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Piston = Accuracy?

This is a discussion on Piston = Accuracy? within the Accuracy forums, part of the M14 M1A Forum category; Purchased a unitized gas system from Brownelles. I understand how not having a loose fit can improve accuracy. But Im having a challenge in understanding ...


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Old March 14th, 2017, 10:31 AM   #1
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Piston = Accuracy?

Purchased a unitized gas system from Brownelles. I understand how not having a loose fit can improve accuracy. But Im having a challenge in understanding how a "match grade' piston can make a difference. I know very little about my new rifle system, but this piston thing just makes no sense to me. Can anyone explain?

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Old March 14th, 2017, 11:03 AM   #2
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Short answer- we don't know why just changing pistons affects accuracy. There are a number of theories but as far as I know, no one has been able to verify exactly why one piston tightens a group and another from the same manufacture will open it out. It may have something to due with weight of piston or shape or spacing between gas cylinder wall and piston or simply voodoo. Gun Professionals like Gus, Art, Ted, and Tony have hashed this over in several threads on here and I think they are still not ready to say for sure.

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Old March 14th, 2017, 11:08 AM   #3
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sadlack match gas piston has front to back groove to relieve gas pressure.(long barrel only)
there have been several threads stating in the old days armorers would go out with the rifles and try different pistons to see which riflr performed better with what piston. might be a barrel harmonic issue

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Old March 14th, 2017, 11:27 AM   #4
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There is the question of dwell. What I've read here seems to indicate that increased rearward op rod travel before the bolt is opened, tends to have a beneficial effect on groups. Also, a grooved piston like the Sadlak, bleeds off some gas, just as a ported or adjustable gas piston does. all of these can have an effect on the function and performance of the rifle.

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Old March 14th, 2017, 11:34 AM   #5
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It's interesting and still baffles me somewhat that a piston can effect accuracy. In reality, the bullet is out of the barrel before the piston starts to move. The only thing I can figure is that piston movement effects the harmonics of the barrel as it vibrates and settles back to rest after the shot. This may have an effect on how the rifle moves during the next firing cycle. With a gas operated rifle accuracy is dependent on consistency and some piston/gas cylinder combinations may work better than others. Note that there is not really a "match" piston. They are all basically the same. The longitudinal groove was included in an attempt to ease up some of the forces on the rifle's action during firing. it has the added benefit of keeping pressures down in the gas system when shooting heavier bullets, however the M14 can handle bullet weights up to 185 grains without problems. The grooved pistons also can contribute to short cycling which we don't want to happen. We tried them back in the early 80's and found no benefit in our NM rifles so they were replaced with standard pistons after only about a season of use.

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Old March 14th, 2017, 11:34 AM   #6
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It's the magic, vodoo , mojo, folklore of the platform.
Nothing scientifically explained with supporting data as far as I can tell.

Guys trying multiple Pistons until they get good groups.

It's the opposite end of the spectrum as computers where everything has a reason.

Just go with it.

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Old March 14th, 2017, 11:45 AM   #7
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It's the magic, vodoo , mojo, folklore of the platform.
Nothing scientifically explained with supporting data as far as I can tell.

Guys trying multiple Pistons until they get good groups.

It's the opposite end of the spectrum as computers where everything has a reason.

Just go with it.
Aaaand roger Capona....just going with it.

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Old March 14th, 2017, 11:52 AM   #8
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Certainly not sufficient info to make a conclusion, but switched from the issue piston to the Sadlak grooved one and rifle is a 1moa+ rifle with either one. With good loads the accuracy is a bit better with either piston. Mostly seat of the pants conclusion but the rifle does seem to offer less felt recoil and action is smooth with no reliability issues. Generally use 150-168gr bullets for loads at ranges from 100 to 200+. I have noticed that doing the tilt test with both pistons, the Sadlak seems to glide quicker than original, perhaps due to the coating??

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Old March 14th, 2017, 12:18 PM   #9
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Certainly not sufficient info to make a conclusion, but switched from the issue piston to the Sadlak grooved one and rifle is a 1moa+ rifle with either one. With good loads the accuracy is a bit better with either piston. Mostly seat of the pants conclusion but the rifle does seem to offer less felt recoil and action is smooth with no reliability issues. Generally use 150-168gr bullets for loads at ranges from 100 to 200+. I have noticed that doing the tilt test with both pistons, the Sadlak seems to glide quicker than original, perhaps due to the coating??
I'd like to see you turn off the gas for 5 rounds cycled manually and see what you get after your normal grouping.

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Old March 14th, 2017, 12:47 PM   #10
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Quote:
Originally Posted by Ted Brown View Post
It's interesting and still baffles me somewhat that a piston can effect accuracy. In reality, the bullet is out of the barrel before the piston starts to move. The only thing I can figure is that piston movement effects the harmonics of the barrel as it vibrates and settles back to rest after the shot. This may have an effect on how the rifle moves during the next firing cycle. With a gas operated rifle accuracy is dependent on consistency and some piston/gas cylinder combinations may work better than others. Note that there is not really a "match" piston. They are all basically the same. The longitudinal groove was included in an attempt to ease up some of the forces on the rifle's action during firing. it has the added benefit of keeping pressures down in the gas system when shooting heavier bullets, however the M14 can handle bullet weights up to 185 grains without problems. The grooved pistons also can contribute to short cycling which we don't want to happen. We tried them back in the early 80's and found no benefit in our NM rifles so they were replaced with standard pistons after only about a season of use.
Well, the piston has moved about 0.050 inch and gained about 10 to 12 fps at bullet exit.

The time to the bolt roller contacting cam on the op-rod ('dwell') is around 2 milliseconds, the bullet is in the barrel for about 3/4 of a millisecond.

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Old March 15th, 2017, 07:42 AM   #11
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TedBrown said:
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It's interesting and still baffles me somewhat that a piston can effect accuracy.
It baffles me, too. Many talented people have tried to solve this mystery without reaching a conclusion. The answer might reside elsewhere.

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Old March 15th, 2017, 07:59 AM   #12
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And to throw another variable into the equation, gas plug torque can also affect accuracy.

Tony.

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Old March 15th, 2017, 08:14 AM   #13
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tedbrown said:
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In reality, the bullet is out of the barrel before the piston starts to move.
lysander said:
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Well, the piston has moved about 0.050 inch and gained about 10 to 12 fps at bullet exit.
It's interesting to me that this difference of opinion still exists.

I have read some anecdotal reports that the oprod moves before the bullet leaves the barrel, but I have seen only one report of reliable testing, which is quoted occasionally on this forum: Springfield Armory Technical Note SA-TN11-1094 dated 16 Dec 1957

Here is one link to it, posted by different in 2009:

Timing Analysis of Unlocking Cycle for M1 and M14

Just last night, I started searching the SA Museum site for that photo test. I am still looking.

Several weeks ago, a TV pundit was discussing something political. He said, the plural of anecdote is not data.

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Old March 15th, 2017, 08:14 AM   #14
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I have a few USGI pistons, 1 Sadlak TiN grooved as well as a Bula Nitrated piston. I have done the piston swap-a-rooo on a good performing rifle set up for Across The Course use. All pistons measure in spec but there is a slight differences on target, nothing dramatically noticeable in my rifle but there can be real wide accuracy swings in other rifles.

Out of all the pistons I've tried the black Bula is the pure accuracy winner, however I'm willing too sacrifice a little of the rifles accuracy because I like the recoil impulse of grooved piston better. I may at some later date risk grooving the Black Bula and see if its accuracy remains the same....

Decisions, decisions, we are all at a cross roads......

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Old March 15th, 2017, 09:12 AM   #15
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I have tried three different US GI pistons and a Sadlak TiN-coated piston in my tuned Loaded. I could detect no noticeable (measureable) difference between the four pistons UNTIL I put in a Shuster vented/overbored plug. The the Sadlak was a bit better for accuracy. All fired strings were Federal GMM to keep the variables as few as possible.

I fire seven shot groups and discard the two outliers and measure the best 5 shots. This seems to give me a better representation of grouping by eliminating most (well, some) of my bad habits on the trigger.

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