I have questions about bullet physics brought up by the HAT bullet test thread by Goodgrouper. I did not want to ask these ?'s on that thread for fear of being misunderstood in my intentions. My ? is in no way directed towards GG's testing.
What I want to know is, what is the rate of decline for the bullet spin as it travels down range? I have never heard that a bullet has to start off w/ enough rpm's in order to have enough down range to remain stable.
Does rpm slow down at the same rate that velocity slows down? The only info that I know of is that it takes a certain bullet stabilization factor, depending on the length of the bullet. That it takes less stabilization for longer ranges, but that a bullet w/ proper stabilization for 500yrds and beyond will not function well for hunting under 500yrds. The longer the bullet gets the more critical this becomes because of the issue of tumbling on impact. In my simple mind, the more spin the better. I know this doesn't work, but it seems that way. I know some bullets will come apart due to centrifugal force if over ratated.
I am getting a little long winded for a short question. How much rpm is lost per yard down range?
What I've heard is that rpms don't drop much with distance. There's not much to slow down the spin other than the roughness of rifling engraved on the bullet. Would seem to be a pretty hard number to measure. But guess you could use a high speed camera.
I'm thinking it's important to get the needed rpm out the muzzle. When the speed has dropped to half muzzle velocity, the rpm is probably much above half. That's my thought anyway.
Last edited by Varminator 911; 08-09-2008 at 12:14 PM.
My take from reading the discussion AJ posted is that most believe that spin slows very very little with distance. This is probably especially true for a long streamlined bullet like the HAT. But there were no absolute numbers given probably because they would be very hard to obtain and aren't significant enough to affect anything. My take is that the decay in rpm over 1000 yds is probably less than 1 percent and certainly much less than the loss in velocity.
You need to read the discussion AJ posted. Because the forward speed decays much faster than the spin, as the bullet travels away from the muzzle, the bullet will spin increasingly faster than 1 revolution per 10 inches. The rpm stays nearly unchanged while the forward speed rapidly decreases.
Pretty simple really, but not important.
With regards to bullet testing, it is important that the necessary spin be imparted at the muzzle. Spin is a combination of twist and muzzle velocity. You need an adequate mix of both to stabilize the bullet. The bullet remains stable downrange because the rpm remain virtually unchanged as the velocity decreases.