Bullet overspin?

[ezmobile]

Hi all.

Just a qiuck question about something I've not considered before which is, is it possible to "overspin" a bullet? I've been handloading for ages but I'm having a new .30 cal barrel made up for one of my rifles with a fast twist (1:9.5) for heavier, longer bullets, ( I already have a 1:12 Palma barrel for it). My question is, would it be o.k. to drive say, a 155 Sierra MK through it at high velocities when the bullet is better suited to a 1:13 or so? I've heard stories of bullets "throwing off" jackets before, but never seen it and would obviously prefer to avoid it if it does happen from time to time.

Apologies to anyone thinking this is a stupid question, its just something I have no knowledge of.

Any thoughts?

[brown dog]

Overspin is quite possible, but I can only comment on it from the point of view of increasing precessional drift.

[VarmLR]

There has been some evidence presented for very high MV loads with shorter (ighter) bullets causing jacket separation before the bullet has exited the muzzle, but I doubt you'd have cause for concern with 155 SMKs and a 1/12. I use them for highish velocity loads from a 1/11 twist barrel as do many sport shooters. The only issues that I've come across were related to smaller calibres using thinner bullet jackets where at (presumably) some predetermined spin rate, the centrifugal forces caused the issue. However, I think that just about all the cases that I've read about were related to older bullet manufacture where the bonding of the jacket to the core wasn't as good as it is today.

 

As Brown Dog says, processional drift and a tendency towards increased inaccuracy beyond certain spin rates, rather than bullet/jacket separation might be more relevant. There is the case of over-stability, where the attitude of the bullet nose remains too high during the descent of the bullet from it's trajectory apex due to gyroscopic forces imparted from too much rotational acceleration for the bullet. The rotational speed for jacket separation is pretty phenomenal. I read somewhere what for something like a 5.56 shooting a thin jacketed FMJ, the rotation had to exceed 300,000 RPM before jacket separation occurred (I don't remember which bullet, only that it was fired from an AR15 during tests, that the MV worked back to calculated rotational speed).

 

I don't think that you have any concerns with a 1/12 barrel and 155 SMK. Plenty of people develop 3,000 fps plus loads for these shot from 1/12 or tighter twists, intended for 1000 yard shooting.

[duey]

The OP has stated that he is moving to a 1-9.5 twist and if that is suitable for 155

I would say it would be a case of try it and see and it will also depend on how hard you push the 155' s, if you use a palma case stuffed to the max as per F class shooters then it would be a possibility

[ezmobile]

Hi Varm. Sorry, I don't think I explained it properly. Obviously a 155 is fine in a 1:12, that goes without saying. The actual question was, how they would go in a 1:9.5 ? (the one I'm having made).

To clarify; I'm not intending to push a 155 through a 1:9.5 tube, I simply wondered what would happen, if anything at all, if this were to happen by mistake? I've put the nombers through the Berger stability calculater, with a M.V. of around 2950 / 3000 fps and its coming back with an S.G. of 3.15. So, would this be something to be wary of, precessional drift aside.

[VarmLR]

Sorry, I mis-read the original post. I agree with duey. Possible, if driven hard enough but never heard of a 155MK jacket debonding.

[ezmobile]

O.K. thats good news. It was just something that popped into my mind when there is the possibility of having two separate batches of handloads which look virtually identical, so making it easy to mix up one for another. One, a batch of .308 fast target rounds for my 1:12 T/R barrel & another batch of heavy (200 grn +) .308 rounds for the 1:9.5 barrel, and what may happen if they were fed into the wrong tube.

I guess thats always going to be a possible problem when you are able to "switch barrels" for a rifle.

[dannywayoflife]

I had a 1in8 twist 308 barrel on my DTA. My load of choice out to 800 was a 155 Optimus HBC Pushed at 2730FPS. It that which promotes growth and vigour incredibly well........

[Andrew]

I would shoot 150's from my 1-10" 30-06 at2900 fps and didn't get any form of destabilization. I can't see a 1:9.5 causing any problem from a 308 with 155's. I might be wrong, but I doubt it.~Andrew