Barrel break in / usage velocities over time?

[Pappabear]

So when you first buy a, lets say Colt AR or maybe even a Bolt gun ( Bolt gunners can chime in too). How do velocities change over the life of a barrel. So as the journey goes from brand new barrel to being completely shot out, how do velocities change over time.

We shot a bunch of 14.5's today and there were no real surprises but it made me wonder. So anyone in the know, help a brother out.

PB

[Red*Lion]

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.

https://bulletin.accurateshooter.com/2018/06/tech-tip-velocity-increase-in-new-gun-barrels/#:~:text=There%20are%20several%20thoughts%20on%20why%20this%20may,as%20it%20exits%20the%20bore%20thereby%20increasing%20speed.

[opngrnd]

I have a barrel with 13k on it that is now 100fps slower that a newer Colt barrel. When I tested it at around 10k rounds it was about 70fps slower. At this point it'll be used for CQB and then a dedicated dry fire upper with the Mantis system. I'll know more on bolt gun uppers after the winter.

[markm]

is now 100fps slower that a newer Colt barrel

Can't compare one barrel against another. We just shot 6 barrels of the same length today and got an 89 fps spread.

[opngrnd]

Understood. I've been comparing these two barrels for a while. That particular Colt barrel is my usual mule, since it's almost only used for reference.

(With -51 windchill here, I'm envying your barrel comparison event.)

[Pappabear]

Understood. I've been comparing these two barrels for a while. That particular Colt barrel is my usual mule, since it's almost only used for reference.

(With -51 windchill here, I'm envying your barrel comparison event.)

Yea, it was freezing first thing then got very nice as the day went on. Turned out to be a glorious day and the reason people live here, other than work. Watching the KS game, it would be chilly getting hit in that weather.

PB

[sinister]

The longer you shoot the more the throat and leades wear. Once you get to a certain point you get a lot of "Free-bore jump" where the bullet isn't touching rifling leades or engraving and you get some gas blow-by (like you would in an automobile engine cylinder once your rings are worn) as the bullet clears the case mouth and your velocity drops.

If you watch high-speed videos of rifle bullets leaving the muzzle with worn barrels you can see quite lot of gas blow-by as the precursor gases blow past the bullet, seeping through the gaps between lands and grooves (why 5R barrels are considered better for precision over the long term).

Watch the gases clear from a .45 pistol:


https://www.youtube.com/watch?v=REcuV86X_Eg