Effect of Primer Selection on Muzzle Velocity

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   Got to the range today to test fire the T that I recently removed the new CBI barrel to check the “breeching-up” angle with a digital Angle Gage.  The breeching-up angle is the angle of underturn when the barrel is hand tight on the action body relative to the vertical position when fully torqued and aligned (typically known as “indexed” here in the US). See previous posts. 
To summarize the previous post, I found that the barrel did not have sufficient underturn, less than 10 degrees. I went up two sizes (+0.002” thicker) in breeching washer to get 18.5 deg of underturn. 

The effect of the tighter barrel was to move the group MPI up by a bit more than 1 minute. And it shrunk the group considerably. 

Here is the first 300 yard group fired with the thicker (0.039”) breeching washer.  7 of the 10 shots grouped at just over 1/2 MOA. That’s quite remarkable, but I am not at all expecting this rifle to consistently hold a minute of angle, but the improvement is noticeable. 

It was quite windy today, so 600 yard groups were a bit scattered horizontally within the black 7 ring aiming mark of the Mid Range Target. 