What causes a slow barrel?

[fatbacks]

I was hoping some folks here more versed in the mechanics of a rifle could shed some light on this.

My buddy and I have the exact same factory rifles. Same caliber and roughly the same manufacturing dates. The only difference is his is a lefty. We are shooting 30 Noslers and mine is consistently shooting 5-7% slower than his. We have swapped the same chrono (magnetospeed) and shot out of the same lots of factory ammo. This appears to be the same with all my hand loads.

With the factory Nosler Trophy Grade 180 Gr AB, they say on the box they should get 3200 FPS. He gets an average of 3215 FPS and I am only getting an average 3009 FPS. Again, same day shooting, same ammo lot, same rifles, etc.... I have more rounds down the pipe than he does as well.

Any ideas on what's going on? I know some barrels are faster/slower, but this seems to be a significant gap in velocities.

 

[Cahunter805]

Many factors can contribute to this especially in a factory barrel. Minor differences in the chamber and throat, slight difference in bore size and lands of the barrel would be my guess.

 

[Tumbleweed]

If your throats are very similar then I would say it's a difference in bore dimensions. Only .0001-.0002" difference from SAAMI spec can cause issues.

I have a factory 24" 300 Wby Vangaurd I'm working with right now that is super slow. Builds pressure soon. This was with two different powders. Mild pressure signs show up around 3025-3050 fps with 185 Berger Classic Hunters. Forget it shooting there, accuracy is down in the low 2900's. You can't force them, some are just tight and they will tell you where they want to be shot.


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