7prc Reloading Question (Barrel Break in or Problem?)

[gwalshmt14]

I have a 7prc that I started loading for about a year ago. Developed a load with it using N560 and 180 Berger Hybrids. This past spring I was getting somewhere in the low to mid 2800's for velocity. Never put it through a chronograph, just used the ballistic calibration on Applied Ballistics without adjusting the custom drag coefficient. Came out to 2825 fps. Shot long range accurately with it, no problems, had light ejector mark on virgin ADG brass. I pulled the same load back out a couple weeks ago, loaded at the same time this spring. Temperature was comparable, within 20-30 degrees of this spring. Load was shooting noticeably high at 500, about 3 tenths high of where it had been shooting. Had to adjust to 2.1 mils to get it back on target from the 2.4 that I was used to at 500. Ran it through a chronograph (Caldwell Ballistic Precision, nothing fancy) and got back an average velocity of 3040. That's cooking for what I was expecting. When I entered the new drop without adjusting the velocity in Applied Ballistics it came back with a velocity of 2925. All this to say why did I experience such a drastic velocity change so abruptly with the exact same load, loaded at the same time and with the same components. Could it be a temperature thing? A barrel break in thing? The load is still holding .5 moa accuracy at 500 and I have no additional pressure signs on the brass. I haven't noticed the bolt lift being hard to lift. Just wanted to get the opinion of some who are more experienced and maybe have a better answer than I do. I do recall possibly hitting a little high as I shot this load which could explain some of the speed up, and the ballistic app is lowballing my actual velocity now, so maybe it was originally and I was shooting closer to 2900? Regardless when I last shot in June 2.4-2.3 was an accurate come up, now it is 2.1-2.0. Just feels like it sped up way too fast in two range trips that were a couple months apart. I do believe my barrel was speeding up and I may not have been fully accounting for that but this seems to abrupt. I have 9 bullets left so I was waiting to finish shooting these 9 to do brass prep with every piece of brass I have after the season but this does concern me for shooting the rest. Any input would be appreciated, worst case I pull the last 9 and work on load development for my once fired brass and crank that out before the season.

Rifle: Factory Bergara Wilderness Ridge 7 prc. 22" Barrel. Found the load with the barrel shot count at about 125. Current shot count is about 225-250. Not cleaned for about 150 shots. Virgin ADG brass. Remington 9 1/2 magnum primers.

Below are pictures of my unfired brass, and a couple pieces of brass that have been fired with the 3040ish velocity.

 

[gwalshmt14]

I can try to get better pictures if needed, hard to see the light ejector mark

 

[gwalshmt14]

Forgot to mention that I am currently running 66.0 grains of N560

 

[bigunit]

The sun or shade can really mess with the optical style chronograph. Try shooting over a garmin or athlon.

 

[Kimber7man]

I’m shooting 66.0 N560 with 175 elite hunter, haven’t had any problems either. MV around 2,880 in a 21” barrel with 150 rounds down the pipe.

Might scrub the throat to guard against carbon ring?

 

[Quandary]

I have found that my 7PRC fouls very easily and requires a through cleaning every 50 rounds. If I don't clean it I get drastic increases in velocity as you describe. I would suggest you get a bore scope and clean to metal. Check with bore scope. Mine also has a tendency to carbom ring which requires a bit of effort to remove. I use 4831SC for my 7PRC load.