# How to test bullet expansion?

#### Wedgy

##### Well-Known Member
. At 500 yards with the velocity slowed significantly and the rotation slowed slightly the bullet maybe doing one rotation every 5”.

The bullet rpm does not slow as fast as the velocity. to put it in simple terms with a 1:8 twist barrel has the bullet leaving the barrel making one rotation every 8”. At 500 yards with the velocity slowed significantly and the rotation slowed slightly the bullet maybe doing one rotation every 5”.
I think you mean the bullet slowed rotation but slower is going to be one rotation every 9 or 10 inches, 1:5" is faster.

BTW this was a Berger 6.5 140 grain at 3,018 fps hitting a ~150 lb deer 550 yards broadside lower chest(12 inches max) ribs, heart, lungs, no spine contact, found in the offside hide, big lump in the skin, I thought it was a tick at first. I have had these exit thru an elk on the same shot similar distance but this one totally came apart

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##### Well-Known Member
Rotational speed=twist rate*muzzle velocity*unit conversion factor

Rotations/minute=(rotations/inch)*(feet/second)*(12 inches/1 foot)*(60 seconds/1 minute)

So as MV increases, so does rotational speed.
Yes, but does rotational speed (rpm) decrease at the same rate as travel velocity? It seems there are far fewer forces acting to slow rpms than the many forces acting on travel velocity. (Air friction, bow wave, base drag, parasitic drag & perhaps others)
Just thinking....do any laws of physics demand that the slowing of forward momentum relate directly to a rotational inertia? I see that being a direct relationship, whilst still in the barrel, but once free of the barrel's influence, what says that the rotational speed is directly geared to forward speed, (especially in lieu of the aforementioned exterbal forces). Good discussions amongst friends....

#### Tank308

##### Well-Known Member
Rotational speed=twist rate*muzzle velocity*unit conversion factor

Rotations/minute=(rotations/inch)*(feet/second)*(12 inches/1 foot)*(60 seconds/1 minute)

So as MV increases, so does rotational speed.
I guess that's where I was confused. I thought that if a barrel twist was 1:8". That the bullet flew a 1:8". But because flight time was less with higher velocity that was the greater RPM numbers. Man my head hurts.

#### Calvin45

##### Well-Known Member
Ballistic Gelatin, and lots of youtube videos on how to make it. Only real thing to show true expansion and be consistent across the board. Just down load it to the velocity you want and shoot it at 5 feet....ok ok...10 feet. Simple minded Fella on youtube has a lot of lower velocity expansion. So far my minimum FPS and energy on target requires the big powder eaters to get past 800yds.
I like that Simple Minded Fella’s channel too, lots of good stuff, no nonsense

#### skipdavidson

It doesn't decrease it's rotational speed nearly as fast as the velocity, mainly the difference is the rotational translation doesn't have to overcome pressure/form drag.

An approximating formula Geoffery Kolbe published for spin decay is:

Where:
Nm = the spin rate the bullet had at the muzzle
N = the spin rate of the bullet after your time of flight to the range of interest
t = time of flight in seconds from the muzzle to the current bullet position
d = bullet diameter in inches
e = natural logarithm base, 2.71828…

N = Nm × (-0.035 × t / d)^e

#### Calvin45

##### Well-Known Member
Problem with "loading down" is that you drastically slow down the rpm of your bullets too, this messes up the expansion .as rpm at impact is a major factor of expansion
Very true, as others have said here.

However…for those who want the exact velocity at distance I think we’d all be a tad nervous about turning our chronographs into targets haha

#### skipdavidson

When we were into this stuff, use a program to calculate velocity at impact for test distance, or specific test velocity.

For simulated test distance of say 50 yards, use the same program to solve the required muzzle velocity.

Use the previous equations to solve for twist rate required to create the correct rotational speed at the simulated test distance.

Order the barrel, load the ammo to the correct mv. And shoot!

After doing a bunch of this, I decided I would rather test in the field.

#### LongBomber

##### Well-Known Member
I think you mean the bullet slowed rotation but slower is going to be one rotation every 9 or 10 inches, 1:5" is faster.

No- I meant what I said, the bullet at 500y will be more like a 1:5 twist.
Yes the rotation slowed, but the velocity slowed realtively more. The bullet that started at a 1:8 twist from the barrel will be closer to a 1:5 twist when it gets to 500y. The rpm is going to be close (slightly slowed) but the velocity is slowed. The bullet will travel less distance per rotation. Hence the rate of twist will actually be “faster”.

#### Wedgy

##### Well-Known Member
No- I meant what I said, the bullet at 500y will be more like a 1:5 twist.
Yes the rotation slowed, but the velocity slowed realtively more. The bullet that started at a 1:8 twist from the barrel will be closer to a 1:5 twist when it gets to 500y. The rpm is going to be close (slightly slowed) but the velocity is slowed. The bullet will travel less distance per rotation. Hence the rate of twist will actually be “faster”.
OK, got you !

#### SSgt G Cody

##### Well-Known Member
Simplify! US Army ballisticians state that rotational spin will never increase after bullet leaves barrel, but will slowly decrease through its trajectory. It decreases at a percentage rate approximately 1/3 that of slowing velocity of bullet over the same trajectory. In other words, if bullet velocity has decreased by 1/3 at the target, rotational spin will only have decreased by 1/9. Bullet designers increase jacket thickness for hunting bullets to counteract forces exerted by bullet's spin energy upon impact. This also increases penetration.

#### BFD Guns

##### Well-Known Member
So, the RPM of the bullet, shot at full MV at a target at distance, will be greater than a bullet fired from a reduced charge that matches the same impact velocity. Correct?

#### Tank308

##### Well-Known Member
So, the RPM of the bullet, shot at full MV at a target at distance, will be greater than a bullet fired from a reduced charge that matches the same impact velocity. Correct?
That's what I interpreted from longbomer's post. With the help of cody's input Because Even at impact velocities. The rpm numbers are different. With a lower divider the farther you go out.

#### snox801

##### Well-Known Member
I joke about this all the time. "Hey ya'll, we are over run with feral hogs destroying everything! So, come pay us to help us out with our problem!" I equate it to having the plumber pay me to fix my broken pipe. LOL

I might be one of the last lucky guys in the world. The past 20yrs I've been able to vermin reduce hogs on a ranch for free. The rancher is a logical man.
Well part of the problem we all know is the sac faces that don’t treat land as if it was them who purchased it. Center pivot shot garbage and cows all add up to not letting someone hunt.
I do understand that but I always have on this site offered to help with the problem as it my main hunting passion. Even to the point of buying insurance to cover any accidents that may happen. Still solid no. So it is easy to see if they let the crops get eaten they collect insurance money for what they could have harvested but now don’t have to spend time or fuel to do that. Government checks are the best.