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This is a discussion on Piston chat.. within the Art Luppino forums, part of the Gun Professionals category; Originally Posted by JayKosta I realize that the precise details about the testing are probably not available, but it would be interesting to know - ...


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Old December 10th, 2016, 07:47 PM   #16
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Originally Posted by JayKosta View Post
I realize that the precise details about the testing are probably not available, but it would be interesting to know -

1) Did ALL of the rifles with 'fast' pistons perform better than ALL the rifles with 'slow' pistons?

2) Were any piston changes done to test 'slow' pistons in rifles that formerly had 'fast' ones? And the other way too.

And who knows, maybe it wasn't the pistons at all - maybe it was something with the gas cylinders instead. e.g. having the direction of the piston's movement be on the same axis as the oprod. Or perhaps the 'bore' of the cylinders being more 'straight' and uniform.

I accept the results of the test for what they are, but it's sometimes difficult to determine WHAT actually caused the results.

Jay Kosta
Endwell NY USA
Jay,

I don't have the answer to any of your questions, the information was passed on to me from Sgt. Donovan.. I did test fire a number of the concerned rifles at the 300, prone, approx one shot every 10 seconds, but not for the same purpose, we were testing Mexican Match, the old pull and push stuff, using the M118 powder and primers, it was outstanding. Art

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Old December 11th, 2016, 12:11 AM   #17
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Pistons

Today I was planning on testing a few rifles, but the alternator in the Range Van (my friends call it something far worse) imploded so I spent the daylight hours changing the alternator and thinking about pistons.

I agree with Art that free-flowing pistons anecdotally shoot better scores, but the question of why is something not often addressed. It is my opinion it has to do with the higher probability of pressure spikes in more tightly fitting pistons, but not all "high vacuum drag" pistons cause pressure spikes or worse scores, so maybe there is something more.

After I finished up with the van, my shop was a mess and I set out to cleanup. One thing that I have been intending to do was to align one of my Hardinge 3-jaw adjustable chucks. Once I had the shop straightened out, I started on the chuck.

One thing you need when you true an adjustable 3-jaw is a straight rod; at least that is the way I like to do it. The last time I made a near perfectly straight rod was when Matt, my machinist and I made a 12-inch rod for aligning Remington 700 receivers for blue printing. We could have just bought one, but they are expensive and we figured we could do it no problem. Nearly 8 hours later, we had accomplished our task and we were able to finish it to within .0004". Not sure it was worth our time, but we still talk about the all-day nearly-straight rod. We eventually just bought one, that turned out to have zero run-out and it was hardened.

I pulled out our nearly-straight rod and began the process of aligning the 3-jaw. An hour or so later it was done.

My 3 other machines verified the work, my test bed verified my machines and then I decided to do some testing. I started with M14 pistons.

First, of all the pistons I measured, only the USGI piston had near zero run-out from the 6th groove to the end of the piston. For example, one other piston measured .001" TIR (total indicated run-out) across this short distance. That does not mean the diameter changes, in fact I have a calibrated V-block diameter gauge and the piston was within .0003" across the same length, but what it does mean is that the end of the piston is not on the same plane as the rear portion, and .001" across this short distance is a lot.

A slight variation in the angle of the piston when the gas is flowing results in either a higher spike or more extreme drop from the mean pressure. High vacuum drag pistons would amplify this variance because they don't have as much tolerance. Whereas a looser fitting (freer flowing) piston inherently bleeds the gas in a smoother curve, and smoother is what you want in the gas system.

The freer flowing gas systems decrease the variance from the mean pressure, which can be caused by ammunition and a poor fitting piston and therefore is simply a variance reducing modification.

A semi-scientific way to verify all of this would be to take a sample of at least five M14 rifles, and use 5 pistons with the highest TIR, and 5 with the lowest TIR. That's 50 variables, five weapons and 10 pistons each. The control would be that the pistons would all have to be the same diameter, length and have the same mass within a reasonable tolerance. It would be a great deal of work, but maybe someone with the time and resources will undertake it and give us the results.

If a simple regression analysis shows a statistically valid positive correlation between straighter pistons and more precise targets, the question will be answered. I believe the freer flowing pistons are usually more precise in terms of group size, but I have seen a number of rifles that performed better with high vacuum drag pistons, so there must be another variable within the gas system.

In my opinion, the freer flowing gas systems eliminate more of the variation in pressures caused by pistons that are not perfectly straight. However, if one could also figure out if that second variable statistically correlates, we might have an answer to the why.

Then again, it could be something to do with the mass of the piston, which is also worthy of further investigation.

Art, thanks for starting this discussion.

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Old December 11th, 2016, 11:11 AM   #18
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Worksheet

Added a piston worksheet. Headed to the range and will update later.

Measured diameter at port, diameter at tip, total indicated runout from those locations, face concentricity and mass.

Note the difference in diameter for Brookfield Piston.
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File Type: jpg pistonworksheet.jpg (32.1 KB, 36 views)

Thanks from tonyben, nf1e, XM25Ren and 3 others
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Old December 11th, 2016, 11:17 AM   #19
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Quote:
Originally Posted by jywolfe View Post
Added a piston worksheet. Headed to the range and will update later.

Measured diameter at port, diameter at tip, total indicated runout from those locations, face concentricity and mass.

Note the difference in diameter for Brookfield Piston.
Great that there is some science and data that will lead to a more informed conclusion.

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Old December 11th, 2016, 11:23 AM   #20
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Quote:
Originally Posted by jywolfe View Post
Added a piston worksheet. Headed to the range and will update later.

Measured diameter at port, diameter at tip, total indicated runout from those locations, face concentricity and mass.

Note the difference in diameter for Brookfield Piston.
Hope you are headed to the range with my rifles. Never mind, I saw Brookfield piston so I know you are!

REN

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Old December 12th, 2016, 02:37 PM   #21
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I noticed last night while I was cleaning an M25 I bought from someone, that it had a grooved sadlak tin coated piston in it as well as a spare.
So I guess I need to know a bit on the grooved piston and what ammo I'm supposed to shoot with it.i recall the groove was for heavier ammo like 168 and 175 but need to confirm with the resident experts.

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Old December 12th, 2016, 02:50 PM   #22
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The piston is the last word in M14 precision, even the rejects may find a home in a rifle and will produce some phenomenal precision. I tried replacing the Art Luppino rejected piston with a NIW USGI, I could not duplicate the performance of the old rusted, pitted reject.

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Old December 12th, 2016, 02:58 PM   #23
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Quote:
Originally Posted by Capona View Post
I noticed last night while I was cleaning an M25 I bought from someone, that it had a grooved sadlak tin coated piston in it as well as a spare.
So I guess I need to know a bit on the grooved piston and what ammo I'm supposed to shoot with it.i recall the groove was for heavier ammo like 168 and 175 but need to confirm with the resident experts.
Not an expert but I do know that the NM grooved piston was meant for the heavier bullets ie. 168 and 175. Anything lighter may not cycle the action.

REN

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Old December 12th, 2016, 03:49 PM   #24
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Originally Posted by XM25Ren View Post
Not an expert but I do know that the NM grooved piston was meant for the heavier bullets ie. 168 and 175. Anything lighter may not cycle the action.

REN
If the action is smooth and everything is functioning properly, even 125's work with slotted pistons. My M25/5R/JAE has a Sadlak slotted piston and it shoots well with 125 TNT's over 45 grains of IMR-4895 in commercial cases and Rem 9-1/2 primers. The brass falls nicely on the bench but I've never had one fail to eject. It's quite fun! It's like shooting an AR!

XM80 ball cycles flawlessly.

Tony.

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Old December 12th, 2016, 04:11 PM   #25
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Quote:
Originally Posted by tonyben View Post
If the action is smooth and everything is functioning properly, even 125's work with slotted pistons. My M25/5R/JAE has a Sadlak slotted piston and it shoots well with 125 TNT's over 45 grains of IMR-4895 in commercial cases and Rem 9-1/2 primers. The brass falls nicely on the bench but I've never had one fail to eject. It's quite fun! It's like shooting an AR!

XM80 ball cycles flawlessly.

Tony.
Tony,

I was just parroting what I have read as I have never used one. After all I am no expert.

I did sleep at a Holiday Inn Express once.
XM25REN

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Old December 12th, 2016, 04:12 PM   #26
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tony, it doesn't eject brass as far? Or at a different clock position?

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Old December 12th, 2016, 04:47 PM   #27
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Quote:
Originally Posted by tonyben View Post
If the action is smooth and everything is functioning properly, even 125's work with slotted pistons. My M25/5R/JAE has a Sadlak slotted piston and it shoots well with 125 TNT's over 45 grains of IMR-4895 in commercial cases and Rem 9-1/2 primers. The brass falls nicely on the bench but I've never had one fail to eject. It's quite fun! It's like shooting an AR!

XM80 ball cycles flawlessly.

Tony.
Tony,

I shot 110 V Max through my number 1 gun, it has a grooved piston and bored out vented gas plug. The 110 did very well in the rapids, 2 and 300. Fun shooting 8X cleans with the 14.

Nez

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Old December 12th, 2016, 05:08 PM   #28
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I just got a grooved tin one to experiment with handloads as well. I will be trying the 140-155 gr, and will report when available.

From what i read, like tony has said, it should operate fine with the "lighter" load, with the added benefit of a more gentle push of the op rod, ie softer recoil, less wear and tear on metal, perhaps a softer ejection pattern to boot. We'll see.

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Old December 12th, 2016, 07:37 PM   #29
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Quote:
Originally Posted by jywolfe View Post
Added a piston worksheet. Headed to the range and will update later.

Measured diameter at port, diameter at tip, total indicated runout from those locations, face concentricity and mass.

Note the difference in diameter for Brookfield Piston.
Also interesting how many exceed the .4975 max dimension.

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Old December 12th, 2016, 08:14 PM   #30
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Quote:
Originally Posted by tonyben View Post
If the action is smooth and everything is functioning properly, even 125's work with slotted pistons. My M25/5R/JAE has a Sadlak slotted piston and it shoots well with 125 TNT's over 45 grains of IMR-4895 in commercial cases and Rem 9-1/2 primers. The brass falls nicely on the bench but I've never had one fail to eject. It's quite fun! It's like shooting an AR!

XM80 ball cycles flawlessly.

Tony.
HA! I got that beat I load 125's under 41.5 IMR/H-4895 in a LC like case and my junk runs just fine.

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