Suppressor Causing Groups to Grow

[Lunchbox37]

Posted this in a discord chat but still looking for some insight and experienced advice…

Here’s the background, got a Xbolt Speed suppressor ready in 30-06. Developed a load running Lapua brass, pushing 178gr ELD-X with 54.7gr of H-4350. MV avg is 2705, SD 8-9 and ES’s in the high teens to low 20’s. This load consistently groups around .79” if I do my part.
So here’s my issue, I just got a TBAC ultra 7 out of jail, took it to the range with this load expecting a POI shift and groups went to . After 4, 5-shot groups here’s what was observed; groups opened up to ~2” at 100yds, poi shift dropped 2” and 1.5 to right, MV went up 25fps, SD’s up to 17-19, es up to 40+. I’ve double checked for baffle strikes with a bore scope (none found) and validated alignment with Huxwrk rods prior to squeezing the go button.
I’d like to get a sanity check here mainly cuz there’s not a ton of info out there for this issue. So I figured I’d go back to load development to find the accuracy. I’m leaning towards lowering my charge weight OCW style with 10rds per step in .2gr increments (starting at 53.7 to 54.5gr) keeping all loads .030 from jam. Figured I’d capture chrono data and see how each “step” groups (slow 5 shot groups)…. Does this seem like a good approach? Open to opinions and any one else’s experiences.

 

[TaperPin]

That’s interesting, but it makes sense that it’s acting like a tuner and it’s just dumb luck if it does or doesn’t effect groups. I suppose you could move the can out in steps with shims, but it als makes sense to just re tune your load with powder and or seating depth if magazine allows.

 

[deadwolf]

Double check that your suppressor is tight. It can and will back off of tight after multiple shots if it is a direct thread. Happens all the time to my ultra 7. I double check tight after 4-5 shots.


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[Lunchbox37]

Double check that your suppressor is tight. It can and will back off of tight after multiple shots if it is a direct thread. Happens all the time to my ultra 7. I double check tight after 4-5 shots.


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It’s a CB can, but I was checking it every couple of shots to make sure it was tight.

 

[wind gypsy]

It consistently groups around 0.79" with 5 round groups without suppressor? Have you validated that it still does since shooting with suppressor?

 

[Lunchbox37]

It consistently groups around 0.79" with 5 round groups without suppressor? Have you validated that it still does since shooting with suppressor?

I did with 2, 5 shot groups. 1st group was .74 and the second was .86 with the same load it had been shooting.

From what I've gathered on the interwebs, predominantly lowering a charge will often improve accuracy and some people have had this issue then reduced the powder charge (but never said my how much or their approach). Really trying to sanity check and see if I'm going about resolving this best way before wasting components.

 

[Lawnboi]

How many rounds do you have on your barrel?

 

[9.3koolaid]

Maybe try a wrap? Could be a mirage issue

 

[Lunchbox37]

How many rounds do you have on your barrel?

I’ve got 340 rounds down the pipe.

 

[Weldor]

Harmonic's, I had issues with a ____ carbon fiber barrel. Would shoot great all day long without a suppressor. I tried all 3 of mine nada. It just didn't shoot well with that extra weight on the end. Swapped it out for a shorter steel bbl. and no more problem. Jmho.

 

[ResearchinStuff]

I'm guessing your barrel is now sagging down and contacting your stock intermittently. Poi changes are expected when putting a weight on the end of a barrel. Also, saying consistent groups down to the hundredth of an inch sounds like small shot # groups, and a small number of them. Unfortunately 3 or 5 shot groups aren't very repeatable, and neither is the OCW methodology.

 

[Lunchbox37]

Harmonic's, I had issues with a ____ carbon fiber barrel. Would shoot great all day long without a suppressor. I tried all 3 of mine nada. It just didn't shoot well with that extra weight on the end. Swapped it out for a shorter steel bbl. and no more problem. Jmho.

I'm guessing it has something to do with harmonics, but since its a browning the barrel swap option is almost out of the question lol... Now the question I have is would an adjustment to the load (powder charge or seating depth) be able to increase its accuracy? if so where would one start, charge weight or seating depth?

@ResearchinStuff

I'm guessing your barrel is now sagging down and contacting your stock intermittently. Poi changes are expected when putting a weight on the end of a barrel. Also, saying consistent groups down to the hundredth of an inch sounds like small shot # groups, and a small number of them. Unfortunately 3 or 5 shot groups aren't very repeatable, and neither is the OCW methodology.

Now this is an interesting point, The barrel sagging down... any idea on how to check for this? For grins and giggles should I just pull the action and check for rubbing?

I will say the group size of .79" I posted was derived from a statistical sample size of 30 rounds, comprised of 6, 5 shot groups that were then measured with the Hornady 4DOF app group analysis and then all data was averaged together to determine the average group size. It doesn't always print groups at .79, some are just over 1moa and others are under .75", these are well within a standard deviation of .79.

 

[ResearchinStuff]

with the can on, see if you can slide a piece of paper under the barrel, or if it gets pinched. if it gets pinched, remove the barreled action and do some sanding.

and props to you for shooting enough to actually have a real group size, most guys that claim .79 moa all day have something like 12 shots through the gun.

 

[Weldor]

I would start by dropping .5 grain at a time and see what happens. I found a increase of 1 grain on my 308 made a world of difference. Shot the same 41 gr load through 2 different rifles and neither one would group under 2-3 inch. Bumped it up 1 gr. and 1" groups. I switch my cans between 6.5 PRC and 308 both took some load development. The 308 is more forgiving than the prc as far a small groups are concerned at least on my rifles.

 

[wind gypsy]

The plastic xbolt stocks do have a pretty flexible fore end. With mine i have noticed it more flexing away from the barrel when loading a bipod than anything. I'd think you'd have to be putting some down force on the top of the rifle/scope or something to make it touch just based off of the 1 i have but it's worth checking to see if there is any contact.

Otherwise, it's hard saying. Might be able to tweak the load a bit and get it there but I wouldn't bet on it and might be a little leary of the load staying consistent if a little tweak causes over 1 MOA difference. Might have to try a different powder.

 

[huntercole]

I had the same thing happen on my Tikka. Shot factory 308 hornady superformance sst around .5 moa, so I bought several boxes. Got my suppressor approved and that opened up to around 1.5 moa. Tried it with two different suppressors and got similar results. I'm not reloading for 308 currently and just tried 3 more factory offerings and found that Federal's Berger load shot well so I'm just going to go with that.

I haven't had any issues with suppressors causing groups to change on my ARs or my 22-250, just POI.

 

[Lunchbox37]

Just an update on the troubleshooting...
- Was able to run a dollar bill between the barrel and stock all the way to the recoil lug with out an issue.
- Pulled the barreled action and visually inspected the barrel & stock. Found no sign of contact on either, re-
assembled and torqued to 35in/lbs.
- Remounted the scope and rings just to insure torque specs etc.

Went ahead and loaded up some rounds to perform an OCW test to test the theory of backing down the charge weight to improve accuracy while suppressed. Previous sweet spot was 54.7gr, but on average the OCW is usually every 3% or so (That would make it ~53.1 on the lower and ~56.3), and every 1.5% you'll have a "scatter node" (as Dan Newberry calls it). So when adding the suppressor I increased velocity/SD/ES and it looked like a scatter node.

With that in mind I'm going to play around in that -1.5% range of the charge weight to see if the node moved (via harmonics change w/ suppressor). I'll round robin 53.7 to 54.5 in .2gr increments (5 rounds for each step X 2 = 10 rounds total for each charge weight). If I get lucky and find that this test produces tighter grouping "nodes" in the 1 MOA range , then great. If it doesn't I'll expand the test to "+" 1% (staying under pressure with the 178gr ELDX) and "-" 2% of the charge weight.

 

[ResearchinStuff]

Ocw is not repeatable, and therefore a waste of components, range time, and barrel life.

 

[Lunchbox37]

Ocw is not repeatable, and therefore a waste of components, range time, and barrel life.

How so?.... I mean the idea is that OCW with a large enough sample size you'd be able to gather enough data for a good statistical analysis of the performance and be able to extrapolate what the standard deviations would be for group sizings.

 

[ResearchinStuff]

The nodes don't repeat though. Every time through ocw produces a new "best."