New barrel blues

[constructor]

I need some opinions. Twice in the last year have received new barrels,installed them loaded up 10 to start the break-in and shot the first 5 or 6 after a sighter into less than 1" at 100.
The next 10 groups 1-1/2-2" What is going on here? exact same load, same length.
This time Derrick at accuracy speaks shot a bunch of mil.surplus through it without cleaning and sent it dirty. I received it at work and went straight to the range. I should have cleaned it first but, the first 5 were through a very dirty barrel.

Can moly build up and cause bad accuracy and high pressure?
The primers seem to be showing more sign now than the first group.

 

[alf]

Moly cannot build up on it's self, or cause higher pressure.
You've got other issues.
Al.

 

[constructor]

update. I cleaned the crap out of the barrel and then shot 40 moly 155 scenars w/ 4895 yesterday, groups were not great. I pulled the upper off to clean at the range and it looked like a mirror after 40 rounds. While looking through the barrel it looked like the brake was at an angle. Upon closer inspection I found copper shavings much smaller than a hair. I removed the JP brake and continued to shoot. Groups were back to 5/8- 3/4.
acceptable for now from an AR. I reamed the brake a little but, I hate fixing products that should work from the factory.