Barrel “speed up”

[Markg308]

I’ve heard all of the barrel speed up in first few hundred rounds talk forever and I have noticed something since I have started cleaning my barrels with abrasives to the point of removing all of the hard carbon out, that they tend to speed up some around 70-100 rounds after cleaning. My question is, is barrel “speed up” due to the metal changing inside or is it due to hard carbon build up that most normal shooter don’t clean out and that increases the pressure?

 

[K22]

I do not have a definitive answer to your question but when I stop aggressive copper removal a few years ago, I experienced an almost immediate, the degree varying between rifles, velocity increase, the most pronounced was about 100 f/s. Since then, the velocity has seemed to have stabilized.

I clean every 50 to 60 rounds with a general solvent and bronze brush.

 

[ronsatspokane]

In the days before cheap bore scopes it was a mystery. With the advent of cheap bore scopes the answer is obvious, carbon buildup resulting in increased pressures.

 

[rijndael]

In the days before cheap bore scopes it was a mystery. With the advent of cheap bore scopes the answer is obvious, carbon buildup resulting in increased pressures.

I won't discount carbon buildup as one cause, but I've had a speedup persist after scrubbing to bare metal.

 

[ronsatspokane]

I won't discount carbon buildup as one cause, but I've had a speedup persist after scrubbing to bare metal.

I always go back to the properties of the components involved. Did the metal shrink and reduce the diameter of the bore? Possible I suppose but shrinking metal usually requires physical contact such as via a hammer or an English wheel. When we look at welding techniques involving shaped charges to join two pieces of metal, it could be that there is an alteration of the barrel metal properties resulting from the explosion and heat that could explain that but I've never seen it.

 

[rijndael]

I always go back to the properties of the components involved.

I can easily spitball at possibilities, like things smoothing out. It doesn't necessarily have to be about bore diameter changes.

 

[Larryh128]

Even tough the barrels are smooth after lapping, More than likely they smooth more after a hundred bullets iron the surface. My guess.

 

[Murray Brook]

Probably a little of all the above posts.

 

[ronsatspokane]

So the contention is that reduced friction is resulting in these speedups? Possible. But like I said, I use labradar and I've never observed this anomaly,

 

[swd]

I believe it’s from the smoothing and burnishing the barrel and changing the friction coefficient. It may or may not be true but it sounds good, lol.

 

[davidjoe]

I believe the hand lapping process, which is physically strenuous, uses a different but very slippery, proprietary liquid solution at each facility.

I speculate that the surface residue of the solution and what remained in the pores of the steel slowly burns away, and as this is happening, bullet friction increases. It also stands to reason that the steel is more perfectly smooth in the first rounds fired, and only gets rougher from there.

 

[JFrank]

I’ve heard all of the barrel speed up in first few hundred rounds talk forever and I have noticed something since I have started cleaning my barrels with abrasives to the point of removing all of the hard carbon out, that they tend to speed up some around 70-100 rounds after cleaning. My question is, is barrel “speed up” due to the metal changing inside or is it due to hard carbon build up that most normal shooter don’t clean out and that increases the pressure?

My unscientific speculation is that after the throat breaks in and copper ceases the grooves although lapped still have some room to smooth out, creating less friction.
My chronograph is a POS so I have no speed up numbers but I can see that after just a few rounds my copper is mostly gone.

 

[Jud96]

I believe I remember reading @Alex Wheeler suggested the lapped surface finish being changed from shooting the barrel could be the reason barrels speed up and then they plateau once that surface finish equalizes, if that’s the correct word. I remember reading that barrels that are setback and rechambered don’t experience much if any speeding up, which would suggest it’s not burrs or surface finish from the chambering process being the culprit. That idea makes the most sense to me and an interesting test would be to see how barrels lapped with a more aggressive grit would act.

I’ve also heard some people claim barrels don’t speed up and I’ve heard from a respectable gunsmith that Hart barrels in particular don’t speed up. This might be dependent on how people clean, how some barrel makers lap, or just random occurrences that are the exception not the rule.

 

[alf]

I shoot nothing but moly plated bullet, and never see any significant increases in barrel speeds.

On the flip side, the one barrel I chronoed at the end of it's life never showed any velocity loss either.