Diagnosing a bullet strike

cdhknives

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So i have a 300 Blackout barrel that shows accuracy issues...bullet holes are oblong (and shifted point of impact a lot) in a target with the brake on, perfectly round (and on target) with the brake off. After a bunch of testing I have narrowed it down to light bullet contact in the muzzle brake. I need to figure out if the machining problem is the barrel threads or the muzzle brake. YHM brake. AR Stoner factory threaded cromoly barrel with 'Melonite' treatment.

Suggestions? No I don't have spares of either for comparison, and the brake is a suppressor mount so I am not going to risk my suppressor on live testing either.

I can't see how to directly measure concentricity in existing muzzle threads without cutting/chasing them and thereby removing the barrel nitriding.

Same problem on internal threads for the muzzle brake.

Maybe easier to just part off the barrel threads and recut?
 
Have you cked the bore on the brake maybe it's off. Happens you can try a bore laser it may show deflection in the beam to help pinpoint the problem. It may just need a little oversize bore on the brake . Just my thoughts
 
I was going to suggest the bore laser as well. Another option is take a piece of scrap and thread it for on and see if it is off, or make a piece to screw the brake on and see if it is off. If you did one for the break, you could cut a rod down so that the end is the same size as the end of the brake, then put the threads far enough back that the end would stick out of the brake and cut the threads. If it screws on without hitting then look at the barrel threats and vice versa. Even a piece of Al should work to make them for testing.
 
Why not put them in the lathe and indicate the bore to zero and then indicate on the outside of the threads ( or the barrel itself would probably show enough runout if it is that ). Do the same to the the inside of the muzzle break with a long reach indicator.

It should show if one of them is way out.

I don’t think a laser will work. Not thick enough to mimic a bullet.
 
6" length of drill rod that fits the bore and try to pass it through. It will show an even gap all the way around at the brake exit. If not then you have a concentric issue. If the brake bore is off, an option is the bore it with a piloted reamer.
Pierre
 
After thinking about it the problem I see with just using a rod on the bore is that it will indicate that something is out, but will not indicate what part. and did I see something about the brake also the mount for the suppressor If so you need to figure out which part is not in line or your suppressor will be out of line when attached. If you just bore the brake so that the bullet does not touch then you still have to problem when the suppressor is mounted unless the problem is fixed. Just my 2 cents
 
Yes, boring the brake larger is not the fix if it is in fact threaded out of spec. If you can determine it is the brake, I would contact the manufacturer and have them send you a new one.
 
While I was eating supper I had two bright ideas...with a little help from another forum. First, shim the brake a half turn and see if the strike follows or stays put. Second, indicate the rifle bore true to the spindle and gauge the ID of the brake and the suppressor mount surfaces. If both surfaces are out in the same quadrant the brake is ok but the barrel threads are likely crooked. If the cant/runout is different on the bore and suppressor mount surfaces the brake is mis-machined. Sound reasonable?
 
McMaster sells a reamer blank in .2969" for under $5...that is right in the middle of the three 30 cal sizes PTG offers (at $35 each). Good enough?
 
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