Effect of Bullet Spin on Terminal Performance

[Tiny Tim]

I was wondering if anyone has any insight into this. According to a test done at the Aberdeen Proving Grounds, a bullets spin decay rate is approximately 2% per 100 yds. Bullets shed velocity at a much greater rate than spin. I understand that a bullet may only make 1-2 revolutions while passing through an animal, but what effect is there on the terminal performance. Using a 1:10 twist barrel of .308 for each. If we plug some basic rough numbers into a comparison we find a 300 WM shooting a 200 gr bullet at 2900 fps mv spins at approx. 208,800 rpm, 2500 fps @300 yds turning 194,000 rpm, and 2200 fps @ 500 yds turning 185,800 rpm. A 308 Winchester would have the same mv as the 300 WM @300 yds but would be spinning 180,000 rpm (7% slower for same velocity), and would have the same velocity at 200 yds as the 300 WM at 500 yds but turning 172,800 rpm vs the 300 WM at 185,800 rpm (again, about 7% slower).

All this to ask, with this roughly 7% difference in rotational velocity, would a bullet perform significantly different out of a 308 than a 300 WM at an equal velocity but different rpm? In other words, would a bullet fired from a 308 Winchester have the same terminal performance at 200 yds as a 300 WM at 500 yds (same velocity)?

 

[Ingwe]

I always wondered about this myself. 300,000 rpm sounds pretty impressive...but what bullet is in the air one minute. I think that RPM is irrelevant.

A bullet fired from a 1:9 barrel rotates once every 9" so I can't see how rotation has ANY effect on an animal...let alone different twist rates.

I'm not an engineer either but this makes sense to me

 

[Deleted member 110997]

Ingwes use of logic buys my ticket. And that's assuming the bullet Spin even reaches the barrels rifling to begin with.

Or

Maybe it's the diff of getting a titty twister and getting your Titty pinched.

Look at ballistic gel and you'll see that it probably doesn't matter much either on the tgt.

 

[Ingwe]

Ingwes use of logic buys my ticket. And that's assuming the bullet Spin even reaches the barrels rifling to begin with.

Or

Maybe it's the diff of getting a titty twister and getting your Titty pinched.

Look at ballistic gel and you'll see that it probably doesn't matter much either on the tgt.

I agree. I think that guys have 300,000 RPMs in mind and picture the bullet impacting like a buzz-saw and forget that it's only spinning one time every 9 inches

 

[bocajnala]

I wouldn't think that it would make any difference.

But I've been wrong before.

 

[Tiny Tim]

I realize that each example seems to be at opposite ends of the spectrum (one rev /10" vs 200,000 rpm). But it is also the rotational force that assists in the radial dispersion of bullet fragments and formation of the expansion. I was just wondering if any one knew if there was any significant effect. I admit that velocity is the primary determining factor.

 

[Hatrick]

I would like to se a test where the same bull it is shot into gelatin at the same velocity. One time with rifling and one time smoothbore. I think a lot of that hydrostatic shock you see at slow motion is spin. It also keeps the bull it straight instead of deflecting from instability.

 

[dennisinaz]

I can tell you with 100% certainty that a 53 grain Vmax from a 1:8 twist AR15 has a much different terminal effect and result than the same bullet fired from my 1:14 .223.

A very noticeable difference on both reactions of the coyote and autopsies done. I can't see why the same result wouldn't hold true in larger calibers.

 

[Tiny Tim]

Dennis, could you explain your experience on yotes?

 

[HARPERC]

......I would like to se a test where the same bull it is shot into gelatin at the same velocity. One time with rifling and one time smoothbore. I think a lot of that hydrostatic shock you see at slow motion is spin. It also keeps the bull it straight instead of deflecting from instability......

I think the effect is 1) Keeping the bullet face square on impact 2) If it's not completely stable in flight, it has poor chances at penetrating in a straight line.

In think there are videos I just haven't looked for them in a long time.

 

[dennisinaz]

For starters, the bullets fired from an AR15 rarely exit, they always exit when fired from my bolt gun. Coyotes would run a ways before dropping. Unless you shoot them in the arse, they don't run with the 8 twist.
Internal damage is more widespread with 8 twist. You have to see it to believe it.

The load was Hornady Superformance 53 grain Vmax loads.

 

[JCow14]

All this to ask, with this roughly 7% difference in rotational velocity, would a bullet perform significantly different out of a 308 than a 300 WM at an equal velocity but different rpm? In other words, would a bullet fired from a 308 Winchester have the same terminal performance at 200 yds as a 300 WM at 500 yds (same velocity)?

My understanding of your question is similar to a football same weight at same speeds and faster or slower rotations- I would think RPM is velocity/fps at some point and vice versa, barrel twist which is very important i think is more important in stabilizing the bullet rather than speed. The 300wm and 308 win are very different in case size and case capacity. You could very easily have two bullets faster or slower than each other -that do or don't stabilize well at a higher velocity or lower velocity. The 300wm at its full opportunity with a good barrel and good load is really non comparative to the 308win at long distance-IMO.

 

[Tiny Tim]

[QUOTE="JCow14, post: The 300wm at its full opportunity with a good barrel and good load is really non comparative to the 308win at long distance-IMO.[/QUOTE]
I agree there is no comparison between the two cartridges as far as performance goes. But for instance, some people claim to have problems with Berger bullets at close range at magnum velocities. So would the same bullet fired at a lower velocity in a 308 for woods hunting perform the same terminally as a 300 WM at a longer distance that has an equivalent velocity even thought the rotation is significantly less? Both bullets being stable, for the sake of argument.

 

[JCow14]

Yes this is true on Berger bullets at close ranges but after 200 yds they really expand well on an animal. I shoot the elite hunter 195s. I think they are so fast coming out they act as a spear tip at close range and go straight thru- really depends on how fast it's going.

As for the same bullet between a 308 and 300wm I would think they both would have the same terminal effects you are looking for, given they are the same Ft lbs at whichever yardage you are comparing.