Accuracy loss and pressure at 400rounds

[Southern Custom]

I have some questions for the guys who have been through all of this and come out the other end. I’m not new to reloading but I’m only a year in to seriously loading for a precision rifle.
So here’s my story. I have a 6mm Creedmoor Bergara HMR pro rifle that has been shooting under .5 without effort and .3 all day as long as I’m on my game. That’s the best I could hope for out of a factory Bergara and I’m happy with it. At the 400 round mark I suddenly started getting accuracy issues. 3/4”-1” groups. Best I could do yesterday was .5. I was just about to start shooting 600yd field precision matches and 1” won’t cut it.
I’m looking for advice on how to proceed. I’ve ruled out gun/scope torque etc. I’m fairly certain this is a powder/barrel issue.
Ive been shooting IMR 4350, Peterson brass, Hornady 108 ELDM. The node I’ve been at was around 2950fps with 40.4g. Gun liked bullets seated 15thou off lands. I have 2thou of throat erosion so far.
At around 400 I suddenly jumped 100fps to 3050. This also came when I started a new 8lb jug of IMR 4350 so there’s that. Last week I blew 1 primer and started getting occasional sticky bolt. Also Ejector marks on brass at 3100.
Just spitballing I switched to H4350 at the same 40.4 which I’ve done before with only 25fps speed increase. Now that same load gives me 3100fps and minor pressure signs. That’s nearly 100-125 faster than that powder switch used to give me. And this in mild 80deg temperatures.
Barrel was cleaned of copper at around 250 and again at around 450. I’m at 500 rounds Last 100 rounds have me totally scratching my head. At 400 rounds and shooting well, I had just decided to do a ladder test with seating depth to see if I was in the best place. This was when things started going south and nothing, even my go to load, seems to work.
Barrel was cleaned using Barnes to remove copper. Wet patches run through and left for 10min twice. Bergara barrels are lapped and I never seem to get any copper buildup but without a bore scope I’m going on my patches for signs.
So my questions are, can I have a serious carbon ring at 400 rounds? With a new jug of IMR 4350 should I be redoing a ladder test from say 39g to 40.4. (First work up showed 3 nodes at 39.6, 40.4 and 41.5) there’s no way I could even shoot 41.5 now considering I’ve sped up 100fps. If the gun seems to like 2950 should I try and get back to that speed? Can there be that big of a discrepancy between powder lots?
I have a bore scope on the way so I’ll be able to get a better look at the bore. It’s possible my barrel is not as clean as I think it is given my fairly gentle cleaning regimen.
Just trying to get some confirmation I’m approaching this from the right direction so far. I have a new Brux barrel waiting in the wings bu I’d planned to milk this factory barrel for all it was worth while I learn.

 

[IHFarmer07]

I’d say you fell out of your “node” and will have to go back through load development and possibly a seating depth test. But it is very possible of a carbon ring ect. I have been using clr for my stainless barrels to remove carbon now after I heard about professional shooters using it, also bore tech is an amazing cleaner of everything in a barrel.

 

[Darkside-Six]

Carbon ring. If you have a bore scope run it down there. Sure it could be the different lot of power but that much speed and pressure increase tells me it’s a carbon ring. My 6GT gets them every 500 rounds like clockwork.

 

[Dthomas3523]

It’s definitely not a “node” issue. You don’t “fall out” and gain 100fps. That’s not even a thing with most 6’s.

Almost always going to be carbon buildup.

 

[Dthomas3523]

Does a tight patch have the same feeling from the chamber down to the bore? If cleaned will it will feel very smooth without any tight spots or different feeling areas.

 

[Average guy]

Give it a good scrub with CLR. I usually follow that up with some Hoppes type. Of bore cleaner to get rid of all the CLR.

 

[Lunchbox27]

Just going to echo what everyone else already said... because I’m having the exact same issue. 600 rounds on my 6 creed. Same brass, primer, bullet and powder lot as the load workup. Even the temp was within a few degrees. 1 hole load went to a 1.5” load. Threw the chrono on and it was about 80 fps faster and getting some heavy bolt lift.

Just pulled the barrel off and threw a scope down it. Nice carbon ring. Plugged it, poured BoreTech C4 into the barrel and let it sit for 3 hours. Scrubbed the shit outta throat with a nylon brush, patched out and hit it with the bore scope again. All clean.

It’s been close to 200 rounds since my last cleaning, but I don’t pussy foot the rifle.

 

[IHFarmer07]

It’s definitely not a “node” issue. You don’t “fall out” and gain 100fps. That’s not even a thing with most 6’s.

Almost always going to be carbon buildup.

True, but the OP said that he lost accuracy and then he noticed a 100fps gain, no one said that you fall out of a “node” and then gain 100fps like you had said, that is just backasswords, everyone knows that when you gain 100fps you fall out of a node that way, something else happened.
Op said at the 400rnd mark, is that all that has been down the barrel and this happened or does this only happen every 400rnds???

What I meant is that it could be the barrel just picked up speed and fell out of the accuracy node that he was at, pretty simple and it happens all the time.

Since it’s a factory barrel it might take more rounds down the barrel to fully brake in, barrels do that at different numbers of bullets down the barrel. If it doesn’t end up being a carbon ring, it is possible for it to have fallen out of the “node” but if it does end up having a carbon ring, yes, there’s the issue. No one will know for sure until a bore scope gets the full story. Just saying.