When I adjusted my scope 4" to the right, I remembered when I last adjusted the left/right adjustment; when I started using the 220gr bullets again. It doesn't make sense to me, but I think those bullets fly differently than the others. Maybe someone will be along that can explain it.
But, tomorrow I think I'll be on target again!
180gr Sierra, Scope Off; NO, Gyroscopic (spin) Drift
(About 4 hours after I wrote the above.)
I figured it out!
I couldn't sleep thinking about why there is an at least a 4 inch difference at 400 yards. There had to be a reason, and I had to know that reason!
That is what it is! Barrel twist!
The 220 RN bullets are in the air almost twice as long as the 180s and the round nose slows it down considerably. So the 220s spin many more times. Coupled with the more spins and the greater area spinning, I think a 4 inch difference at 400 yards may be in order.
The comment above, "when I last adjusted the left/right adjustment;" is what got me thinking. I made a 2.25" left adjustment when I switched from shooting the 190s to the 220s at 300. That is 3 clicks, one more click would put a bullet that gets there a little faster, (180s,) centered.
I thought to myself, which way would the twist need to turn for my theory to be correct. I thought the bottom of the bullet would have more resistance/friction. Therefore the bottom would need to be in a left direction to force it right. If my twist was clockwise, my twist theory would have to be the factor. I couldn't remember of which way it turned so i got up and tested, Sure enough it is a one in ten clockwise twist!
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I always knew about the twist making the bullets trajectory head in a lateral direction, but I never seen any numbers on the subject or a detailed explanation; so I thought it was a negligible amount. It seems as if I was in error!
[EDIT] As I am uploading the video, I am searching for info on bullet drift and twist rate. If I find some info I'll post a link to it.
Found one! (Example: A .308 Win shooting a 175 Sierra MK in a 1:12 barrel will experience about 8" spin drift in 1000 yards; the same bullet will drift 12" (a 50% increase) in a 1:10 barrel.)
It doesn't say which direction but I think I must be correct.
I am correct, Bullets fired from right twist barrels drift to the right,
but for the wrong reason. It makes better sense now. Much more drop, much more drift.
I love missing targets! I learn so much!
All my 600 yard shots were all right about the same amount, (around 6 to 8 inches from center.) I was kicking myself in the behind for not calculating the wind drift correctly. Now I suspect it may have been a little wind and a lot due to twist!
I would have never came up with this epiphany if it weren't for me deciding to be more precise in my shooting at these shorter distances to get better at longer distances. 4 inches difference at 400 yards, most people wouldn't have thought anything of it.
I'm going to hit that paper plate at 600 yards next time, you just watch!
Gyroscopic (spin) Drift, who could have guessed?