6 GT Load Suggestions & Suppressor Pressure Signs!?

[juice-barrel]

Looking for some advice/thoughts on this load. Broke in a new barrel this weekend. Groups were great, SD not so much. Most of the details below. I included photos of the composite group and the 8 shot group. Not sure what happened with the literal flier, might have been me, might not have.

I'm not sure if I should just chalk this up to barrel break/cold weather or not. Not going to lie, I was expecting a lower SD with how well it was grouping. The 5x5 groups were the 32nd to 56th shots through the new bore. I shot another 17 shots of a different load after the 5x5 group, then the 8 shot group, so shots 74-81.

Oh, and btw, I was getting pressure signs with the suppressor (OCL Polonium); heavy bolt lift and some ejector swipe. Here is the break-down of the suppressor groups. From my limited research, I guess this is a thing where some suppressors will cause pressure signs with an otherwise normal load. I have a Silencerco Omega to try out next time.

Shot a 5 shot group (great group - 0.3 MOA no pressure signs)
Shot a 5 shot group and had pressure signs, group was terrible, worst of the day
Cleaned chamber/barrel
Shot 5 shot group pressure sings went away, good group 0.5 MOA
Shot 2 more, pressure came back, sadly took the suppressor off and put it away
Shot 8 more shots no pressure signs with brake

Right now, I'm leaning to loading another 50 of the exact same load and retesting (with brake and Omega suppressor). I don't live close to the range and with the family I don't get many days at the range, so I try to bring a lot of ammo for thorough testing/practice, even if it means using more components than I need to.

5x5 and 1x8 groups for a total of 33 shots
Avg. Velocity: 2829 fps
Velocity SD: 12.7 fps
Case: Virgin Alpha
Primer: CCI 450 OEM
Powder: 32.5 gr. Varget
Bullet: Berger 105 Hybrid Target
Distance to lands: ~0.055
COAL: ~2.535
Barrel: Proof 6 GT 7.5 Twist (pretty sure Proof has the .120 FB)
Muzzle Device: 6mm Hellfire Match
Range: 101 yards
Ambient Temp: 37 degrees Fahrenheit
Chronograph: Garmin Xero mounted to ARCA
All loads weighed on A&D FX120i

 

[Kickin45]

Tight bore?
Sounds like copper fouling.
Try some factory Hornady ammo and run 2 rnds. Thorough cleaning. 2 rnds, clean.
If the factory ammo pressures after one or two rounds, then you definitely have a tight bore. Could be a tight chamber as well, but I’m leaning toward the bore.

If you have pilots, try and get an internal measurement at the muzzle

 

[juice-barrel]

Tight bore?
Sounds like copper fouling.
Try some factory Hornady ammo and run 2 rnds. Thorough cleaning. 2 rnds, clean.
If the factory ammo pressures after one or two rounds, then you definitely have a tight bore. Could be a tight chamber as well, but I’m leaning toward the bore.

If you have pilots, try and get an internal measurement at the muzzle

I'll have to get some factory ammo, that's a good idea. It has been a while since I have bought any factory ammo for any of my bolt guns, lol.

When you say pilot, you mean like a gauge pin, right? I have been meaning to order some, maybe now I finally have a good excuse.

What is the theory on why a tight bore would cause pressure signs with an otherwise normal load?

EDIT: I guess you could have meant like a pilot/mandrel for neck turning and/or expanding. I do use a mandrel for setting neck tension 0.242" I will see if it goes in. I think I'll order some size specific gauge pins around 0.243 +-0.0005

 

[Kickin45]

Yes, the mandrel. Sorry I was driving and voice texting, and couldn’t remember the right terminology lol

 

[Cascade Hemi]

Can you drop a bullet freely into a fired case?

Are you leaving the bolt open and retracted between strings?

 

[Kadams1563]

How many rounds in barrel?

With my GT I ran it slow for 200 rounds and then did load development. Got up to 2950 with a 108 but since have backed it down to 2850.

 

[juice-barrel]

How many rounds in barrel?

With my GT I ran it slow for 200 rounds and then did load development. Got up to 2950 with a 108 but since have backed it down to 2850.

Only 81 total rounds fired on the barrel to date. I was expecting some variability during break in but not sure what’s to be expected from velocity SD during break in.

 

[juice-barrel]

Can you drop a bullet freely into a fired case?

Are you leaving the bolt open and retracted between strings?

No a bullet does not drop into a fired case

 

[Cascade Hemi]

No a bullet does not drop into a fired case

You have inconsistent pressure with minimal loads. It is most likely from a tight chamber neck.

 

[juice-barrel]

You have inconsistent pressure with minimal loads. It is most likely from a tight chamber neck.

Yes I am leaving the bolt up and back between strings.

So what are my options neck turn, have the chamber reamed out a little more? See if it gets better after break-in?

 

[John Glidewell]

Yes I am leaving the bolt up and back between strings.

So what are my options neck turn, have the chamber reamed out a little more? See if it gets better after break-in?

Those are your 2 options. It will not get better, if anything it will get worse. I just had similar issue with one of my GTs running suppressed. Started seeing pressure around 175 rounds between cleanings. Being new to running suppressed all the time I asked around and was told it happens and I need to be cleaning a little for frequently so it does not happen. So far so good, it has not come back and I have not run past 100 rounds before cleaning. I did notice it had heavy carbon fouling in the throat, beginnings of a carbon ring. Suppressors really bake that shit in the throat. You might try giving it a really good cleaning and use some JB bore paste in the throat to make sure you get all the carbon out and see how it runs.

 

[Cascade Hemi]

Yes I am leaving the bolt up and back between strings.

So what are my options neck turn, have the chamber reamed out a little more? See if it gets better after break-in?

Neck turn or ream the chamber.

 

[juice-barrel]

Those are your 2 options. It will not get better, if anything it will get worse. I just had similar issue with one of my GTs running suppressed. Started seeing pressure around 175 rounds between cleanings. Being new to running suppressed all the time I asked around and was told it happens and I need to be cleaning a little for frequently so it does not happen. So far so good, it has not come back and I have not run past 100 rounds before cleaning. I did notice it had heavy carbon fouling in the throat, beginnings of a carbon ring. Suppressors really bake that shit in the throat. You might try giving it a really good cleaning and use some JB bore paste in the throat to make sure you get all the carbon out and see how it runs.

Yeah that all makes sense.

I guess the thing that confuses me is that I cleaned the barrel and chamber multiple times that day. And when I saw pressure I cleaned it again. I could get 5 shots off suppressed before I would I see pressure signs. The barrel only has 81 total rounds on it right now. So the carbon is baking on that fast to cause issues with the suppressor?

 

[John Glidewell]

You stated you cannot drop a bullet in a fired case. How tight is it? What is the measurement of the neck without bullet, with bullet and fired? What is the neck wall thickness? It is sounding like you either have thick brass or a tight chamber neck. If this is the case you will need to neck turn brass or ream chamber, as has been stated by others.

 

[Cascade Hemi]

Yeah that all makes sense.

I guess the thing that confuses me is that I cleaned the barrel and chamber multiple times that day. And when I saw pressure I cleaned it again. I could get 5 shots off suppressed before I would I see pressure signs. The barrel only has 81 total rounds on it right now. So the carbon is baking on that fast to cause issues with the suppressor?

No, you probably have inadequate neck clearance. The brass can't release the bullet so it's building additional pressure. It shows up as inconsistent pressure signs. Carbon build up will make it worse but not in the number of rounds you fired. Ideally you should have .003" to .005" of clearance between the loaded case neck and the chamber.

Pressure should be linear; more powder, more pressure. Random pressure usually means there is a clearance problem or water in the bore.

 

[juice-barrel]

You stated you cannot drop a bullet in a fired case. How tight is it? What is the measurement of the neck without bullet, with bullet and fired? What is the neck wall thickness? It is sounding like you either have thick brass or a tight chamber neck. If this is the case you will need to neck turn brass or ream chamber, as has been stated by others.

Here are those measurements:

Loaded case neck diameter: 0.269
Fired case neck diameter: 0.273
Average case thickness: 0.0132

Used a ball anvil style micrometer and followed the AMP annealing methodology for neck wall thickness measurements.

 

[juice-barrel]

No, you probably have inadequate neck clearance. The brass can't release the bullet so it's building additional pressure. It shows up as inconsistent pressure signs. Carbon build up will make it worse but not in the number of rounds you fired. Ideally you should have .003" to .005" of clearance between the loaded case neck and the chamber.

Pressure should be linear; more powder, more pressure. Random pressure usually means there is a clearance problem or water in the bore.

See measurements in above post, I guess I have at least 0.004" of clearance then. Which should be at least 0.002" on either side of the neck.

Also, to throw another rub into this whole mystery, I went back to the range today to test varying seating depths. All depths grouped great again, but the velocity SD was pretty interesting. Didn't get a chance to try out a suppressor today, I was more concerned with seating depth testing. Note, these are shots 101-200 through the barrel. I did a thorough cleaning at home before heading out, but the barrel wasn't cleaned at all today, last cleaning was after shot 81.

5x5 groups for a total of 100 shots across all 4 seating depths. The loads identical other than seating depth.

Approximate distance from lands / velocity SD / avg velocity / shot number on new bore

Shot a 19 shot warm up with a StaBall Match load then:
0.075" / 14.0 / 2852 /101-125
0.060" / 13.2 / 2879 / 126-150
0.045" / 12.7 / 2885 / 151-175
0.030" / 6.9 / 2894 / 176-200 - either the barrel finally stabilized or we found a winner.

So this reply is getting pretty long now, so I'll summarize:

• Gun shoots great groups with low SD with the right seating depth and no signs of pressure, no charge weight tuning has been performed
• Seems to be +0.002 clearance on either side of neck
• Suppressor specifically OCL Polonium known for high back pressure caused pressure signs in previous testing after 5 suppressed shots

Do we still think the barrel has irregular pressure spikes, which is the underlying cause of the pressure signs when shooting suppressed?

I know I owe everyone a range trip to test out a different suppressor. And thanks to everyone so far for your input.

 

[Cascade Hemi]

Do we still think the barrel has irregular pressure spikes, which is the underlying cause of the pressure signs when shooting suppressed?

I'm going from what you've posted, if there's no pressure signs then no. Your charge weight is pretty low but you have decent velocity.

 

[Lawnboi]

My theory, because I have had the same head scratchers, and I shoot suppressed almost all the time.

The temp your shooting in, combined with gasses left in the barrel leaves some moisture and carbon dust in the chamber. This causes bolt thrust, as your not seeing a spike in pressure but just signs the case is being pushed into the bolt head. It’s not a problem until the temperature gradient in your chamber is enough to attract the carbon.

I have had this exact issue and remedy it by putting a fan in my chamber immediately after my string, and letting the rifle cool completely. Keeping smoke from settling in the chamber area.

I don’t have any evidence beyond seeing this bunch when shooting those 32-40 degree, high humidity times, especially in a head wind.

Good luck. I bet you screw on that suppressor when it’s warmer and the problem disappears.

 

[John Glidewell]

The temp your shooting in, combined with gasses left in the barrel leaves some moisture and carbon dust in the chamber.

I have seen this twice now shooting in cold weather. Was not sure why it was happening. I did remember it was after I took a break and let the gun / can cool. I think next time out I will take a cleaning rod and run a couple dry patches before I start my next string. This makes a lot of sense, thanks for posting.