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- Jan 26, 2015
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Thanks JCow. I wish I understood terminal ballistics better. I was simply questioning this on aspect. Thanks for all the replies!
I know there are people out there that calculate spin drift. I'm just not one of them. I do know as your Velocity changes so will the rate of the spin drift . It's really hard to calculate spin drift when you are messing with wind. So unless there is a way out there to eliminate ALL wind while taking that 2000 yards shot, how can you tell what the twist rate of the barrel dos to the bullet? You can't. It's just all Theory.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)?
I'll chime in here without getting too physics-y, on two counts: first, the force involved, and second, the rotational decay.
In the first case, the bullet is held together (stopped from flying apart) by the material strength of the bullet - lead, jacket, copper, whatever. For a given caliber, the force required to hold a bullet together increases as the square of the rotational speed - a bullet rotating at 40% greater rate than another requires twice the force. That outward force bears on the expansion rate of the bullet as it deforms and "mushrooms." Higher spin, more rapid or greater total mushrooming, all other things held equal. Conversely, the more rapid the mushrooming, the greater the braking force on the rotation rate as tissue drags on the increasing diameter of the bullet. So the wound channel profile differs for different spin rates.
Second, while it is true that a bullet is decaying in spin rate in air slowly, and its maximum rotation rate appears to be that of the barrel twist times the velocity, you can see that the ratio of rotation to velocity increases with distance up to the point of contact with the animal, in effect increasing in spin rate as measured in rotations per linear foot; a 1:10 twist might be a 1:8 at impact. At the point of contact, the bullet decelerates dramatically - but the rotational rate does not. Accordingly, while a bullet leaving the muzzle might spin one time in 10 inches, inside an animal that would change, because of the rapid linear deceleration compared to the slow rotational rate deceleration, to a full rotation in perhaps 2 or 3 inches, with mushrooming affecting the rate of decrease of rotational speed.
If the bullet opens like a Black Talon, then, the aggregate tissue damage from the "buzzsaw" effect would be far more pronounced at a higher rotational speed. You can see this from the wound cavity geometry in clear gel. The spiral track of the cavity might show a half-dozen spins within a distance of 16 inches, yet no barrel has a twist rate of one in 3 inches.
The net: higher destructive force with a greater spin rate, all else held equal; and expansion brings a measure of inequality. The above does not account for tumbling - only for stable transit of a bullet.
This is a formula determining twist rate RPM's. The RPM's for bullet core separation/blow-up which occurs at 310,000 to 330,000 RPM's.
(Muzzle velocity) x (720) /twist-rate =RPM
Blasting a 6.5mm lead core bullet at 3300FPS with a fast twist(7.5), would result in 317,000 RPM's ...potentially a recipe for erratic bullet performance.