effect of fast twist on light bullets?

[JeffG]

As I've read and experimented it's obvious that, in any given caliber, certain weight bullets have a recommended barrel twist rate (i.e. 6mmx65g do well in 12-14 twist, 105g requires 8-twist...doesn't seem to matter whether it's 6mmbr or 240 weatherby).

My question; what effect does shooting a light weight bullet in a (too) fast twist rate barrel have?...other than losing some velocity that you might get in a slower twist barrel.

I have a .277 9-twist barrel, for a hunter project, and I'm thinking 270-308 improved, but I want it to handle 120g as well as 150g.

 

[Phil3]

The bullet can blow up. It can only spin so fast before the centrifugal force is so high, it will fly apart. If a 52 grain 223 bullet comes out of a 1:7" twist barrel at 3000 feet/second, the bullet is spinning over 308,000 revolutions per minute. That is not a misprint. At that amount of rpm, if the bullet was rolling on its side on a road (like a wheel/tire), it would be traveling at 205 mph! The G force (force of gravity or "Gs") would be so high, it may just pull the bullet apart. I can't speak to reduced velocity or other effects.

Phil

 

[DRNewcomb]

One of the side-effects of the fast twist needed to stabilize the long very low drag bullets that are popular these days is that the high RPMs tend to exaggerate the effects of small imperfections in the bullet's jacket and core. A short bullet that is good enough to perform well from a, say, 1:12" barrel might be all over the target when shot from a 1:7" barrel, at the same velocity. I personally have observed this with some 55 gn .223 bullets of dubious provenance. Assuming a high quality bullet and reasonable velocities (See JeffG above), there should not be any problems (but you still might not obtain the best accuracy doing this). IMHO.

 

[BradY]

Ive had funny things happen to 40gr nosler BT's in a 223 1:8 twist krieger. Ok to 200 metres, then after that things go pear shaped. Shots going high, low, way right... all unexplained. Yet brilliant half to quarter inch groups at 100 metres and sighted in 3/4 inch high for an approx 200 zero. Switching back to 50 and 55gr bullets for now and ultimately going back to 1:14 twist for the lighter varmint weight bullets.

 

[Luke_NL]

Have personally witnessed 40 (maybe even 50) gr .223 bullets not making it across a 50 m range when fired from a 1:8 twist barrel. Stumped the pretty decent shooters, had us in stitches when we figured out what was going on. Since it was a new rifle, a few shooters came over for 'looksies' so we had fun with it. My buddy would demonstrate the rifles capability shooting 69 grainers, and would then cordially invite the range's loudest tool to have a crack. Loading him a magazine of 40 grainers. The tool was quick to critisise both rifle, scope and setup - whereupon I would offer to try, and group 69-ers again. To this day he probably has no idea.

 

[kentuckytroutbum]

I had a similar experience with Winchester 55 gr. .223 FMJ's (Vietnam era M193 Ball) in an 1:8 match barrel. My friend who was spotting for me said that about 10-12% of these bullets would blow up leaving a gray puff of smoke at a range of 40-50 yds. and no hits on the target.

After talking with the Winchester tech rep, he told me that the twist was too fast at that MV to keep the jacket together. Hence the blow ups.

Bill

 

[Turbulent Turtle]

Ive had funny things happen to 40gr nosler BT's in a 223 1:8 twist krieger. Ok to 200 metres, then after that things go pear shaped. Shots going high, low, way right... all unexplained. Yet brilliant half to quarter inch groups at 100 metres and sighted in 3/4 inch high for an approx 200 zero. Switching back to 50 and 55gr bullets for now and ultimately going back to 1:14 twist for the lighter varmint weight bullets.

Two words for you: dynamic instability.

I discussed this issue in another thread on the "Small stuff" forum about a month back.

What is happening is that somehow your light bullets are getting damaged at firing and the damage causes the bullets to wobble in flight due to the very high twist rate. You can picture this by putting a piece of tape on a top and spinning it very fast. This dynamic instability is very small at the start but it grows as time (distance) goes on. So your bullets are fine at 1-200 yards and then the instability becomes sufficient to make the bullet nose wobble and make the bullet hit not quite point on. It will also deflect the bullet and make it inaccurate.

My current theory on the onset of the damage centers around the initial engraving of the bullet into the lands. It may be that the lands angle may not be optimal for that bullet, either too great or too shallow.

 

[Luke_NL]

The bullets were dirt cheap Prvi stuff, made in Eastern Europe somewhere. They differ in weight, shape, and length so I would not be surprised if these are way more likely to spin wildy out of control and even frangeate when spun too hard.