, test fire them, keep the best one, and sell the rest to us addicts. Chances are that a piston that didn't shoot the best groups will be perfect in somene else's rifle.
Yeah I caught that it could be read that way. I liked the way I was reading it since there would be a limit on the expense. But now you've burst my bubble so I'm just going to really hope you're wrong USNA and Murphy doesn't read this thread cause he's been rewriting his law to "whatever on an M1A can cost you more money will cost you more money" USNA

I can't really tell if they tested one from each size group or multiples from each size group per rifle.

The tolerance for the gas piston is a range of .0005”. We used a precision set of micrometers to separate the pistons into groups by .0001” “sizes,” as even the best Dial Calipers are not precise enough for this kind of measurement. Then we tried pistons from the different “size” groups until we found the size piston with which the rifle shot best. You actually see groups shrink or swell, depending on which size piston is used. Generally, but not always, the pistons in the middle range of the manufacturing tolerances would make the rifles shoot the best. Only every now and then would a piston from the smallest or largest size shoot the best in a rifle.
 

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There is more involved than just the OD of the piston - other variables are the ID and depth of the 'chambers' that are inside the piston.

It's similar to buying shoes - keep trying until you find a pair that feels good.

Jay Kosta
Endwell NY USA
 
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