suppressor inconsistency with subsonic rounds?

[Jasper1573]

Acquired a suppressor within the last few months. This suppressor shot great when I first received it, and still does very well with standard velocity rounds in 308 Win. However, its performance with subsonic rounds has deteriorated in a strange way. When I first received the suppressor, the group size for subsonic rounds at 1050 fps was about 1 MOA at 100 yards. Now, with exactly the same loads, it will shoot a few rounds in a 2 inch group, then they will begin to go high and right as much as 12 inches from the original zero.

Any idea why this might be occurring?

 

[TNBilly]

Just a thought I guess but, just from the way you're putting the problem everything is fine with standard rounds but goes haywire with a different loaded round? The fact one is sub the other not I should think is not relevant, the problem is only with the sub load so it's a load/cartridge problem at face value. If it were a mechanical to the firearm or supressor problem than I'd think it would affect both rounds. I'd suspect some type of fouling or unusual heating problem with the sub load, though I can't provide a solid reasoning for heat, perhaps a result of a powder burn problem.

 

[cheeze]

But these loads shoot fine without the suppressor?

 

[Jasper1573]

It has been a while since I checked these loads without the suppressor, so I intend to load up some more and try with and without the suppressor. I posted this same topic over on snipershide and no conclusion there yet either.

Have to fire more rounds with/without suppressor later in the week.

Will post how it comes out.

 

[Captains1911]

Is it a QD or screw on can? Any chance the can is loosening?

 

[Jasper1573]

Again, I really appreciate all the info and advice...you all are a good crowd to converse with.

So I did as was directed...I loaded up some rounds with the original load that worked so well originally, 175 grn SMK, 10 grn TB, WLR, and Win brass. I fired 5 rounds without the suppressor...nice group of about an inch with one flyer a couple of inches away, but that was a problem with the trigger mechanism, that is, my trigger finger;-) No issues with tumbling...nice clean holes at 100 yards. All rounds were carerfully single fed.

Next 5 rounds through the suppressor...again, nice group of 4 rounds at about an inch or slightly more with one flyer a few inches away, but not 7-8 inches as before. Now I am beginning to think it was all operator error.

So I loaded up 10 more just as before, but before going out to shoot I put my stock between my legs and gave a little pressure to the suppressor to ensure it was good and tight. On the first group I had only snugly hand tightened without wedging the stock between my legs.

So I go out and shoot...first two rounds about an inch and a half apart and grouping as before. Next three rounds fly 7-8 inches high and right, and I begin to think to myself, "self, you tightened that can down differently this time." I loosened the suppressor, then snugly re-tightened using only my right hand on the suppressor as with the first 5 rounds. The next 5 rounds grouped at about 1.5 inches...if I throw out the flyer, the four were within 1 MOA.

Now to explain...I gave my rifle to my gunsmith/machinist to do a little work on the threads so that the suppressor didn't work itself loose with standard velocity rounds, and he did some machinist's trick so that when I tighten the suppressor down, there is extra space now and that causes the suppressor to stay in place without working loose. Before he did this work, I had been accustomed to really cranking down on the suppressor with the stock between my legs.

Apparently, since the new thread work, I have been cranking down way too hard on the suppressor and likely putting it off center. With the suppressor snugly tightened with only one hand on it and one on the stock, the issue is resolved.

Good gracious, who would of thunk it?

Jasper

 

[Zak Smith]

Now to explain...I gave my rifle to my gunsmith/machinist to do a little work on the threads so that the suppressor didn't work itself loose with standard velocity rounds, and he did some machinist's trick so that when I tighten the suppressor down, there is extra space now and that causes the suppressor to stay in place without working loose. Before he did this work, I had been accustomed to really cranking down on the suppressor with the stock between my legs.

I was talking with the other guys in the shop about this and the first thing came to mind was that the subsonic rounds were not totally stable and that you were getting some contact. I still think that is the most likely explanation if full power rounds are 100% accurate.

However, this paragraph raised some red flags. First of all, you shouldn't have to really crank down on a screw-on suppressor (holding the stock between your legs and using two hands) to get it to stay on. Second, the thing about the "machinists trick" and the "extra space" is concerning. You want a screw-on suppressor to abut directly, firmly, and symmetrically to the barrel shoulder behind the threads. Third, if the threads are concentric to the bore and the shoulder is cut at a right angle to the bore, then there is no way it can get "off center" by simply applying more torque to it.

I would start off my investigation by seeing if it really is shouldering up right, and then I'd look at the run-out of the suppressor once mounted.

 

[TNBilly]

I'm with Zak.... perhaps let another "machinist inclined" person have a look at it and see what exactly the first smith actually did. I'm suspecting the threads have been slightly flattened or otherwise deformed.