"graceful" cycling

[stagpanther]

I mentioned this on another thread to someone who was having accuracy issues in their AR10 build depending on whether or not he had his gas system "turned on" or not. He mentioned his groups opened up significantly--though not terribly, when the gas system was cycling.

The hardest part for me doing AR10 builds has always been getting the cartridge to cycle all the way through with minimal "damage" on its way into the chamber. Among other things, there's the extension lip and lugs that the bullet will at least drag by, maybe smack right into, on the way to the chamber.

Today I was out shooting my 7mm-08 build and noticed the brass was getting scraped pretty good while cycling, one cartridge I did a test drop of the bolt and then ejected showed a deflection in the bullet's tip.

Here's a group of 120 vmaxs I shot at 113 yds using the burris RT3 prism sight. The first two shots out of the magazine are the higher ones that hit the target circle. The last three shots I hand fed directly into the chamber and they impacted a bit lower but made a ragged hole. Could be a coincidence, but I don't think so.

 

[tangolima]

Cold bore shift is possibility. Need more experiment to determine.

In my case, I didn't single load. When the gas was off, the rounds were in the magazine. Feeding was by pulling charge handle all the way back and release. So it is very similar to the semi auto action when the gas was on.

I also need to experiment more. Will do that when we are back in town.

-TL

PS. I have found myself doing more and more single loading. Shoot slower to conserve components, and to make each round count more.

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[Recycled bullet]

I wonder if the impact on the bullets tip that happens during the feeding cycle is introducing variable run out into the assembled cartridge and you are seeing this on the target.

 

[stagpanther]

I wonder if the impact on the bullets tip that happens during the feeding cycle is introducing variable run out into the assembled cartridge and you are seeing this on the target.

That's along the lines of what I suspect.

 

[rickyrick]

I have tossed around the idea of blocking a gas port and making a straight pull bolt action upper; Considering the cost of some decent bolt actions.

Sorry, I couldn’t resist.

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[stagpanther]

Shot another magazine load today (in the rain) and seeing "a bit more than usual" scrapes on the ejected cases. My motto with AR's (actually works for most firearms I think) is "brass tells the story."

 

[tangolima]

I have tossed around the idea of blocking a gas port and making a straight pull bolt action upper; Considering the cost of some decent bolt actions.

It works, but it is not ideal. The mating between upper and lower receiver in AR makes it a hit and miss affair for accuracy. Cycling is certainly not as smooth (graceful) as in a real bolt gun. It lacks the proper primary extraction. I was concerned with possible hard extraction. It turned out not bad. The inertia of the BCG did an sufficient job after firing. A muzzle brake definitely helps too. It is just like mortaring to extract a stuck brass.

I quite like the flexibility of this approach. I can turn on the gas to go semi auto if I need it. Need another caliber? A new barrel / upper will do, plus all the winky dings for AR platform.

-TL

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[tangolima]

Shot another magazine load today (in the rain) and seeing "a bit more than usual" scrapes on the ejected cases. My motto with AR's (actually works for most firearms I think) is "brass tells the story."

A full magazine in semi auto, I suppose. Shoot same number of rounds single loading when you have chance to compare groups. The best is to shoot the groups when the bore is warm.

I'm going to do similar test when I'm back in town next week.

-TL

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[Shadow9mm]

There are 3 theories I have heard explaining cold bore shift. The first is a cold shooter, the second is fouling, lastly its just part of a larger group.

it was one of the believe the target podcasts. the guest stated that he had his trainees do exercises, dry fire, before shooting their cold bore shots. He stated that this almost always eliminated cold bore shift.

The host stated he did believe in cold bore. He stated that when shooting longer ranges the first shot was consitently 1moa low. He stated he believed it was due to cleaners or oils left behind or possible the bore not being fouled, that the fouling in the bore causes the gun to shoot differently.

Lastly is the Hornady theory somewhat put forth by hornady. Your sample size is too small. Shoot a group of 20rnds, see where you cold bore spots wind up within that group. It just may be a statistical anomalie that appears to be a shift, but its just part of a group that is larger over all.

 

[stagpanther]

I should have mentioned it--there was a full 5 shot magazine shot before I did the picture above--that's when I noticed the brass coming out a bit damaged. I've been through this many times before with other builds. There's "get it to work" and then there is "get it to work optimally."

 

[stagpanther]

I went out today and shot 8 shots from the magazine and 8 shots hand-loaded into the chamber. Distance was 102 yds into a moderate quartering headwind. Neither group was especially good, but the magazine-fed shots obviously were dispersing wider--by a factor of more than 2x.

mag-fed:

hand-fed:

I don't think that this one case in my build necessarily proves anything--this difference is actually more profound than what I've seen in my other builds--but it does point out that I need to do some sleuthing to figure out what the issue is. For what it's worth, I don't recall pulling any shots.