end cap strike, 9mm threaded can

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mr fixit
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end cap strike, 9mm threaded can

Post by mr fixit »

As I posted previously, I'm working on a 9mm can for a Glock AR SBR. I have a Griffen style taper mount I made from 17-4 for the brake mount. I made the endcaps and threaded the tube myself. This afternoon I finished the distal end cap. I threaded it all together and put it on the AR just to shoot it, no baffles inside since I haven't made any yet. The end cap has a .406" bore.

I had previously sighted the gun in with a Sig red dot and it grouped well. Put the muzzle brake mount on and checked it, which also grouped well. This afternoon when I shot the empty can, I had bullets hitting several feet low. Checked the end cap and saw a slight crescent of brass on the inside, obviously a strike. There was no damage or deformation to the cap, just a slight dusting off brass.

I'm asking for advice on how to proceed. Would opening the bore on the end cap to say .420" be enough, as long as the baffles are also matching? Should I try to somehow realign the tube? I'm kind of waiting and thinking on what I should do.
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Re: end cap strike, 9mm threaded can

Post by mcrump »

Something is way off. I'm guessing it's an alignment problem. I would order a proper size alignment rod from McMaster-Carr and check that the suppressor is in line with the rifle bore.
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Re: end cap strike, 9mm threaded can

Post by CMV »

an 11/32" aircraft extension drill will give you a close approximation w/o buying drill rod.
mr fixit wrote: Wed Jun 10, 2020 10:06 pm snip... I have a Griffen style taper mount I made from 17-4 for the brake mount. I made the endcaps and threaded the tube myself.
You did all these with your 4 jaw chuck and indicated the work every time? Also, did you torque the bejeezus out of both caps?

I would say you threaded an end with too much runout, have an [both?] endcap shoulder not square, or didn't check barrel threads for concentric alignment. No telling without physically having it to play with & see.

If it were mine (and it HAS been mine - had same issues and if you saw what I grenaded 2 weeks ago and why that happened, you'd just shake your head) I would indicate the barrel in 4 jaw, then keep adding 1 piece at a time indicating those to see where the problem starts.
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Re: end cap strike, 9mm threaded can

Post by mr fixit »

CMV wrote: Mon Jun 22, 2020 4:39 pm an 11/32" aircraft extension drill will give you a close approximation w/o buying drill rod.


You did all these with your 4 jaw chuck and indicated the work every time? Also, did you torque the bejeezus out of both caps?

I would say you threaded an end with too much runout, have an [both?] endcap shoulder not square, or didn't check barrel threads for concentric alignment. No telling without physically having it to play with & see.

If it were mine (and it HAS been mine - had same issues and if you saw what I grenaded 2 weeks ago and why that happened, you'd just shake your head) I would indicate the barrel in 4 jaw, then keep adding 1 piece at a time indicating those to see where the problem starts.
I used a 5c collet in the Logan 12" for the brake. Caps and tube were done in the 4 jaw, indicated each time.
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Re: end cap strike, 9mm threaded can

Post by mr fixit »

The endcap had no damage. just a brass kiss I found after the bullets were hitting a foot low at 25 yds. I opened the endcap bore to .422 and now all is good
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Re: end cap strike, 9mm threaded can

Post by CMV »

I'd still find where the issue is. .060" is a safety margin, not min clearance to work. Even if baffles have same bore size and bullet clears everything, being so far off center thru the suppressor bore is likely to play hell with accy. Again....ask me how I know..... :)

Maybe not every single instance....my sample size of 1 time it happened.....no baffle strikes or rubs anywhere, but it just spewed bullets all over the place. Like 12" pattern @ 50 yds. That was from crap alignment and incrementally increasing bore helped some, but didn't fix.

Might also want to try with a big long spacer inside, end caps torqued down on it vs an empty tube w/ just endcaps. A full tube w/ tension from the endcaps makes a difference. Doesn't have to be an interference fit spacer like you'd use for construction, just a length of slightly smaller diameter tube, cut square, and long enough that the caps put it in tension. Torque it down good like you would if it was a baffle stack in there. No idea if any kind of tape would stick over exit hole, but if it could, you'd see how close you are under that condition to being aligned.
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Re: end cap strike, 9mm threaded can

Post by mr fixit »

I did wonder if shooting through an empty tube would have any effect on the bullet. Since the bullet never outruns the gas in the singe chamber a tube with no baffles creates, would it push the bullet at all?
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Re: end cap strike, 9mm threaded can

Post by mr fixit »

How do I check the tube to see if it is 'right'. If i had to bet that's where I would wager the concentricity is off, I had to thread it in the 4 jaw with a steady rest.
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Re: end cap strike, 9mm threaded can

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You'd know that indicating it in 4-jaw. Gets down to almost no runout, move dial indicator back on piece a few inches and now there's runout. If no matter what there's runout when measuring a few inches apart your stock is out of round. If you have the needle type dial indicator vs the plunger type you could also indicate the OD then move it to see if same amount of runout on ID.

Me, I'm lazy and would just roll it on a surface plate and watch closely :)

To be sure, you'd clean up the OD & ID with skim cuts. Sounds nice, but takes a beefy machine. My 2HP 13x40 won't do it. I get a boring bar that far out from my cheapie toolpost & would just make it worse most likely. I like DOM tube for this reason - it's generally pretty true.
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Re: end cap strike, 9mm threaded can

Post by mr fixit »

CMV wrote: Tue Jun 23, 2020 1:51 pm You'd know that indicating it in 4-jaw. Gets down to almost no runout, move dial indicator back on piece a few inches and now there's runout. If no matter what there's runout when measuring a few inches apart your stock is out of round. If you have the needle type dial indicator vs the plunger type you could also indicate the OD then move it to see if same amount of runout on ID.

Me, I'm lazy and would just roll it on a surface plate and watch closely :)

To be sure, you'd clean up the OD & ID with skim cuts. Sounds nice, but takes a beefy machine. My 2HP 13x40 won't do it. I get a boring bar that far out from my cheapie toolpost & would just make it worse most likely. I like DOM tube for this reason - it's generally pretty true.
The exterior of the tube is good. When cutting threads on the interior, I made a skim cut to keep the baffles from dragging on the threads, then threaded.

I'm wondering if I cut the threads off somehow
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