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rsterne · Moderator

I've been playing with some numbers this afternoon, and I'll throw this idea out there just for discussion.... It is similar to the long standing idea of having a minimum of 1 FPE of energy per pound of body weight of the target.... I am a firm believer in the relationship between momentum and killing power.... While things like the Meplat diameter affect the diameter of the wound channel, and the sectional density (or the sectional density but using the meplat diameter instead of the caliber) affects the penetration.... you still have to hit an animal hard enough to kill it.... and the bigger it is, the harder you have to hit it....

The momentum of a bullet at impact is the weight in lbs. times the velocity in fps.... That works out to the weight in grains divided by (7000 times 32.4) times the velocity in fps.... ie W (gr.) x V (fps) / 226,800.... The resulting number would be about 0.8 for a 200 gr. bullet travelling at 900 fps.... pretty hard to relate to the weight of an animal.... If instead of dividing by 226,800 we divide by 1000, the number is now 180.... hmmmmmmmmm.... would a 200 gr. bullet travelling at 900 fps (at impact) drop a 180 lb. animal?.... I would think that would be pretty close....

Here are some other examples....

8 gr. @ 450 fps = 3.6 lbs (low powered .177)
8 gr. @ 900 fps = 7.2 lbs (high powered .177)
16 gr. @ 900 fps = 14.4 lbs (high powered .22)
30 gr. @ 900 fps = 27 lbs (high powered .25)
40 gr. @ 1200 fps = 48 lbs (eg. .22 LR rimfire)
78 gr. @ 900 fps= 70 lbs (low powered 9mm)
158 gr. @ 900 fps = 142 lbs (high powered 9mm)
230 gr. @ 900 fps = 207 lbs (low powered .45 cal)
170 gr. @ 2000 = 340 lbs (eg. 30/30 Winchester)
450 gr. @ 900 fps = 405 lbs (high powered .45 cal)
220 gr. @ 2500 = 550 lbs (eg. 30-06 Springfield)
570 gr. @ 2000 = 1140 lbs (eg. .500 Nitro Express)

I think those numbers are fairly realistic, maybe a bit on the low side.... Want to make it even simpler?.... How about if you have an air rifle that shoots in the high subsonic (say 900-950 fps) you just use the bullet weight in grains for the animal weight in pounds....

8 gr. pellet - 8 lbs.
16 gr. pellet - 16 lbs.
30 gr. pellet - 30 lbs.
100 gr. bullet - 100 lbs.
200 gr. bullet - 200 lbs.
400 gr. bullet - 400 lbs.

Since we have pretty much found out over the years that we don't want to push velocities over about 1000 fps (but we want to get close).... could you ask for a simpler guide to killing power?.... Incidently, at 950 fps, the energy is 2 FPE per gr. of pellet weight.... so we have values about twice the bare minimum FPE if you go by the above numbers....

Bob

radar · Silver Status Member

Interesting viewpoint. Yes, I'd say a 200 grain bullet of reasonable diameter at 900 fps could kill a 180 lb (or even larger) beast with proper placement. That's like a .45 Colt revolver.

GoodEnough · Member

Pretty obvious, since accuracy was not raised as a concern.

radar · Silver Status Member

Bob, just for fun, I'm working on an airgun sized ballistic pendulum that will measure momentum directly, and from there one can calculate velocity based on its measured momentum with a known pellet weight...You know, what they used for chronographing before the chronograph was invented.
If I am successful, I'll post about it separately.
Any advice on this project?

-Rob

rsterne · Moderator

I would think that what happens to the pellet when it hits is important.... I think the energy has to be absorbed without being converted to heat.... I do remember that there is a difference in Physics between elastic and inelastic collisions....

Bob

radar · Silver Status Member

The heat does not matter as that, as well as (and is a partial result of) the deformation, and is an effect of KE which is not conserved in the collision. The momentum will be conserved as long as the pellet doesn't bounce back. I am using Duct Seal in a soup can for the catcher.

-Rob

rsterne · Moderator

Makes sense, the KE is converted to other forms but the Momentum is conserved, and since you are capturing the pellet you know the mass of the pendulum before and after, so measure the swing, calculate the velocity of the pendulum, and you can calculate the velocity of the pellet....

It will be interesting to see what kind of accuracy you can get.... How are you going to measure the first swing, photographically?....

Bob

radar · Silver Status Member

I was using a bore-sight laser attached to the rear of the can on a prototype, but it was hard to read the peak of the swing. So I will use a standard method of a lightweight sliding needle indicator moved by the pendulum rod with just enough drag on its pivot to stay at the high point of the swing on a degree scale.
Obviously, with the sum of all the small errors in measuring the center of gravity, rod mass, etc. I will not be able to just shoot at it and directly measure the pendulum's momentum until I try to first calibrate out the errors by moving the degree scale around a bit.

-Rob

radar · Silver Status Member

Well I think due to time constraints right now, I'm going to put building a pendulum from scratch on hold, and get this one and modify it for my needs:

http://www.sciencefirst.com/Ballistic-Pendulum.html

Can't beat the price, and building a frame and all that from scratch is going to take some time and money for materials.

radar · Silver Status Member

Bob, here is an interesting webpage: http://user.xmission.com/~fractil/math/kp.html

I haven't yet read the whole page, but the part about the total volume of a "wound channel" being the same for a given energy, but a projectile the with higher momentum will go deeper, with less "splash" seems to jibe with my tests on soft clay.

-Rob

rsterne · Moderator

I didn't even look at the article yet, but the concept of "wound volume" being proportional to FPE at impact is quite familiar to me, and I would for the most part, agree with it.... I have done penetration tests in various materials, from cedar, to balsa wood, to paraffin wax, to playdough, and typically the volume stays pretty constant, for a given gun, pellet to pellet.... Wadcutters and hollow points have a large diameter wound but shallower, while round nose pellets drive deeper, and points go the deepest, but with the smallest diameter affected by their passage.... Likewise, heavier, slower pellets drive deeper while lighter, faster pellets disturb more "flesh".... and pellets that expand make a shallower, wider wound....

I haven't taken the time to compare different guns with the same pellet but at different velocities.... mostly due to the inconsistencies in the target medium.... and the way different pellets expand differently at different velocities.... Plus, it's hard to get an accurate measurement on the volume, other than by calibrated eyeball....

Bob

radar · Silver Status Member

radar · Silver Status Member

radar · Silver Status Member

To reiterate, I always hear it takes only about 1 ft. lb. energy per pound of animal (at the impact range, of course) to kill it with a well placed shot...but that's just a rule of thumb. Is your Venom shooting at its normal power level, or doing something weird?

Cheers,
Rob

radar · Silver Status Member