Why does velicity increase with # of shots?

[Pdvdh]

Full auto!

 

[Frank in the Laurels]

Yes lance, they use a return to fire type rest, all they do is once they adjust it is push forward and load at the same time, the action opens toward your face so you can use your free hand to load, the shells are on a little stand so there's no wasted time...go to the Williamsport PA 1000 yard match home page, they usually have someone featured on video in action, You'd never have time if you had to align sights, return to battery rest makes it possible...

 

[FURMAN]

I can tell you every custom barrel I have owned has speed up, somewhere around 50-100fps depending on cartridge, at around 100 rds. If I found a sweet spot in the first 100 rds I have had to adjust powder charge down to the preferred velocity. It is very repeatable and I only use chronographs for extra data. The impacts on distant targets don't lie, chronographs do.

 

[WildRose]

No, no, no.....when the chamber heats up it transfers that heat into the new cartridge while your sitting there readjusting everything... The new unfired round goes from air temp to hot very quickly which affects the velocity, the more you shoot the hotter it gets inside the case causing pressure increases which in turn causes velocity change and impact difference....what a thousand yard shooter in action, not only does he want as many as he can get in the same air but he doesn't give the new cartridge time to heat up...I've watched many that have two or three in the air at the same time!!! I've heard shooters say they've seen four but I've never seen that many...just another perspective to drive you nuts.....lightbulb

There's a lot of truth in this.

Also, if you are shooting the same rifles all day running a large number of shots through them once you get the burrs smoothed out which will certainly help you pick up speed you then also have to deal with carbon and copper building up.

Shooting hotter ammo (not letting the chamber cool down between shots) and tightening the passage crating more pressure is almost certainly going to speed things up at least until i reaches a point at which there is so much resistance from the build up to slow the rounds down.

Keep them cool and clean them reasonably well aftee fifty rounds or so and you will probably see much better consistency.

I try very hard to keep my groups to 3 shots fired fairly quickly but then open the bolt and allow it to cool back down for five minutes before repeating.

Remember heat and pressure are what kill barrels so there's nothing of value to be gained by getting them hot and then continuing to shoot them hot.

 

[Greyfox]

Other than a new barrel for the first 100 rounds, I have not experienced increasing velocities with my rifles. A 50-100 FPS increase is very high and would be a killer for any long range work. I do believe that the combination of a hot chamber with a case that uses a powder charge that does not utilize all the case capacity can see a pressure/velocity spike, especially if the case sits in the hot chamber for any length of time. Also, when using the Magnetospeed I have found velocity can change due to the bayonet positioned too low relative to the barrel, small changes in the bayonet sliding on the barrel due to recoil, or carbon build up on the surface of the magnet. The change is usually progressive when this occurs. I'm not saying these factors are the reason for the OP's increasing velocity but it's something to keep an eye on when velocities are out of control.IMO.