Affect of rotational velocity on bullet performance

[elkaholic]

This topic has been discussed before, but I think this data backs up the fact that rotational velocity does indeed affect bullet upset, and likewise, wound channel and tissue damage. I took these pics while testing the new ELDX bullets for terminal performance. The relevant bullet in the first pic is the one fired into damp clay at 1095 yards with a mv of 3155' and an impact velocity of 1859'. The lower pic shows 2 bullets fired with reduced loads point blank into the same consistency of damp clay. Both bullets were fired from an 8 twist 6.5 SS with muzzle and impact velocities noted. The only difference was the rotational velocity, but the affect on the bullet is very obvious! I did not have identical impact velocity data, but you can see that even the bullet fired with nearly 300' more velocity, at point blank range, did not expand nearly as far down the bullet body. Reduced load expanded bullets will sometimes be larger in diameter because there is not high enough rotation to rip off the petals. These pics also show that affect. Wound channels will be larger resulting in more tissue damage when bullet rotation is higher. I believe that the minimum velocity required to expand a bullet is also lowered with higher rotation. The bottom line in all this for a long range hunter, IMO, is that higher velocity rifles like the 6.5ss do not just exceed the killing distance of say a 6.5x47 by merely the distance of the impact velocities. The fact that the rotational velocity, with like twists, is so much higher with the ss, will cause more tissue damage at lower velocities. Therefore; instead of saying it has a 400 yard farther killing range, it may well be 500 or more! Look at the pics and fire away......Rich

 

[Edd]

Maybe you've discovered the lead gets hot and becomes soft in flight. You need to develop a new heat resistant lead.

 

[elkaholic]

Maybe you've discovered the lead gets hot and becomes soft in flight. You need to develop a new heat resistant lead.

Ya think!

 

[Timber338]

I also agree that higher rotation rates/inertia will cause more dramatic bullet expansion. Great that you have some evidence to back that up. I'll always remember my dynamics professor in college and his story about how to convince people that rotational inertia is real... Whenever he had a student that didn't understand, he told them to go jack up the rear wheels of their car, get them spinning to about 50mph, and then grab onto one of the wheels with both arms as hard as they could. Don't think anybody was dumb enough to try but it got the point across.

 

[elkaholic]

I also agree that higher rotation rates/inertia will cause more dramatic bullet expansion. Great that you have some evidence to back that up. I'll always remember my dynamics professor in college and his story about how to convince people that rotational inertia is real... Whenever he had a student that didn't understand, he told them to go jack up the rear wheels of their car, get them spinning to about 50mph, and then grab onto one of the wheels with both arms as hard as they could. Don't think anybody was dumb enough to try but it got the point across.

Maybe you can get Edd to try it!

 

[Timber338]

Maybe you can get Edd to try it!

Ha!

I also think it's cool how all of this sort of falls into place for long range hunting, like you have pointed out. We shoot heavy, high BC Bullets and need fast twist barrels to stabilize that also helps the terminal ballistics... I bet there is a sweet spot for muzzle velocity vs bullet weight for a given cartridge... For example, my 338 RUM that doesn't quiet have the horsepower to push a 300 grain pill over 2800 fps...I wonder if a 250-280 grain bullet in the 2900+ range would give better terminal performance at long range?

 

[elkaholic]

Ha!

I also think it's cool how all of this sort of falls into place for long range hunting, like you have pointed out. We shoot heavy, high BC Bullets and need fast twist barrels to stabilize that also helps the terminal ballistics... I bet there is a sweet spot for muzzle velocity vs bullet weight for a given cartridge... For example, my 338 RUM that doesn't quiet have the horsepower to push a 300 grain pill over 2800 fps...I wonder if a 250-280 grain bullet in the 2900+ range would give better terminal performance at long range?

All good things to consider? It's what makes shooting so interesting and it is not always the obvious that works best. It all boils down to what the bullet does when it arrives though.....Rich

 

[RockyMtnMT]

Rich,

This is the same thing that we found doing low velocity impact testing. My thinking is that the higher sg keeps the bullet point forward causing it to better deform. Or in other words a bullet that is marginally stable will tip on impact and inhibit expansion. Bullets can be very accurate with marginal stability, but from my lessons, I don't believe they should be hunted with. Keeping a hunting bullet at 1.5sg or higher is needed for proper bullet function. I think this goes for all bullets. I also think that many of the bullet failure stories that we have all heard here are due to poorly stabilized projectiles.

My .02

Steve

 

[elkaholic]

Rich,

This is the same thing that we found doing low velocity impact testing. My thinking is that the higher sg keeps the bullet point forward causing it to better deform. Or in other words a bullet that is marginally stable will tip on impact and inhibit expansion. Bullets can be very accurate with marginal stability, but from my lessons, I don't believe they should be hunted with. Keeping a hunting bullet at 1.5sg or higher is needed for proper bullet function. I think this goes for all bullets. I also think that many of the bullet failure stories that we have all heard here are due to poorly stabilized projectiles.

My .02

Steve

Another very good observation Steve! Makes some sense.......Rich

 

[tbrice23]

Rich, your 6.5 eld bullets held up much better in water and clay (more similar to tissue) than mine did in sand. Like we predicted.

Good info and recovery.

 

[elkaholic]

Rich, your 6.5 eld bullets held up much better in water and clay (more similar to tissue) than mine did in sand. Like we predicted.

Good info and recovery.

It's nice when something works!......Rich

 

[Predator 22]

Tag

Makes me think that manufacturers should have a rpm threshold to work in for proper bullet performance.

 

[elkaholic]

Using the MV and twist rate for my SS in this data, here is the difference:

mv=3155 x8 twist(1.5)x60= 283,950 RPM

Using the impact velocity above at 1859 fps and using a reduced load to duplicate you get the following:

MV=1859x1.5 x60= 167,310 RPM

Rotational velocity decreases at a MUCH lower rate than forward momentum (impact velocity) so it does not take a rocket scientist to see how much more disruption of the bullet might take place with an additional 116,640 RPM......Rich

 

[Predator 22]

Using the MV and twist rate for my SS in this data, here is the difference:

mv=3155 x8 twist(1.5)x60= 283,950 RPM

Using the impact velocity above at 1859 fps and using a reduced load to duplicate you get the following:

MV=1859x1.5 x60= 167,310 RPM

Rotational velocity decreases at a MUCH lower rate than forward momentum (impact velocity) so it does not take a rocket scientist to see how much more disruption of the bullet might take place with an additional 116,640 RPM......Rich

My point exactly. Thanks for the info Rich informative as always