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Piston = Accuracy?

This is a discussion on Piston = Accuracy? within the Accuracy forums, part of the M14 M1A Forum category; There exists a relationship between the piston and the cylinder walls, the best accuracy happens when this relationship is at point X. Nobody knows what ...


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Old March 15th, 2017, 09:48 AM   #16
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Once more into the----

There exists a relationship between the piston and the cylinder walls, the best accuracy happens when this relationship is at point X. Nobody knows what or where point X is.. By piston change-out the best relationship between the pistions on hand and cyl walls can be found, sometimes.

There is also two another factors. That's the air in the bore being forced forward rapidly, it expands and acts on the awaiting pistion and flash suppressor.

Time for a BIG RED.. Art

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Old March 15th, 2017, 11:54 AM   #17
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Question

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Originally Posted by art luppino View Post
There exists a relationship between the piston and the cylinder walls, the best accuracy happens when this relationship is at point X. Nobody knows what or where point X is.. By piston change-out the best relationship between the pistions on hand and cyl walls can be found, sometimes.

There is also two another factors. That's the air in the bore being forced forward rapidly, it expands and acts on the awaiting pistion and flash suppressor.

Time for a BIG RED.. Art

Art, I understand what your saying but I also have a counter idea.... A grooved piston would allow more air too slip past, then the weight of the op-rod and the strength of the return spring pushing against the piston would still be the same. I'm not saying there is zero movement, but less than is the norm.

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Old March 15th, 2017, 12:16 PM   #18
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As i see it, the air in the tube would take the path of least resistance. Get pushed out the front of the barrell. The gass behind the round, once it seals in the barrell, may escape through the grooves but still will go out the end of the barrell easier than diving through the gas port. As the round passes over the gas port the resistance of the seal between the bullet and barrel will send the gas douwn the gas port. Short of pulling the shot in that last millisecond the piston has no influence to this point. Its an oblect not in motion.....i think

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Old March 15th, 2017, 12:58 PM   #19
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I've been giving this piston business some thought. We'd have to get the engineering design notes but I'm certain field functionality and reliability were the prime goals with accuracy not a factor as long as the overall rifle accuracy metric was achieved.

What happens when the piston returns to its "A" position?
Is it placed against the gas plug and kept there by pressure from the spring?




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.

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Old March 15th, 2017, 01:52 PM   #20
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That may be over simplifying it. The M14 is a creature of many moving parts and accuracy is dependent on how those moving parts relate to one another. Throughout the firing cycle there are numerous interactions that effect the rifles harmonics, not just the barrel harmonics. It would fill a book to try and explain it. The relationship of the piston is but one very small part of a larger picture. It's contribution to accuracy is not all that significant, but it makes a difference to Master Class shooters.

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Old March 16th, 2017, 10:27 PM   #21
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All is great! Keep shootin x ring and don't worry! Ya if the wheel isn't Broken theory! Just have fun eh!

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Old March 17th, 2017, 03:23 AM   #22
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Quote:
Originally Posted by shooter86314 View Post
tedbrown said:

lysander said:

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.
I posted some graphs from Technical Report SA-TR11-2610, Gas Systems - Caliber .30, T44 Rifle.

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Old March 17th, 2017, 06:00 AM   #23
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OP, Gus Fisher has a great post if you do a search on the impact of Gas Pistons and accuracy. Basically what he writes is that when he was the armorer for USMC rifle team they discovered that certain pistons "fit" with a particular rifle. They would take 5 pistons (not necessarily new) and try them in a rifle changing no other variables (e.g. torque kept the same, etc.). One piston of the five, for example, would produce noticeably tighter groups.

As Ted mentioned (and others) no one knows why. It is just a fascinating platform...so many moving parts and so many inputs to accuracy (piston, stock, headspace, sights, etc.) but when you get it right it is so sweet....

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