Editor: Many new barrels will deliver higher velocities with the same load after 100-150 rounds through the bore. The exact reasons for this speed-up are not 100% certain, and velocity increases (if any) will vary from one barrel to the next. But this “speeding up” phenomenon is common, so be prepared if this happens with your next barrel. If you do experience a significant velocity increase you should probably re-tune your load AFTER the velocity stabilizes at the higher level.
From the Sierra Bullets Blog
Article by Mark Walker, Sierra New Product Development Director
In a previous post, I discussed a couple of methods to tune a load to your barrel to help achieve the best accuracy possible. People most often work on load tuning if they get a new rifle or have a different barrel installed. In both instances, the barrel is new and has not been fired very much. According to most competitive shooters, this is the most accurate your barrel will ever be, so getting it tuned and shooting accurately is a priority.
The Speed Up Phenomenon After 100-150 Rounds
Even though after you work up a load and your new barrel is shooting great, a lot of shooters notice that at around 100 to 150 rounds their rifle may stop shooting as accurately. I had this happen to a rifle and I was confused as to why something that worked so well to begin with would all of a sudden quit shooting. I decided to break out the chronograph to do another load work up to see what was going on. To my surprise, the velocity had increased around 80 fps over the original velocity! After performing another ladder test and adjusting the seating depth, the rifle was once again shooting well.
There are several thoughts on why this may happen, however, you can rest assured that it does happen. One thought is that as the barrel breaks in, the tooling marks in the throat of the chamber smooth out and allow less resistance to the bullet as it exits the bore thereby increasing speed. Another idea is that the throat area starts to get a little rough which in turn causes more resistance which increases pressure and therefore more velocity. I’m sure there are some out there who have a better understanding as to why this happens, but it can definitely affect the accuracy of your rifle. So be aware and never be afraid to rework a load to keep your rifle in tune.
Experts Confirm That Barrel Speed-Up Is Common
Two respected shooters have observed an increase in velocity with new barrels, typically after 100 rounds. Gunsmith and Hall-of-Fame benchrest shooter Thomas “Speedy” Gonzalez has documented barrel speed-up with testing. Moreover, Speedy’s bore-scope barrel inspections revealed a smoothing of the barrel lands. Jim See, a top PRS competitor, has encountered barrel speed-up many times. Accordingly, he re-tunes his load at 150 rounds.
“Alex Lipworth and I documented this phenomenon about four years ago and I have told all my customers about this. My son Mikee would shoot 100 rounds through all new barrels we planned on shooting before we would begin to do load development. We had a shooting snail that caught all the bullets set up in front of an indoor bench. We called it a wear-in process because upon careful examination of the bore when the ‘Speed Up’ takes place the cut-rifled bore resembles that more of a button-rifled barrels with the lands taking on more the softer look of a buttoned bore.” — Speedy Gonzalez
“Seen it [barrel velocity increase] too many times to count. All my match barrels get a ‘generic round’ loaded for them, which has worked well in barrels historically. After I hit 150 rounds I fine-tune the load and never look back, until the tube starts to slow down at it’s life end.” — Jim See