6.5mm/.257 Roberts

[vintage hunter]

.257 bullet a tight squeeze in a 6.5mm bore? I think you have it backwards. I'm quite surprised your getting any accuracy at all. A 6.5mm bullet measures .263-.264 dia. while a .257 bullet measures .257. Appearently your gun's bore is just tight enough for the lands to grip the projectile. If you were to shoot it over a chronograph you'd see a lot of velocity loss due to blow by from the undersized bullet. The .2559 measurement is actually the BORE diameter for a 6.5mm barrel not the GROOVE diameter. Get yourself a set of 6.5/257 dies and use the correct dia bullets and you will get better accuracy.

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[GUTS]

.257 bullet a tight squeeze in a 6.5mm bore? I think you have it backwards. I'm quite surprised your getting any accuracy at all. A 6.5mm bullet measures .263-.264 dia. while a .257 bullet measures .257. Appearently your gun's bore is just tight enough for the lands to grip the projectile. If you were to shoot it over a chronograph you'd see a lot of velocity loss due to blow by from the undersized bullet. The .2559 measurement is actually the BORE diameter for a 6.5mm barrel not the GROOVE diameter. Get yourself a set of 6.5/257 dies and use the correct dia bullets and you will get better accuracy.

Your right, I was thinking backwards, anyway I don't plan on shooting it anymore but if I do I'll take your advice, thanks.