What would cause inaccuracy when hot?

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CMV
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What would cause inaccuracy when hot?

Post by CMV » Thu Jul 02, 2020 8:58 am

My 9mm is Ti tube & endcaps, 7075 K's. I *thought* it has been good to go all this time....nice sound & just fine running on a Beretta 92 at typical pistol distances ....~10 yds. Got a CZ scorpion & it sounds great there too. But now I'm shooting at 50 yds & notice after about 10 shots my accy goes haywire. Played around yesterday because i thought it was my 147 gr subsonic handload, but turns out the load is crazy accurate unsuppressed out of the Scorpion.

So unsuppressed, 50 yd groups go from about 1.25" (which is great for just a red dot) to about 3" with the suppressor. Not so great, but I can live with that & POI s--t is minimal. But then after can gets warm, I start seeing vertical stringing. Windage stays fine, just group turns into vertical line about 6" or so long.

viewtopic.php?f=10&t=137407&p=940765#p940765 this is it. Pretty simple build, just basic Ks. 0.400" bore.

I see no signs of strikes or rubs anywhere. Rounds make perfect circle holes on paper at all ranges - no keyholes or anything. Just groups open up with suppressor, then open up more as it gets hot. End caps are torqued down good and staying tight. Direct thread suppressor, twisted on the gun real tight & not coming loose. Some FelPro copper anti-seize on barrel threads & suppressor hand tightened down. Takes 2 hands to get it loose, so I don't think it's an issue of something just getting loose form recoil.

Kind of sucks.....all this time I was real happy with that 9mm can until I started using it at a longer range.
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Re: What would cause inaccuracy when hot?

Post by #40Fan » Thu Jul 02, 2020 11:48 pm

Mirage from the heat of the suppressor?

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Re: What would cause inaccuracy when hot?

Post by alordnapa » Fri Jul 03, 2020 1:57 pm

Are you using cast slugs? The additional back-pressure caused by the can may be causing your barrel to overheat, and this could cause erratic performance. Even without cast slugs, an over-heated barrel will cause accuracy problems.

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Re: What would cause inaccuracy when hot?

Post by CMV » Fri Jul 03, 2020 5:11 pm

147gr ACME flat point bullets. They're coated cast.

I can remove suppressor and immediately go back to good groups, so I think issue is in the can. Just weird. And can only tell it at 50 yds. Not bad enough to miss steel targets at 10-12 yds, smallest is 6" though so would take a lot to miss at that range I guess.
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Re: What would cause inaccuracy when hot?

Post by Ridge Runner » Sat Jul 04, 2020 12:19 am

A 6-inch offset at 50 yards sounds to me like a Bernoulli effect. The bullet isn't rubbing baffles, it's just getting so close that the pressure on the edge is pushing against the bullet and changing its course slightly the air is acting like a bearing. By way of example if you have a mild rub it would likely destabilize your bullet to the extent that you should be able to hear it as it will be tumbling shortly after it leaves the barrel. Also, a bullet that has any meaningful rub inside the suppressor is going to be off by more than 6 in in 50 yards.

You didn't mention the length of your suppressor. My guess is that if you pull all the baffles out and reinstall them randomly things will change. no I can't be sure whether they'll improve or degrade. There are two obvious solutions in my mind. The low-tech solution is to increase the bore size of all your baffles and endcap. .40 inches is a significantly close tolerance fit for a .358 in projectile you're only looking at .02 in clearance and that is assuming perfect alignment. No one gets perfect alignment.

I would recommend if you choose this course that you select two or three baffles or at least one as your blast baffle since it's close to the entrance end there's little reason to enlarge that clearance hole. after you get past the baffles that are always going to be on the entry end enlarge the clearance holes on the rest of your baffles by at least one 64th of an inch possibly one 32nd. This will reduce the efficiency of your suppressor a little bit perhaps. I'm betting it's less than one decibel. The other solution which is a lot easier to accomplish if you have a lathe would be to Mount your suppressor on a straight rod and mount that in a lathe and measure total indicated run-out. 4 one hundredths of an inch equals a baffle strike with your current design. See what you can do to remove run out or rather to mitigate it. One final area where you're losing clearance will be any space between your baffles and the inside diameter of your tube. In a tightly compressed suppressor that space could be top or bottom in a more loosely compress suppressor that space is always at the top bringing the clearance holes slightly below center line of the suppressor.

Sorry for the poor writing I'm sending this from my phone.

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Re: What would cause inaccuracy when hot?

Post by mcrump » Wed Jul 15, 2020 10:03 am

What type of baffles and how are they clipped? Single or double clip?
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Re: What would cause inaccuracy when hot?

Post by ECCO Machine » Thu Jul 16, 2020 10:23 am

Ridge Runner wrote:
Sat Jul 04, 2020 12:19 am
A 6-inch offset at 50 yards sounds to me like a Bernoulli effect. The bullet isn't rubbing baffles, it's just getting so close that the pressure on the edge is pushing against the bullet and changing its course slightly the air is acting like a bearing.
I agree. .400" is pretty tight for 9mm. Double check your alignment, but consider opening the apertures up to .415-.420.
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Re: What would cause inaccuracy when hot?

Post by T-Rex » Thu Jul 16, 2020 2:37 pm

Not sure of OP's design, but I've done 9mm K's (what the OP is using) w/ a bore of ~.386. That's only 16thou clearance, per side. No issues w/ accuracy and marginal POI shift.
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Re: What would cause inaccuracy when hot?

Post by ECCO Machine » Thu Jul 16, 2020 3:44 pm

T-Rex wrote:
Thu Jul 16, 2020 2:37 pm
Not sure of OP's design, but I've done 9mm K's (what the OP is using) w/ a bore of ~.386. That's only 16thou clearance, per side. No issues w/ accuracy and marginal POI shift.
I'd say that was more luck than anything.

I would never go less than .040" diametral clearance, usually run .050"-.060". No suppression is gained by going tighter, just the potential for issues like the OP's.
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Re: What would cause inaccuracy when hot?

Post by T-Rex » Fri Jul 17, 2020 7:39 am

ECCO Machine wrote:
Thu Jul 16, 2020 3:44 pm
I'd say that was more luck than anything.

I would never go less than .040" diametral clearance, usually run .050"-.060". No suppression is gained by going tighter, just the potential for issues like the OP's.
I didn't chose that bore arbitrarily, it was a decision based on previous results.
If dealing w/ cones, I'd agree about tightness. However, K's rely on trapping gas and the more you trap the greater the design's efficiency. I've found that they (K's) can benefit greatly from a tighter bore.
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Re: What would cause inaccuracy when hot?

Post by ECCO Machine » Fri Jul 17, 2020 11:30 am

T-Rex wrote:
Fri Jul 17, 2020 7:39 am
I didn't chose that bore arbitrarily, it was a decision based on previous results.
If dealing w/ cones, I'd agree about tightness. However, K's rely on trapping gas and the more you trap the greater the design's efficiency. I've found that they (K's) can benefit greatly from a tighter bore.
Depends on the design of the K. The more tunnel there is, the more the aperture size matters. I don't use them anymore because cones metered better across the board, but my previous core for the Phoenix IX was full of these with minimal tunnel:

Image

The baffles have a .410 aperture. This is a K-baffled Phoenix running 158 gr. GECO .38 spl FMJ:

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