Diagnosing a bullet strike

[Tozguy]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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.