762-SDN-6 Possible Endcap shave/strike. Off-center bore?

[pneumagger]

I experienced some erratic grouping when sighting in an optic when shooting my SDN-6 for the first time. This was the first 50 rounds through the silencer, so it is essentially still brand new. After getting home, I noticed a little copper rubbing on the endcap bore. I cleaned off the copper rubbing and noticed a bit of pitting on the inside lip of the endcap. When looking across the top of the endcap and placing a flat surface on it there is slight protrusion right at that point of the bore hole. So I'm 99% sure there was some sort of endcap rubbing/striking happening.

I took some pictures and caliper measurements
The endcap bore seemed incredibly uniform at 0.368" +/- <.0005"
The OD of the suppressor near the endcap was also uniform at 1.443" +/- .002"
The little raised edge at the rub mark is pushed about .005" above the flat endcap face.
Measuring the endcap dimension from the bore edge to the tube OD of the tube beginning at the rub mark and going clockwise 45deg each yields:
1) .547" (at rub mark)
2) .539
3) .530
4) .523
5) .530 (opposite rub mark)
6) .535
7) .546
8 ) .546
9) .547" (at rub mark, again)

Given the OD uniformity and bore uniformity, it would look like the center of the endcap hole is off center about 0.12"
Given that the rub mark IS along the thicker side, could this cause the problem?
I only put about 40-50 rounds through it. I do have another rifle with a 20" the same style brake mount I can try the SDN6 on too. I also have a brand new 762-SD I can try on the original 14.5" rifle. Perhaps this could rule out the rifle/mount combos.

As a background:
I ran some rounds though the rifle at a couple close distances to make sure the bullets was stabilizing. I was shooting factory 155gr Amax and 168 Amax shot from a 14.5" 1:10 barrel - nothing exotic. The silencer had some very minor wobble at full lockup, less than a few other AAC 51T cans I've seen. Looking down the bore to give it te old eyeball-alignment test; while it wasn't "perfect" it certainly seemed centered decently enough. I used the "o-ring method" to remove the wobble. I 100% know the can was installed completely because before I ever added the oring I tightened the can down as much as possible to make a point of reference on the mount indicating where the can should rotate at full lockup - so made sure to tighten it 100% and it did not back off at all.

[RJT]

AAC bores are EDM'ed concentric to the mount, so an off center bore at the end cap doesn't make a difference. My cyclone is off center, and I haven't had any issues. When I called, AAC said shoot it, IF there were no other alignment issues. I also have a Specwar 5.56 with an off center bore, and no issues with it either.


"Looking down the bore to give it the old eyeball-alignment test; while it wasn't "perfect" it certainly seemed centered decently enough. I used the "o-ring method" to remove the wobble."

This is probably what caused the strike. All of mine are "perfect", when looking down the bore. Maybe your host wasn't threaded concentric to the bore, or the o-ring threw the alignment off.



ETA: There is still a possibility the suppressor bore is "off", I'm sure AAC will get you taken care of either way.

[RJT]

Borrow a loose mount, and see if the bore looks perfect with it attached to the suppressor only. That should tell you what the problem is real quick. I've sent back 3 barrels because they were improperly threaded. The can screwed on perfectly, but the bore was offset at the muzzle. Some makers don't index off the bore when they cut threads. Just tossing out some ideas, hope you get the issue sorted out.

[pneumagger]

The mount and barrel are pinned, so I can't really swap mounts easily. I'd really like to verify it's not the can before I have to basically get a new barrel.

I put the SDN-6 on a 20" Rem700 SPS AAC that also has a brake mount. It wobbles a bit more than on the 14.5" AR, but appears to be slightly better centered when eyeballing down the bore. I'm going to pick up the longer 762-SD suppressor in a few days and take a look at it on the 14.5" and the 20" as a comparison. The weather should warm up this week and I may be able to get out and see if there are any issues shooting longer distances.

In the mean while, I'm going to get a long piece of .2968" drill rod and drop it down the barrel to see how the suppressor actually lines up to the bore.

[pneumagger]

I purchased a 3 foot piece of 19/64" precision ground drill rod (.2968") to go into the bore of my rifles and check alignment. Man, these are straight and a slip fit. For less than $5, I can't imagine having one for every caliber I suppress.

So I'm pretty sure my barrel threads and mount are both concentric and have minimal runout. Here are pictures of the rod as it sit in the brake mount. For reference, the bullet rub on the endcap is on the "right" side of the rifle/silencer.

Views and measured clearance at brake muzzle: Top (0.023"), Right (0.023"), Bottom (0.024"), and Left (0.026")


Pretty concentric. Looking at the runout, again here's the views of the silencer muzzle (WITH the oring installed).
Everything's looking good and centered so the runout seems decent too.


Below is showing the end view with NO oring but the can is as tight as it can be turned. It has ZERO play/wobble.
Looks VERY well centered. It's about 10% of a click from locking on the next tooth, but if shot it would back off about 90% of one tooth.


These side pics show the suppressor with NO oring installed and aimed straight upward so it sits naturally.
It is screwed in and locked to the final tooth that it can fully engage - so this is how it sits without an oring.
It is not well centered and one side may even be touching. (not the side that has bullet rub, though)


This is an end view with NO oring and the can only engaged fully on the last tooth it can.
With light finger pressure to the top and bottom of the silencer, the rod easily touches when I push the bottom.

[pneumagger]

I attempted the same set of observations on a Remington 700 AAC-SD with similar brake mount using this 762-SDN-6. The concentricity and runout were also excellent (as expected on a factory suppressor ready AAC barrel). The only difference was the mount does not back off as far when ratcheting back after fully installing. The Remington 700 only backs off about 1/3-1/2 of a tooth whereas the 14.5" AR10 backs off almost an entire tooth... so there is less play and I can't get the rod to touch the Rem700's suppressor even pressing firmly on the silencer. I am confident this silencer is just fine and both my 14.5" AR10 and Rem700 barrels threading are great.

I've come to the conclusion my mount and suppressor combo has TOO much wobble/play. I will have to lap/polish the rear mating surface on the mount so I can get that last 1/10th of a tooth to fully engage. Unfortunately, it's pinned and welded to a 14.5" barrel, so I can't just chuck up the mount into my lathe and polish it all concentric-like. I may have to get creative with keeping the material removal controlled and uniform.

So even though an oring holds the can centered at rest, it is just an elastomer oring and can flex when 10000+psi hit the suppressor and push on it. It may keep the user from feeling any wobble and it may even help shot-to-shot consistency by centering the suppressor each time the gun fires; but it may not be good enough to replace tighter tolerances.

[pneumagger]

Well, being unable to chuck a pinned/welded mount into my lathe to reduce the mounting collar for a better fit, I was faced with cutting it off for a new better fitting mount. So with nothing to lose I used a pair of sharpening stones to reduce the surface by hand with no guide or rotation. Going slowly it took about 30 minutes to remove a small amount of material - but just enough to barely get the last tooth to click. I'd say about 95% of the black finish was ground off. There's absolutely ZERO backlash and NO wobble. Checking the runout with a drill rod, it was also near-perfect. I'm super happy with the way it turned out.

Disaster averted... and, most importantly, money saved.

Even though my other can (762SD) fits decently on my Rem700 AAC-SD with only the slightest bit of backlash, I'm going to pull the mount off and hone that one down too. Fitting and mating a mount to a suppressor is just that much nicer. If my Remington 700 mount modification is as successful as my AR10 modification, I'll likely abandon my plans to get a dedicated thread on silencer for precision. I feel that custom fitting the 51T mount to a silencer is pretty much as rock solid a a thread on.

[Conqueror]

Good followup. The mount wobble has been a controversial topic for a long time. Many have had no issues, but some have. The lapping/polishing seems a good fix for those who can do it.

[pneumagger]

I was surprised how well it locks up with less than a half hour of free-hand stoning with the mount still attached to the rifle. Although keep in mind mine was very very close to the next tooth already. So with sharpening stones it can be done by hand with no other tools, So pretty much anybody can do it.

Although with proper tools it was MUCH faster. I used my lathe to match a spare mount for my other supressor and it took about 5 minutes to shave off a half-notch worth of material. I just used the same hand-stone and let the lathe spin the mount. Most of the time was chucking it up and ensuring concentric rotation.

Distributers or SOTs should offer this as a service while people wait for the forms to clear. $25 per mount to mate it to the can and it only takes 5-10 minutes. They would sell more mounts with the suppressor and get some service revenue too.