Posted: 9/18/2016 3:46:56 PM EDT
sorry but pic got blocked originally for some reason :(
It was the owners hope he was building a precision upper :) He ended up with something he claimed shot wildly and had no clue what could be wrong. To eliminate some guesswork I put the upper on my lower and shot it off my bench using my loads. The pic is of a group shot from a cold barrel and the load is 58gr vmax on 25.2gr of N530. I know some are going to ask. As you can see I got it settled down into a 0.75" group with only horizontal stringing and not much vertical. So before I pull it down for him and see what I find I thought I would throw it out to the masses to see what you think might be causing the side walk. Should be fun troubleshooting. |
It definitely could be shooter, bench setup or technique but honestly since I shoot off the same bench, rest and bag with a bolt 6PPC printing around 0.1" groups and one of my gas guns is under 0.25" I don't think I would be off as bad as this. The owner definitely had a problem but now I have it down to a nice little horizontal line and am leaning towards a mechanical problem still in the upper. He didn't check the trueness of the receiver/extension or bed the barrel in. The scope is a second hand that he really doesn't know anything about but it is suppose to be a weaver with micro track which is pretty good usually. FF tube is tight and the gas system seems ok. Waiting on a set of scope rings for my spare 36x scope so I can rule out a scope problem. Stress left in the barrel from machining can make it walk but that usually settles and pulls in tighter on the last 3 shots. Few things to check out I guess before pulling it down. The owner is excited about how it shoots now and wanted it back but I said sorry too late the process of elimination has started 
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Quoted:
Tough to say with one group, are all/a majority of them looking like this? Which way is it walking? I'd also like to see a couple more samples. What was the dispersion like? Did it just walk one way shot after shot or did it go one way then the other then center fill-in? |
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Just mounted a different scope on it. I shot that last target just to snap a pic. Placement starts to the left and continuously moves to right in general but I have had some double backs to the left which could be me. If a scope problem doesn't jump out at me I will shoot 3 groups of 5 shots to give a better picture. Need to process some more 6X45 brass.
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What about a bad barrel stringing the shots as the barrel heats up? Did the shots continuously move right, or was it more random?
ETA: Understand, moves to right each shot. What profile is the barrel? Thinner? Barrel maker? ETA: Usually see this one sporter profile barrels on MANY hunting rifles during the Deer Days sight in at my local range. Lots of guns going through there with thin, factory barrels. Sometimes need to stop for a few minutes to let the barrel cool. Usually helps them get zeroed. |
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Medium contour 416 stainless 0.740" at the muzzle. I have had some chrome moly barrels walk similar to this when heating up but so far haven't noticed it much in a stainless barrel. I had a CM bull barrel that was a tack driver once you put half dozen rounds through it to warm up so I had the bright idea to send it to be cryo treated and hopefully lock it in. It locked it in alright...to a 2" group. Extreme load change wouldn't even tighten the group after that. Not one of my brightest ideas.
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Quoted:
It definitely could be shooter, bench setup or technique but honestly since I shoot off the same bench, rest and bag with a bolt 6PPC printing around 0.1" groups and one of my gas guns is under 0.25" I don't think I would be off as bad as this. The owner definitely had a problem but now I have it down to a nice little horizontal line and am leaning towards a mechanical problem still in the upper. He didn't check the trueness of the receiver/extension or bed the barrel in. The scope is a second hand that he really doesn't know anything about but it is suppose to be a weaver with micro track which is pretty good usually. FF tube is tight and the gas system seems ok. Waiting on a set of scope rings for my spare 36x scope so I can rule out a scope problem. Stress left in the barrel from machining can make it walk but that usually settles and pulls in tighter on the last 3 shots. Few things to check out I guess before pulling it down. The owner is excited about how it shoots now and wanted it back but I said sorry too late the process of elimination has started I would look there first. Look at how tight your vertical is. That's crazy tight vertical, not accidental. Looks like the windage turret is drifting with the recoil impulse from semi auto, return-to-battery stroke beats old school optics around like this in many cases. I've seen in when people try to slap cheap scopes on ARs. |