RE: Too fast of a Twist?
 

Yes speed has as great an effect as twist rate in stability, as mentioned above it is the rpms that do the job, since the twist can't change in a particular barrel, increasing or decreasing the speed varies the rpms.

RE: Too fast of a Twist?
 

I'm guessing that the reason your 200 grain MK was completely stable at that velocity with only a 1:10" twist was because of the different aerodynamic forces experienced by a subsonic projectile, perticularly as it pertains to the bullets center of gravity (CG) versus its center of pressure. If you know anything about supersonic flight you'll know that as an aircraft passes through the sound barrier the center of pressure shifts dramatically forward, which can be a very dangerous condition for an aircraft, as the CG can suddenly be behind the CP, which is a VERY unstable condition. I would think that a similar condition exists with the 300 Whisper. If you pushed the same bullet to just over supersonic velocity you'd probably find that the bullet was very unstable. Aerodynamics at subsonic velocities is too different to make a comparison to normally supersonic projectiles.

Mike

RE: Too fast of a Twist?
 

There may be something to this subsonic stability factor. I have also heard that when a bullet passes below the sound barrier at long range, it can have a temporary effect on its stability - sort of like the bullet had to break through a thin sheet of plexiglas. But if it was never above it to start with . . .

I still disagree with the RPM theory. I have, over the years, rechambered quite a few small case .22 centerfires to big case .22 centerfires. Often times, a benchrest shooter will have a premium barrel in .22 PPC or .22 Rem BR that has opened up its group to .3" - no longer competitive in benchrest, but still a fine varmint barrel. So the take-off barrel is rechambered to .22-250 or .220 Swift and put on a regular varmint rifle. It's still not going to stabilize a bullet much better than the smaller case .22 CF. A little better, but not much. If it is a 1:14 twist, either the .22 PPC or the .220 Swift can stablize the 55 HPBT - and neither will work with the 60 grain Berger. And that's going up in speed almost 1,000 fps.

If the RPM theory held any water, bullets would suddenly become unstable as they slowed down in flight to the target - the faster ones would just get further downrange before encountering this in-flight instability. And we know this doesn't happen . . . .

RE: Too fast of a Twist?
 

Ah but while air resists the bullet in such a way as to slow it down, it hardly effects the rpms at all, they do decay, but not to any great extent. :eek:

RE: Too fast of a Twist?
 

OK - so if slowing down doesn't really effect the spin that much, then going faster shouldn't really affect it that much either. And, based on personal experience, it doesn't have much affect. A little, but no where near as much as the difference in speed would suggest. A .30-40 Krag might need 1/2" faster twist to stabilize the same bullet as a .300 Ultra Mag.

RE: Too fast of a Twist?
 

I guess your response confused me. As the bullet leaves the barrel its rate of spin is determined by the twist rate and the velocity. This rate of spin doesn't decay much as the bullets velocity is shed. But how can you say that a higher velocity wouldn't make the bullet spin faster? I don't recall the twist rate of a .30-40 barrel, but I'm sure you could get by with less in a .300 ultra mag. I don't shoot one of those, so I don't know what rate they come with.

RE: Too fast of a Twist?
 

I know this is somewhat counter-intuitive, but velocity doesn't seem to have a whole lot to do with stability. If it did,rounds with lower velocities would need very fast twists andfaster calibers would have slower twists. There is very little differencebetween them, inmy experience.

Another exampleI would point to would be the 107 Sierra VLD bullet in the 6MM caliber. In a .243,this bullet needs a 1:8.5 twist to be stable. A 1:9 is right on the edge - 1:10 you are out of luck. My 6MM-284 can push the heavier bullets at least 300 fps faster than a .243. If the RPM theory was true, I should easily be able to get this bullet to work with a 1:10 twist in the bigger caliber at 10% higher velocity. But it don't work that way. Bullets hit the target sideways just like they did in the .243 with a 1:10 twist.

RE: Too fast of a Twist?
 

Ok but what about the nearly 1000 fps in your .30-40/.300 RUM comparison. Does the .30-40 require a faster twist?

RE: Too fast of a Twist?
 

From what I have read and seen personally, the slower bullet does require a slightly faster twist - but nowhere near as much as the difference in velocity would suggest. I would expect the difference between the .30-40 Krag and the .300 RUM would be less than 1" of barrel twist. I know that in the 6MM-284, I have yet to find a bullet that I can stabilize that isn't also stable in the .243 with the same twist- at 300 to 400 fps difference in velocity.

I also played extensively with the .30-378 Weatherby and was surprised that it wouldn't stabilize any heavier/longer bullets, with a 1:12 twist, than a .308 Winchester.

RE: Too fast of a Twist?
 

well I just hope ya'll won't tell my Encore 22-250 with a 1-12 twist it's not supposed to shoot the 64gr Winchester PP's like a house a fire... cause that's what I'll be shooting for deer this year when I go upstate...