Why do Barrels Speed Up?

[Brillo]

That's the nub of the question, why does the MV of a new barrel increase after about 300 rounds?

 

I was discussing barrel run in procedures with a fellow shooter A while ago, a guy of some repute I must add, and he told me that a new barrel will increase its MV after about the number of rounds mentioned, but couldn't explain the physics.

I have since discussed this phenomenon with other knowledgable shooters and they all say the same thing.

Interestingly, I have verified this with the (few) new barrels I've owned but thought it just a fluke.

 

Can anyone explain why this happens and the physics behind it?

 

Thanks

[DL.]

Would 300 rounds of barrel wear be the golden window when the rifling is offering less resistance than when new, yet good/efficient stability before declining into being shot out.

[The Gun Pimp]

Would 300 rounds of barrel wear be the golden window when the rifling is offering less resistance than when new, yet good/efficient stability before declining into being shot out.

If the barrel is 'speeding up' then that's because of increased resistance - creating more pressure.

 

The only explanation I can come up with is that with a new barrel, the bullet is riding on 'high spots'. After 200 - 300 rounds these are worn away and the contact area becomes greater - increasing friction/resistance and thus pressure - which gives an increase in velocity.

[MJR]

Ok, I have no evidence to support this but could it be that the barrel breaking in is allowing the bullet to create better seal with the barrel wall. This would reduce the amount of gas escaping around the bullet and produce more velocity. I suppose the bullet fitting better would also create more friction, requiring more force to push it down the bore hence more pressure which takes us full circle to vince's explanation of more resistance?

[andybrock]

I remember reading somewhere that carbon build up can increase MV not sure if this is true or not?

[The Gun Pimp]

I remember reading somewhere that carbon build up can increase MV not sure if this is true or not?

In theory, anything which would reduce the diameter of the bore could increase pressure/velocity. In my case, I'm a bit anal about carbon/copper removal so that's not the reason.

[skany]

No idea but my 6mmbr was around 3750+ wih 55g noslers when I first did the load!(foxing rnd so didn't look again or fuss about it much at all!)

 

checked 600rnds later(although with a newer lot of powder) and was hitting 3900+

 

The increased pressure seems to make sense so ill go with that:-)

[Swarovski1]

Ive not noticed an increase only a decrease in fps, not sure if it was the chrony playing up then did notice the extra elevation needed at 1k, ive upped my loads since and there back up where the were or should of been, will see tomorrow at 1000yds if ive got elevation lower, havent shot at 1k since it all happened, should be a nice morning for it.