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Discussion Starter · #1 ·
I spent the day at the range experimenting with different ammo and gas valve setting combinations.

I took American Eagle (LC), Fiochi, Remmington and Winchester all in 55gr FMJ to the range today. I shot five round groups at all five settings which was 20 groups for 100 rounds.

At higher settings I found all four breeds to perform unsatisfactorily. As the settings dropped down the groupings overall tightened up. On "S" I shot an extra 50 rounds, I did not experience any FTF or FTE in this position as the manual suggest may happen with out a silencer on the end of the barrel. I found the gas valve setting "1" to be the best overall gas valve setting for the overall best groups in all brands.

Groups were as tight as 1.25" out to 4". Best average groups were 1.5" using AE.

Overall ammunition performance in my XCR in order from average groupings from best to worst was American Eagle, Winchester, Remington, Fiochi.
 

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Interesting report...

I wonder why/how the gas setting would affect the accuracy. I can't think of a good reason right off hand.
 

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Discussion Starter · #4 ·
Interesting report...

I wonder why/how the gas setting would affect the accuracy. I can't think of a good reason right off hand.
The gas valve setting will affect the variable amount of spent gases used to drive the piston home to cycle the bolt. The volume and pressure of residual spent gases used to propel the bullet down the barrel will also vary depending on the gas valve setting. The different volumes or pressure of the spent gases going down the barrel will result in different harmonic frequencies in the barrel itself. The speed or energy used in driving the piston and bolt back will also play into the equation as well some what as this can be adjusted via the gas valve. Al these will affect the overall performance and create a different point of impact (POI) and grouping size albeit in some rifles it may be small and others significant.

Bear in mind that all firearms are not created equal that they each have their own personalities on how they react to changes in the firearm. Experimenting with different ammo and gas settings will tell you what combination your rifle likes best.

I hope this answers your question.
 

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I don't believe that a change in the gas setting by itself will affect accuracy to any noticeable degree in a firearm like this. Certainly not in a range of 1.5 to 4" in 100 yards. Something else has got to be going on there. ;)
 

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Discussion Starter · #6 ·
I don't believe that a change in the gas setting by itself will affect accuracy to any noticeable degree in a firearm like this. Certainly not in a range of 1.5 to 4" in 100 yards. Something else has got to be going on there. ;)
???

I did not infer that all 4" groups shrank down to 1.5". What I did say was that groups were out to 4", this was based on different ammo at different settings. The Fiochi for instance was the worst performing for me averaging 2.5" to 4" depending on the gas valve setting from "S" - "4". The AE on the other hand was best at 1.25" at "1" out to 2.75" at "4", again depending on gas valve setting. I hope this clarifies your doubt.

If one methodically experiments and tweaks with one's firearm, they may be surprised at how much they can improve it. But it has to be done methodically and with one change at a time so you know what works what doesn't work. As I mentioned before no two firearms are alike but they may be close.
 

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The gas valve setting will affect the variable amount of spent gases used to drive the piston home to cycle the bolt. The volume and pressure of residual spent gases used to propel the bullet down the barrel will also vary depending on the gas valve setting. The different volumes or pressure of the spent gases going down the barrel will result in different harmonic frequencies in the barrel itself. The speed or energy used in driving the piston and bolt back will also play into the equation as well some what as this can be adjusted via the gas valve. Al these will affect the overall performance and create a different point of impact (POI) and grouping size albeit in some rifles it may be small and others significant.

Bear in mind that all firearms are not created equal that they each have their own personalities on how they react to changes in the firearm. Experimenting with different ammo and gas settings will tell you what combination your rifle likes best.

I hope this answers your question.
I don't mean to dispute your findings, perhaps we are misunderstanding what you are saying in the OP.

I find it unlikely that the gas setting would affect the accuracy potential of the rifle inself. While no doubt the gas/piston/bolt interaction will have an affect on barrel harmonics, it can probably be argued if it has any affect on the bullet. My understanding is that bullet has left the barrel by the time the gas piston begins to move (due to the momentum required to move the piston vs. the bullet), and is certainly out of the barrel by the time the bolt begins unlocking. Notwithstanding that any affect the movement of the action does have on the bullet should be consistent and 100% repeated on every shot, meaning the most that should be seen is a shift in POI but not grouping size.

Can you post more specific data on the variation of group size vs. gas setting?
 

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With something affecting velocity you should see more of a vertical stringing of the group and the shots drifting in the direction of the rifling--spin drift. All of those rounds are of first rate manufacture, but alas, are still shooting a 'plinker' slug. In my humble experience a good bullet can help make up (read: save your string of fire!) for a bad load elsewhere. I'd be interested to see this same test run with some good factory match loads, or if it you've got the tools, some good match handloads because that'll be about as consistent as you'll ever get.
 

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I agree. My rifle shoots much better with handloads of Factory Match than regular FMJs.

Also with the thin profile I get a little bit of vertical stringing as the brawl heats up. It is only really noticeable at 200m or more though.
 
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