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Discussion Starter #1
It’s a long story, but I ended up with a used gas cylinder on my double lug build this spring. However it shot very well with a new LRB nitrided piston so I moved on. Just recently I noticed my brass ejection pattern went from a consistent 1 o’clock to nearly random 1 to 5. I’m also getting vertical stringing on the shots.
The rifle and piston both now have 1200 rounds and you can see the chatter on the piston. Gonna change the piston now and the entire cylinder and piston after my last match on oct 31
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Carl Liesenger has convinced me that minute differences in pistons affect accuracy. The problem is that the only way to find out is to put rounds down range, which can be expensive and time consuming. I've seen him show up at the range with one of his builds, bringing along a 10rd cartridge carrier full of USGI pistons.

As for the nitrided ones, I not convinced that the new production ones are stainless steel, like most others.
 

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Are you referring to the Marks on the Tail of the Piston as Chatter Marks ? There was another thread about the Marks on the Tail of the Piston as machine tool marks & would have no effect on Piston operation . If there is something on the Piston itself , thats another matter.
 

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Are you referring to the Marks on the Tail of the Piston as Chatter Marks ? There was another thread about the Marks on the Tail of the Piston as machine tool marks & would have no effect on Piston operation . If there is something on the Piston itself , thats another matter.
Thanks for mentioning that other thread. I have observed first hand the building of those "chatter marks" over time and concluded, myself, that they do indeed effect accuracy. I am going to guess the piston on the right was similar to the one on the left when it started out. I worked on my own gas cylinder to remove sharp edges on interior and exterior of the D opening, polished the flat of the piston stem and stopped this from happening.
 

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Discussion Starter #5
I should have included a little more information about the picture. As Art surmised, the piston on the right is the one in question and the one on the left is a barely used one. The one on the right was perfectly smooth on that flat surface. I know some stainless can be slightly attracted to a magnet, but that nitrided piston latches up hard to a magnet and I reasonably confident it is not stainless.

The worn piston never malfunctioned, but newly developed wild ejection pattern was an indication that something was changing. The associated gas cylinder is clearly loser than my brand new spare and you can feel the difference in piston wobble. I don’t have gauges but I’m betting it wouldn’t pass inspection.

As far as accuracy goes: I’ve heard and read the stories about the effect on accuracy. I shot a group today with the new piston. Nearly all my brass went to 2 o’clock and to the same distance. A few pieces went to 4 o’clock and I suspect they were the last ones in the mag. Today’s group (single target) did not have any vertical string. The other targets were from last week with the worn piston. Maybe I just shot better today?

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Some years back I came upon the SEI chrome plated stainless pistons and have been using them exclusively ever since. I keep a clean USGI piston in the butt for emergencies. (The rifle butt ;) )
 

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Here is the other Thread , read Ted Brown's reply in post #3 . Though if you believe it shoots better with the new set up , by all means go with it .
 

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Extensive M1 Carbine and Rifle collections with current focus on standardized Army rifled arms
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Government testing revealed that the piston doesn't start moving until the bullet is 15 (or is it 25... been along time since I saw) feet out of the barrel. It's hard to imagine how chatter on the piston would effect anything. Perhaps the chatter marks are caused during piston movement.

I'd be interested in scientifically significant analysis.... data counts.

But in any case, confidence adds to performance. If you feel that a smooth piston helps then it just might.

A lot of really talented great shooters think that a clean piston and cylinder support good accuracy. I suspect they're right.
 

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Discussion Starter #10
Hey thanks for that link. Good read. I sure don’t think mine had the marks to begin with. Regardless, swapping pistons had a noticeable Change on the ejection so I’ll run with this one until I refurbish the whole system this winter
 

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Hmmm..... FWIW. While pistons may measure the same on the outside they are all different on the inside. I have a SADLAC TiN grooved (2nd batch), a early Black Bula as well as a late SAI (both are spares). All three shoot about the same in there accuracy but the SADLAC is softer and shows a slight edge and is the reason it stays in the rifle. I haven't played with increasing the dwell with it (yet).

With the above said, what do your three measure for lenth?

In my #2 rifle it uses a grooved USGI piston that has had the tail trimmed so this rifle has a longer dwell than #1 does, my two rifles weigh nearly the same but #2 is much softer firing and flings out the cases out at almost 3 o'clock using known accuracy loadings. Bump the load up or use a different bullet and ejection pattern also changes.
 

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Government testing revealed that the piston doesn't start moving until the bullet is 15 (or is it 25... been along time since I saw) feet out of the barrel. It's hard to imagine how chatter on the piston would effect anything. Perhaps the chatter marks are caused during piston movement.

I'd be interested in scientifically significant analysis.... data counts.

But in any case, confidence adds to performance. If you feel that a smooth piston helps then it just might.

A lot of really talented great shooters think that a clean piston and cylinder support good accuracy. I suspect they're right.
I pretty much determined the problem was that the chatter marks effected the chambering of the next round which we all know is terribly important for repeatability. I found this by a regiment of single loading after finding the marks on the piston tail. Also found that the marks were chopped into the tail by the exterior of the gas cylinder. Seems the trailing edge of the cylinder becomes burred by carbon over time, and can cause this. I think one would need to do considerable shooting for this to occur. Naturally , if you have a safe queen that gets only an occasional view of the sun, chances are this will never happen to your piece of tail.
 

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I would get caliper measurements of the 2 pistons and compare. I'd be curious to find out the differences. Then I would stone the anti-rotation flat on the piston tale until it dropped freely in the cylinder.
 

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Discussion Starter #14
Following up on Phil's comments I am reminded that back in about 2016 when I was tuning up my first rifle (a Fulton rear lug) I had a thread on this forum about pistons, and specifically the size and weight differences. For whatever reason the photos no longer work.
Piston Discussion from 2016

One photo I was able to recover was a snapshot of the piston weights and dims. Outside Diameters were taken with a micrometer and I feel good about the accuracy recorded. IDs and lengths were taken with digital calipers and are significant to 3 decimals, but not the 4 recorded for length :

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They're are some differences in those dimension readings , could make a difference in operation depending on Gas Cylinder dimensions . I could also believe that if the Marks were on the Piston Tail or anti rotation flat are from manufacturing , that with Carbon build up or a loose fit in the Gas Cylinder , the marks may interfere with its return operation , but don't see how those marks could be made by cycling the Piston , thats would be one loose Fitment, of course anything is possible , just not likely .
I would try to Stone it as said above , that Nitrated surface may be difficult to work on though .
 
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Hmmm, I measured the pistons I have on hand 3, the one that is mia happens to be in a rifle that’s out getting mudded up at Tonyben’s place. I apologize up front I couldn’t get there weights as my scale doesn’t read high enough. You will also notice that the ring spacing of the SAI piston is also in different locations in comparison too the Bula and SADLAK witch are copied from USGI prints.

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The SADLAK piston is from the second production run and has a unknown round count but it’s been used in 6 rifles I’ve owned, and worn out a barrel or two. It does show some light discoloring on the outside I’m too lazy too wipe it off. The Black Bula has a about 200rds under its belt its nitride surfaces does exhibit a deep inky black chrome hue. The SAI piston has about the same round count however it does exhibit a light frosting on the main body. If I were too polish it I doubt it would return.
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As for the angry beaver teething marks my grooved USGI piston does shows a few mars but I’ve always been of the mindset that’s just a USGI trait and not a indicator of a pistons potential inaccuracy.
 
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