Re: Its Science Holmes...mounting a can
My suppressors are all single point thread attachment.
My friends have several of the various QD versions out there; notably the ratchet type AAC(I think?)
My friend was a die hard "suppressors screw up accuracy" guy.
I noted group sizes on his custom built TBA 308 at 100, 300, and 600yd.
Baseline was:
1/3MOA FGMM @ 100yd
1/2MOA FGMM @ 300yd
4" (2/3 MOA) FGMM @ 600yd
The argument that a better shooter could do better is valid, however I wanted to see what would happen with just a suppressor on/off test from the rifle's owner.
With the suppressor on it was
1 MOA @ 100yd
1.5MOA @ 300yd
1.5MOA @ 600yd
The shift was sometimes repeatable, sometimes not. The best groups were routinely found on the "4" thread on the 51t ratchet mount. On the others the groups were double the posted size.
We took the mount off the rifle and shot it.
No suppressor baseline did not change.
With my suppressor cranked on there it showed no group size change, the shift happened but it was entirely repeatable at 100 and 300yd. We didn't get enough time to test with the 600yd burm on the second day.
We ran a bore scope down the suppressor to look for damaged baffles or shaved jacket material that would indicate baffle strikes and did not find any.
I dialed the barrel into the lathe and checked the runout of the mounting surfaces on the muzzle brake/suppressor mount and noted that there was several thousandths of concentricity and angularity runout.
My initial thought was "the guy who installed this screwed up"
I took the mount off and then checked the barrel threads and bore for runout, found none there.
I then checked the mount again for concentricity and found runout between the toleranced surfaces that lined up with the mounting surfaces inside the suppressor.
We pulled a brand new mount out of his bag that had yet to be installed, it was off as well, but only by about half as much (3.5 thou as opposed to the 8 thou on the used mount).
We swapped the mounts and the situation improved, however it was never as good or repeatable as the single point attachment.
Using my 30c suppressor I took my 308 and did the following test:
Thread it, shoot for baseline groups at 100, 300, 600.
All were sub 1/2 MOA except for 600yd. It was 3" for 2 - 5 shot groups.
Attach the suppressor, note the shift, note the index point based upon the suppressor markings.
Shoot groups at 100, 300, 600 again. Note group size --> No change in baseline other than shift.
Check for shift consistency by taking the suppressor off shot to shot.
The group sizes opened up about 1/8th MOA over the previous averages. This could be attributed to suppressor and/or the shooter. Each time I had to take the suppressor off I had to break my cheek weld and move around, there was not the same level of shooting position consistency.
Put the barrel in the lathe, push the sholder back 0.010" and note the index point.
Shoot 100yd group, note the suppressor shift and compare it. The vertical shift was appx cut in half, the horizontal shift appx doubled.
Group size and shot to shot shift were again consistent.
Repeat pushing shoulder back by 0.010"
Horizontal shift was zero now and vertical shift was past zero and in the opposite direction.
Pushed shoulder back to 0.022" to the original clocking position. The zero shift returned to within 1 click (1/4MOA) of the original shift.
At least on my 30c suppressor the shifts were dependent upon the clocking position of the suppressor.
I call the Quick Attach mounts into question because of the thread pre load involved with multi-lead threads and the ratchet stops.
My suspicion is that the suppressors are not being consistently preloaded into the joint on the muzzle tenon and the suppressor is causing shift that varies shot to shot similar to the behavior that Robert Vaughn discusses for shot to shot dispersion due to the barrel/receiver joint destabilizing.
I am not 100% on this and admittedly I have not gotten enough testing time to really make a decision, however what I've seen anecdotally, through the limited testing I mention above, and watching my friend's AAC suppressor have issues I am leaning toward that hypothesis.
