Home Forums Hunting & Shooting Ask The Gunwriters Spin Drift and a Zero?

I just conjured up a load for my Ruger 77ST, 30-06, old tang safety model, mounting a fairly new Weaver Classic K6X38 scope that I don't think I'm going to tinker with anymore (yeah right

It's no screamer but it's really accurate. 48.5 grains of IMR4064 behind a Sierra 165 HPBT Game King in a Federal case, CCI 200 primer, OAL 3.200. I'll get around to measuring it's speed soon.

Several 3 shot groups into nearly one hole or maybe I better call it 1/4" capable if I really do my part. This is unusually very good for me and this rifle.

My question is; the zero for this load is about 1/8" inch from the center of the aiming point at 100 yards. Any idea where this bullet will impact center because of spin drift, if it ever will? ......Thanks, JJ

Shoot multiple 10 shot groups. That will give you a better idea of where the center of your group is. 3 shot groups really tell you nothing. I was at the range yesterday testing some loads with my new to me rifle and some of the groups showed double grouping where the first 3 shots were in one ragged hole, then the next 2 were outside the group by 1/2"+. Getting all fancy and talking about spin drift and bragging about 1/4" groups, "if you do your part" really is a moot point, if you are not shooting more per string. Here's an interesting read for you:

The trouble with 3 shot groups

The effect of spin drift is fairly small until you get a long way out there - less than 1/2 moa at 500 yards in all likelihood, maybe 1 moa at 1000 - easily lost in the effects of other things like atmospherics. There are formulas and calculators. Personally I've never worried about it for a hunting rifle, and even in 1000 yard competition it is a simple matter of recording your sight settings in a notebook (and of course adjusting for the conditions).

Your zero is 1/8” from POA in which direction? What ROT is the barrel? Assuming RH twist barrels, I often favour the left of POA when I zero at 100 meters just to offset spin drift farther downrange. Fast-twist barrels induce more spin drift than barrels with a slower twist rate. In your case, assuming a 11” twist rate, 2975 fps at the muzzle, SAC, and a 100 m zero, spin drift causes 2” of deflection at 500 m and 15” at 1000 m. In angular units this is about 0.1 MRAD/0.3 MOA and 0.4 MRAD/1.3 MOA, respectively. If instead of zeroing right on at 100 m you zero 0.3” left of POA, spin drift will be reduced to 0.5” at 500 m and 12” at 1000 m, or 0 MRAD/0.1 MOA and 0.3 MRAD/1 MOA, respectively.

J O'Connor said, in his opinion, the accuracy of a hunting rifle should be judged by a 3 shot group....

What say you, Mule Deer?

Very good explanations in the above responses.
You mentioned "several" groups produced tight groups. Consistent POI? If so, I would go with it, and test shoot at 3 and 500 yrds. Atmospherics will always be the biggest challenge, IME.
PS, that`s why sights are adjustable.

WAM,

I have also published articles and book chapters about the problem with making accuracy judgments on 3-shot groups. Among other things, a professional statistician I'm acquainted with calculated that the average point-of-impact from several 3-shot groups will vary around .7 inch at 100 yards, which doesn't help much when calculating downrange spin-drift.

Will also note that Elmer Keith, who did quite a bit of long-range target shooting, and what was considered long-range big game hunting back in his day, preferred to sight-in his long-range hunting rifles 3 inches high at 100 yards--and half an inch to the left, to compensate for spin-drift with a typical RH twist barrel.

I've never experienced spin drift in my 338 Laura with the 300 grain SMK. I've spoken to Brian Lietz about this and he said there are explanation why but spin drift is still there

When I started reading this thread, Elmer Keith’s sight-in technique was the first thing that popped into my head. I can’t tell you how many times I read that in his articles 😊

I think this response was directed at the wrong person...

You are correct, wrong person. The rifle has a 1-10" RH Twist and POI IS 1/8" left...O'Conner was talking about one of his custom Model 70, 270s that he used in the Yukon, British Columbia, Iran, Botswana, Idaho and Scotland, and elsewhere . After claiming how it held zero, his load of 62g H4831, Nosler bullet, Western cases and ave velocity of 3,140 in a 22 inch barrel...I quote; "if I'm having a good day, let the barrel cool a bit between shots, and take pains to hold the rifle the same for each shot, it will keep five shots in an inch or slightly over. Contrary to much of what one reads, this is exceptional. With this rifle I have shot many THREE- SHOT groups (which I think should be the standard by which a big-game rifle is judged) of 3/4 inch or a bit less".

Have you ever kept shooting at a particular target until the group hole in the paper stopped growing? If not then try it sometime.

Yes, did that two days ago, not on purpose, but the last three or so of each group did not make the hole bigger.... also, apart from the question, has little to do with this thread.. thirteen shots on the left, 10 on the right.

The
Agreed. I have read as much of your fine works as possible but don’t recall anything on spindrift. As I am familiar with artillery and naval gunfire characteristics, I understand spindrift. I just never gave it much thought to high velocity rifle trajectory at modest ranges since I don’t like to shoot much past 300 yards at game. But since I now have a 6.5-300 Weatherby, perhaps I should visit the spindrift effect for out to 500 yards or less. Any further insight or suggestions would be most welcome! Steve

If you can call wind within 1 1/2 inch, every shot, at 500, you have it figured out.

What does wind correction have to do with spindrift and zero?

While you're fretting spin drift, don't forget about Coriolis, and which direction you're shooting......

^^^^[ Oh, Yeah!!!! LOL!

WAM,

I have mentioned spin-drift here and there in my writings, but for most hunters, spin-drift is pretty much adjusted for for when checking clicks and adjusting zero etc. at longer ranges.

If you really want to read up on it, Bryan Litz's writings are a good resource: www.appliedballisticsllc.com.