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Diagnosing a bullet strike

cdhknives

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#1
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?
 

Silverbullet

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#2
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
 

kvt

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#3
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.
 

Hozzie

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#4
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.
 

pdentrem

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#5
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
 

Tozguy

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#6

kvt

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#7
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
 

Hozzie

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#8
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.
 

cdhknives

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#9
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?
 

cdhknives

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#10
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?
 

Tozguy

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#11
On second thought if you want to determine which is out of spec, the barrel or muzzle brake or both, I agree the rod trick will not help. It would only indicate if there is a problem and you already know that from the oblong bullet holes in the target.

Hozzie's post (number 4 above) would show the culprit part better.
 

cdhknives

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#12
The problem with indicating the outside of the threads is finding an accurate surface, not getting lost in the peaks and valleys. How is that going to work?
 

seasicksteve

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#13
The problem with indicating the outside of the threads is finding an accurate surface, not getting lost in the peaks and valleys. How is that going to work?
Thinking outside the box here and I have never tried this but here goes. So indicate the barrel in the lathe just as if you are going to thread it for a brake of suppressor. Set up as if you are going to thread set thread pitch to match the pitch of the thread already on the barrel. Mount a dti in a toolholder and set it up so the ball of the dti is centered in the valley of the thread, kinda the same way you would if you were picking up a thread. Rotate the spindle by hand with the halfnut engaged and watch the dti. I would then disengage the half nut and check the surface/shoulder that the brake locks up on. this should tell you where the problem lies. If it checks out mount the brake while the barrel is still chucked up and true and check the ID of the brake. if it is not concentric simply buzz it with a small boring bar and true it up. Dont need to be super concerned about maintaining a real tight tolerance between the bullet dia. and the brake id some clearance wont kill the brakes effectiveness.
 

Hozzie

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#14
Is there no relief groove cut near the shoulder? May not be, but would be your best bet probably. You may have to simply take measurements at 4 spots where you are inside (or touching two threads). Not really rotating the barrel. It should give you an idea if the threads are aligned with the bore or if they only centered on the outside of the barrel.

Edit: Steve has a good plan. Checking the shoulder is also a good point.
 

derf

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#15
Easier way......chuck up a piece of scrap and thread it to accept the brake. Use it to determine if the brake is running true. You'll want of minimum of .020" clearance over bullet diameter. While you're at it, check your can at the same time. If the brake runs true, it's the muzzle threads that are off, and vise versa.
 

cdhknives

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#16
Would using a couple of thread wires for the outside allow me to measure the barrel wall thickness to check for the bore and threads being offset?
 

Bamban

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#17
Thinking outside the box here and I have never tried this but here goes. So indicate the barrel in the lathe just as if you are going to thread it for a brake of suppressor. Set up as if you are going to thread set thread pitch to match the pitch of the thread already on the barrel. Mount a dti in a toolholder and set it up so the ball of the dti is centered in the valley of the thread, kinda the same way you would if you were picking up a thread. Rotate the spindle by hand with the halfnut engaged and watch the dti. I would then disengage the half nut and check the surface/shoulder that the brake locks up on. this should tell you where the problem lies. If it checks out mount the brake while the barrel is still chucked up and true and check the ID of the brake. if it is not concentric simply buzz it with a small boring bar and true it up. Dont need to be super concerned about maintaining a real tight tolerance between the bullet dia. and the brake id some clearance wont kill the brakes effectiveness.
Steve,

That is how I check the tenon thread on the AR barrels that I do, substitute the threading tool with a DTI. Without changing anything on the set up I just run the lathe in reverse. The runout should be the same as the chamber. I also re-indicate the shoulder before I call it good.
 

cdhknives

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#18
I got the thread wires and calipers out tonight. Barrel thickness seems uniform to less than .005"...and this is very hard to measure with rifling in the way but I tried 4 points and they were all within .005".

Then I measured overall thread diameter. I came up with .593". This is just BARELY above min for a 2A thread.

I tried the brake on another barrel that was also 5/8-24 and it is noticeably tighter...less wobble when a half turn short of snug.

Enough slop to cause my problem???
 

Hozzie

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#19
Did you check the shoulder? If the thread is loose and the shoulder isn’t square to the bore, that could do it I suppose. I still think the only way to know for sure is to chuck it up.
 

cdhknives

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#20
The combination of a short (10") barrel and 3/4" bore in my spindle means I can't chuck it into my lathe. The chamber end flange (AR15 barrel) is too big and would be inside my spindle. Maybe if I float the whole thing out on a steady rest...ugh.
 

Dabbler

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#22
cdhknives, the only way we ream chambers and crown muzzles and thread barrels is using a steady rest... My toolmaker friend and I tried a bunch of other approaches, but this way we can achieve tenths concentricity and perfect alignment to the bore - both of which prevent problems such as you are facing. Because we are so careful we install all muzzle breaks and suppressors for out city's tac team - they just don't trust anyone else.

BTW you don't need contact to flip your bullet... we tried a few off centre muzzle breaks with .020 clearance to the bullet... With .005 out of concentricity the bullet yaws considerably at 100m. I presume it is the differential gas pressure/gas velocity that does it.
 

cdhknives

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#23
Interesting. The little bit of instruction I got was to use a heavy copper wire around the barrel in the 4 jaw and a spider at the left end of the spindle. Not an option here. I have done exactly 2 re crown jobs on my rifles so I am totally green at this but I have cut some tight threads. If I can dial in the bore I am confident I can rethread this. I assume you are chucking the left end in the 4 jaw to handle thrust, as using a driving plate and dog would not hold it securely I think.

I ran the barrel between centers, just turning by hand. The face was dead on but the relief groove shows 2-3 thousandths float on the dial. I do not have a piloted reamer to insure the crown is square/true though, so if the crown job is bad it would not necessary ride on the live center true...???
 

Dabbler

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#24
The most accurate way to centre the bore we have found is to make a close fitting pilot (we use hard bronze) that fits the barrel perfectly - barrels can vary +/- .0015 from the published spec - we've seen a few +.001. So we always make a tight slide fit pilot and indicate on that. That makes our threading concentric to the bore +/- about .0002 or .0003. Most of the barrels we do (these are very expensive brand new target barrels, mind) are between .004 and .008 out of concentricity (that's how we discovered the bullets 'kicking' sideways in live fire tests). We discovered these concentricity problems over 30 years ago, and some of the manufactures have tightened up their specs, but now the new carbon barrels are worse. The best we've measured has been .008 off concentricity.
 
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