Bullet weight vs twist

[mgva]

I'm new at 6BR shooting. I have 6BR rifle with a 12 twist. I was under the impression that the heaviest bullet that this twist would stabilize was 60 gr. However, this weekend I fires both 75 gr and 87 gr with 5 shot groups less than1 MOA at 100 yds. Nice round holes. Will these bullets still be stabilized at 200 yds, 300 yds, 600 yds? I'm a bit confuse about what is going on here. Thanks Dennis

 

[crashnrondo]

I have a 12 twist as well and shoot 80 grain Bergers most of the time but I have shot 87gr V-maxes as well and they were fine as well.

 

[KevinThomas]

MGVA,

Stability increases as the bullets go downrange, as the rotational velocity doesn't drop off like translational velocity does. Assuming they're stable at shorter range, they should remain so downrange. A 1x12" in a 6mm BR isn't really all that slow, so it shouldn't be too much of a surprise that the 87 grain bullet is shooting. I'd have called it a bit marginal, but if you're dealing with a buttoned barrel, that twist could be off by +/- a half inch in either direction, and this could also contribute to what it will (or will not) shoot. The 75 is no surprise at all, and I'd expect to see this one stabilize properly even out of a true 1x14", no problem.

On the flip side, a round hole doesn't always indicate true stability. I've seen many combinations that were not fully stable, but did manage to punch round holes on target. In those cases, a loss of accuracy frequently preceeded any signs of tipping, much less full keyholing or tumbling. One of the reasons most ballisticians will recommend a 1.3-1.5 SG as a minimum, rather than getting too close to the edge.

 

[mikecr]

Another factor is air density.
Twist requirements are tied to a standard air density at sea level and 59deg(relatively high density).
So unless you're shooting in Nova Scotia today, your air density is likely lower, and stability would be higher with this.