Horizontal drift due to bullet spin and drop?

orwapitihunter · #1 07-01-2004, 01:06 AM

Do bullets drift to the right at long range do to the spin of the bullet acting on the air as it falls? Or something like that that isn't related to wind? The two rifles that I was shooting today were showing about 6MOA of drift to the right at 1000 yrds. Both rifles were shooting mid BC bullets (.4ish) and show about 24MOA drop between 100 and 1000. I couldn't detect any wind and haven't seen this before with higher BC flatter shooting rigs. Can anyone shed some light for me?

climber · #2 07-01-2004, 01:37 AM

Yes, It's called spin drift. It comes into play beyond 600 yards. 2 MOA correction beyond 600 should compensate for the drift.

Richard338 · #3 07-01-2004, 08:46 AM

Spin drift does play a small role. The bullets you are shooting are similar to those that McCoy discusses (.308) in his book. The spin drift at 1000 yrds for these cases would be a max of 1 MOA.
Much more likely to account for 6MOA is some wind that was down range. I have had to correct at least 5MOA for wind at 1000yrds when there was little or no wind at the firing line. Did you have wind flags?

#4 07-01-2004, 08:48 AM

Most of the rifles I shoot at 1000yds will need 1.5-3 MOA to the left to offset the spin drift and I usually shoot bullets with a BC between .5-.6 with a muzzle velocity of 2800-3200fps.
Wayne

orwapitihunter · #5 07-01-2004, 04:28 PM

Thanks guys. I was shooting acrossed a canyon so there may have been a little wind in the middle that was undetected on either side. I would imagine the set ups I was shooting would be some of the worst for exhibiting this "spin drift". 3450 and 3500 fps should give me lots of spin, and light low BC bullets give me lots of drop. In past shooting the effect seemed small enough I questioned it's existence. Once again thanks, and anymore info on this subject would be great.

[ 07-01-2004: Message edited by: Birdshooter ]

meichele · #6 07-01-2004, 10:06 PM

If there was 6 MOA of drift with no detectable wind, was the rifle canted? was the scope adjusted? If so, is the scope level? Between spin drift, coroilis, a canted rifle and/or a scope that is not level and adjusted to -0- at 1K could easily account for 60 or so inches.

Spin drift alone should not be more than 9-12" fired from a 308 at 1K