Suppressed.308 bolt issues

[ASH556]

I have an older 700 PSS that I had cut to 16” and an AAC brake/mount installed/timed. I really like the concept of this short suppressed bolt gun and have had success hunting with it.

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Beyond an initial zero and shooting that one deer with it, though, I haven’t really messed with it much. I needed to confirm/zero a couple new builds so I took the .308 along to see what it would do on paper at 100. I was very pleased with the first 5 shot group at 100 using Lake City 175:

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Then I screwed the can on and fired another 5 shot group, same setup, same ammo:

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I was horrified. 1/2moa to 5moa just by mounting a suppressor (AAC SDN6). Just to double check I fired another 5 and the group was the same or worse. I then decided to try different ammo and fired the last 4 rounds of 168 FGM I had in a box and the group greatly improved, but it was still way low and right from point of aim.

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Now, I understand POI shift is typical for suppressors, but I’ve never seen this much. The same can on my 300 Blackout Handi gives about 1.5 MOA straight down and my M4-2000 on both my AR’s is within MOA.

I suppose I could play with some different loads and try to find one that will shoot accurately both suppressed and not with minimal POI shift. Honestly though, I have a decent amount of the 175 on hand and it upsets me for the gun to be having these issues period.

1. Anyone ever seen something like this and/or have a remedy for it?
2. Would you keep messing with it or just move on?

P.s. when I built this gun, the goal was to have something with which to provide precision fire at distance, suppressed and not, that was also handy and maneuverable enough in the North Georgia Appalachian foothills, and with enough punch to take deer. My last week’s experience taking a 180lb 8pt Whitetail at 100 yds with a suppressed 14.5” AR, an ACOG, and a 62gr Gold Dot (15 yard recovery) has me questioning the need for a .308. Besides that, if it’s just about hunting, I have a 30/06 and a 300 Blackout and a 30/30 that can do the job. So now I am honestly considering going back towards a precision AR setup since I know my shift with a can is minimal, and just keeping a Nightforce 2.5-10 zeroed in a Larue Mount to swap onto the gun when I need/want to.

[JRCHolsters]

Truthfully, that doesn't look like a POI shift as much as contact with the endcap or possibly baffle stack. With the yaw of the bullet, you are probably barely scraping the endcap. Look for copper marks on the passage through the endcap or even baffles with a flashlight.

If it is rubbing, you need to check alignment to see if the can is not mating to the mount correctly, or the threads / mount may not be concentric.

[JodyH]

Truthfully, that doesn't look like a POI shift as much as contact with the endcap or possibly baffle stack. With the yaw of the bullet, you are probably barely scraping the endcap. Look for copper marks on the passage through the endcap or even baffles with a flashlight.

If it is rubbing, you need to check alignment to see if the can is not mating to the mount correctly, or the threads / mount may not be concentric.
^^^ This ^^^
Concentric threads and a high quality suppressor are critical on a precision rifle.
All the long range guys I shoot with are Thunder Beast (https://thunderbeastarms.com/) fanbois and only trust one or two people to thread their barrels.

[ASH556]

Barrel work was done by Accurate Ordnance, which is a well regarded precision rifle shop. I just checked the can and no signs of baffle or endcap strikes.

[JRCHolsters]

Have you used this can on any other rifles? If so, what were the results? You are definitely getting some kind of destabilization. If anything your groups should tighten up. Certain baffle designs can cause such extreme jetting that they create accuracy issues, but I wouldn't think this is the case.

[ASH556]

Have you used this can on any other rifles? If so, what were the results? You are definitely getting some kind of destabilization. If anything your groups should tighten up. Certain baffle designs can cause such extreme jetting that they create accuracy issues, but I wouldn't think this is the case.

Yes, on an AAC/NEF Handi in 300Blk, accuracy is great with minimal (1.5moa straight down) shift. Also, note that the 168gr Federal Gold Medal Group through the .308 was acceptable.

[JRCHolsters]

I may stand corrected on the can causing accuracy issues. I just did a little searching online and this particular model seems to have issues with accuracy. Sounds like it wasn't intended for precision rifle work to begin with. With most can, you get something we called "free-bore boost" usually anywhere from 10-25fps increase in velocity. If the gases are jetted across the path of travel, they can cause more yaw than usual, creating accuracy issues as opposed to POI issues. With the bullet you are using, it may throw it off just enough.

[Clusterfrack]

I’ve seen two AAC cans have very inconsistent accuracy due to mount problems. Both never resolved after multiple trips back to AAC.

And totally agree with Jody about TBAC.

[ASH556]

I may stand corrected on the can causing accuracy issues. I just did a little searching online and this particular model seems to have issues with accuracy. Sounds like it wasn't intended for precision rifle work to begin with. With most can, you get something we called "free-bore boost" usually anywhere from 10-25fps increase in velocity. If the gases are jetted across the path of travel, they can cause more yaw than usual, creating accuracy issues as opposed to POI issues. With the bullet you are using, it may throw it off just enough.

Another thing I just thought of is twist. These older PSS’s are 1:12. Between the velocity drop of the 16” barrel, the gas stuff with the can, and the longer 175, it’s just enough factors to jack everything up. The twist might explain why the 168’s shot better and why I deemed accuracy acceptable with the 150gr Fusions I hunt with. Maybe just trying out some lighter loads is the answer.

[JM Campbell]

Another thing I just thought of is twist. These older PSS’s are 1:12. Between the velocity drop of the 16” barrel, the gas stuff with the can, and the longer 175, it’s just enough factors to jack everything up. The twist might explain why the 168’s shot better and why I deemed accuracy acceptable with the 150gr Fusions I hunt with. Maybe just trying out some lighter loads is the answer.

175 gr needs 1:10 twist or lower to stabilize, 168 shoot well out of 1:12 twist but even better with 1:10


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[LittleLebowski]

Looks like shoulder/mount issues.

[ASH556]

Took me a sec to realize you weren’t trying to tell me I was shouldering the rifle incorrectly, lol.

[Backspin]

Not sure if twist rate alone can account for a dramatic destabilization. I've shot 175's out of a 1:12 twist 24" barrel AIAT with easily sub MOA results (all between 100-200 yards though). I later got a 1:10 twist barrel for it and I did not see a dramatic increase (if any) in accuracy or consistency with both 168 or 175 SMK's.

This guy is shooting a 1:12 17" barrel AIAT at distance with 178's and seems to be doing fine: https://m.youtube.com/watch?t=182s&v=kKQ6qQAsSPU

[Hambo]

Not sure if twist rate alone can account for a dramatic destabilization.

It can, but I'm not sure that's the only problem with the OP's rifle. I see two choices: solve one problem at a time until it's fixed or it's definitely the twist causing the problem, or just jump ahead and shoot the loads it likes.

[StraitR]

If it was the twist rate, the 175gr groups would be similar with and without the can. As it stands, the gun shoots the 175's very well without the can. It's certainly caused by mounting the AAC.

You have this set up as direct thread, correct? Do you have any shooting buddies with .308 cans you can try?

IMO, the fact it shoots 168 FGM much better when suppressed suggests it may be just a matter of finding the right ammo (bullet design/weight/BC).