Many new barrels deliver higher velocities after 100-150 rounds. The exact reason for this speed up is not 100% certain. And velocity increases (if there are any) will vary between barrels. This “speeding up”, however, is a common phenomenon. Be prepared if it happens to your next barrel. If you experience a significant increase in velocity, you should probably retune your load after the velocity has stabilized at the higher level. From the Sierra Bullets BlogMark Walker, Sierra New Product Development director, wrote this article
In a previous article, I discussed two methods for tuning a load to your rifle to achieve the best accuracy. Load tuning is usually done when a person gets a new rifle, or if a new barrel is installed. In both cases, the barrels are new and have not been fired much. Most competitive shooters believe that this is the most accurate barrel you will ever have, so getting it tuned up and shooting accurately should be a priority. The Speed-Up Phenomenon after 100-150 rounds
Many shooters find that their rifles stop shooting accurately after 100 to 150 shots, even though they have worked up a load. I had the same thing happen to a gun and was confused about why something that had worked so well in the beginning would suddenly stop shooting. I decided to use the chronograph and do another load test to see what was happening. To my surprise, I found that the velocity had increased by around 80 fps compared to the original velocity. After adjusting the seating distance and performing another ladder test, the rifle started to shoot well again. You can be sure that this does happen. As the barrel breaks in, tooling marks on the throat of chamber smooth out allowing less resistance to the bullet when it exits the bore. Another theory is that as the throat area becomes rougher, it creates more resistance and increases pressure. There are probably others who understand why this happens better, but it will definitely affect your rifle’s accuracy. Be aware of this and don’t be afraid to rework your load to keep it in tune. Experts Confirm Barrel Acceleration Is Common
Two respected shooters have noticed an increase in velocity after 100 rounds with new barrels. Thomas “Speedy Gonzalez”, a gunsmith and Hall-of-Fame Benchrest Shooter, has documented barrel speed-up through testing. Speedy’s borescope barrel inspections also revealed a smoothing out of the barrellands. Jim See, top PRS competitor has experienced barrel speed-up on numerous occasions. He re-tunes the load every 150 rounds. “Alex Lipworth, myself and others documented this phenomenon around four years ago. I told all my clients about it. My son Mikee would fire 100 rounds through each barrel we were planning to shoot before we began load development. We set up a snail to catch all the bullets in front of a bench indoor. We called it a “wear-in” process because, upon close examination of the bore after the “Speed Up”, the cut-rifled bore looks more like a barrel with button-rifles. The lands also take on a softer appearance of a button-rifled bore.” — Speedy Gonzales “Seen [barrel velocity increases] too many time to count. All my match barrels are loaded with a generic round, which has historically worked well for barrels. After I reach 150 rounds, I fine-tune my load and don’t look back until the tube slows down at the end of its life.” — Jim See
