6GT Barrel Issues - Bolt Thrust?

[rijndael]

I have a new 6GT barrel that I'm breaking in. Here's the load:

32.0 of Varget
107 SMK
Alpha Brass

2760 fps ... so, it should be about 100 fps under a normal load. The load, with this brass, works fine in my Proof 6GT barrel.

I'm getting ejector swipes and very heavy bolt lift. I cleaned the barrel with brake cleaner before the install, so it shouldn't be an oily chamber.

My hunch is that maybe there's a chamber finish issue, causing the brass to not grab the walls properly.

Any advise would be appreciated.

 

[DLT]

I have a new 6GT barrel that I'm breaking in. Here's the load:

32.0 of Varget
107 SMK
Alpha Brass

2760 fps ... so, it should be about 100 fps under a normal load. The load, with this brass, works fine in my Proof 6GT barrel.

I'm getting ejector swipes and very heavy bolt lift.

My hunch is that maybe there's a chamber finish issue, causing the brass to not grab the walls properly.

Any advise would be appreciated.

Would a bore scope show you if it was to slick ?

 

[rijndael]

I bore-scoped it but I'm not sure I can properly identify what's too slick ...

 

[jxb]

I bore-scoped it but I'm not sure I can properly identify what's too slick ...

visually compare to your proof barrel chamber. Prob want to clean/dry both first.

If you've shot this load in both barrels and have shot brass with same count on them, maybe measure both CBTO and OAL of a few cases from each just so you can see how they compare for stretch on say first firing brass with that load. Not sure that helps tell anything about grab but it's data.

Oh and I'm assuming your measured touch on each barrel and set your seating for each based on its own touch. also that you don't share brass between them, checked headspace on each, etc.. all that stuff.

 

[rijndael]

I should be over .020" off the lands, they measured pretty close between barrels.

The headspace between the barrels is on opposite ends though, they're about .006" off from each other, this new one being much shorter.

also that you don't share brass between them.

I've heard this, but I don't understand how it can cause this problem. Would you mind elaborating on it? It chambers fine, after sizing, with the appropriate bump. I can understand it where you have differing amounts of unsupported brass at the end, and the die won't size it, but that's not the symptom here.

 

[rijndael]

The only thing I can think of is ... maybe when I brake cleaned the bore, I didn't get the chamber area and it's blocked with a bore guide in place.

 

[jxb]

I should be over .020" off the lands, they measured pretty close between barrels.

The headspace between the barrels is on opposite ends though, they're about .006" off from each other, this new one being much shorter.



I've heard this, but I don't understand how it can cause this problem. Would you mind elaborating on it?

Best if one of the gunsmiths answers that or someone with far more knowledge than me, but yeah so if you're setting back .002 (or whatever amount you're using) then dealing with a difference of .006 between barrels would make it hard to share brass.

I don't share my brass between barrels for just this reason (unless I've had the barrels done with same action, reamer, and gunsmith). Even then a person needs to measure everything to assure they are close enough.

Well I did get a couple 220 Swift done same time but same gunsmith, reamer, and they assured the headspace matched. Then I verified each as well as touch before loading for them, they share brass because they are almost identical.

 

[jxb]

The only thing I can think of is ... maybe when I brake cleaned the bore, I didn't get the chamber area and it's blocked with a bore guide in place.

I always clean my chamber last for that reason and if there was any leakage into the chamber while cleaning the bore. But I'd think the issue would go away after a few rounds if it was left a little wet.

 

[rijndael]

Best if one of the gunsmiths answers that or someone with far more knowledge than me, but yeah so if you're setting back .002 (or whatever amount you're using) then dealing with a difference of .006 between barrels would make it hard to share brass.

What I did was set aside 100 pieces from the other barrel, resize them, and bumped it back .0025" after I got the bolt to close.

 

[rijndael]

Just found this on the first round I shot today .... I think I screwed up and didn't swab the bore and the chamber was dirty.

 

[jxb]

yeah I guess what I was meaning was, if you were to share brass between them then you would be overworking your brass both in resizing and firing. cases from the longer chamber you resize more, and if you fire cases sized to fit in the shorter chamber in the longer chamber then your always fireforming those. Basically the reason is brass life.

Keeping them separate also helps when you find an issue. Say you share brass and somewhere in your reloading during inspection you see an issue, you have no idea which barrel it originated from. I work in IT, #1 rule is isolate away from the variables. Just makes it worlds easier to find the cause.

 

[jxb]

Just found this on the first round I shot today .... I think I screwed up and didn't swab the bore and the chamber was dirty.

View attachment 1571590

Um yeah agree 100%, that's pretty bad. clean dry brass and same for chamber and giver er another try.

another reason to borescope after cleaning, to check from chamber through bore so you know where you're at before putting it away. Not to mention how good/bad a job you did. I never get my bore super clean but really look at chamber, throat, lands for anything I don't want.

 

[rijndael]

I do borescope after cleaning, but it's usually with the guide in the bore. Lesson learned.

 

[jxb]

I do borescope after cleaning, but it's usually with the guide in the bore. Lesson learned.

Yeah I have to say I'm super anal with any thing match grade, high end brass, etc. It's just so costly a little more time is well spent. My hunting/varminting rifles I just clean them + chamber and shoot em.

 

[CJ6]

I know know this is basic, but any chance of lube left on the cases?

 

[rijndael]

I know know this is basic, but any chance of lube left on the cases?

No, I wipe after sizing.