Rotational projectile drift

[jpretle]

Does it come into play, when shooting long range??? or is it rendered moot w/ sight adjustments?
In other words. in 'no wind conditions', do you put on elevation only, or are adjustments made for the rotational drift of the bullet???

 

[BallisticsGuy]

Keep in mind I'm a target shooter, not a long range hunter anymore so this is from that point of view. Shooting at meat IMHO requires a slightly different approach. So the short answer, generally don't bother but check your spin drift numbers at your planned distance maximum and decide where you're going to cap your range or start adjusting.

"Spin Drift" happens. There's a very un-handy equation that makes a really good fist of calculating it which I'm happy to share for those so inclined.

In general, I don't bother with correcting for spin drift at all until well after 700m when using drop/drift charts and then it's not really usually a factor of importance for me till after 1000m. If I'm using a gadget then I have spin and coriolis included in the result if that feature exists. There's usually no point correcting for spin until things get really long anyway. It's a very small effect. The slop in the average wind call has much more effect. There's also another induced drift I mention above referred to as "horizontal Coriolis" which is dependent basically on time of flight, velocity and latitude. It's not worth bothering with until >1000m for anything I do. There's a slightly less brutal equation to calculate Coriolis.

FWIW, there's also a vertical Coriolis component dependent on time of flight, velocity and heading. It's also pointless to discuss at anything short of 1000m and there's a mostly non-brutal equation to calculate it.

 

[jpretle]

Thanks, BallisticsGuy. I know in small bore target shooting, it is very notable at even the 50yd mark, and needs to be compensated for. I assume most of that, or at least some of it, is due to sub-sonic velocities.

 

[velvetant]

On my 6.5 Creedmoor I need to add 7" (.66 moa) left at 1000 yards no wind for spin drift.
Since I run a 1/4 moa scope it's in the middle of where I need to be, lucky for me there is no such thing as no wind where I live.....
I don't really think about spin drift since my ballistic app automatically corrects for it, but I do have it noted for the times I don't use my app and use a dope chart

 

[Lpart]

My G7 rangefinder does not include spin drift. In lieu of that, I use their on line ballistic calcualtor to get the drift and then I usually end of setting up my 100 yard zero to be one or two clicks left of center and then forget about it. Seems to work for me but I never shoot more than 1000 yards and do that rarely.

 

[BallisticsGuy]

My G7 rangefinder does not include spin drift. In lieu of that, I use their on line ballistic calcualtor to get the drift and then I usually end of setting up my 100 yard zero to be one or two clicks left of center and then forget about it. Seems to work for me but I never shoot more than 1000 yards and do that rarely.

^^^ That's a really smart way of dealing with it. You're at 2-3 clicks of spin at 1000 (according to my math) and substantially short of that it's hard for 2-3 clicks to be of too much consequence. For most shooters just the slop in wind call nets more horizontal error than that.

On my 6.5 Creedmoor I need to add 7" (.66 moa) left at 1000 yards no wind for spin drift.
Since I run a 1/4 moa scope it's in the middle of where I need to be, lucky for me there is no such thing as no wind where I live.....
I don't really think about spin drift since my ballistic app automatically corrects for it, but I do have it noted for the times I don't use my app and use a dope chart

FWIW, I get 6.8" (.82MOA) using my formula for a stereotypical 6.5CM load. This is not saying anything about your data other than, it looks like whatever system you're using and mine are outputting very comparable results, meaning the model is highly effective.

On my .223 running 75gn Hornady HPBT on a 9 twist I get 1 full MOA of spin at 1000yrds (about 11 inches). At 700yrds it's just 2 clicks for .48MOA which is only a bit over 3". At 500yrds it's 1 click meaning about 1.5 inches. On my new 7 twist barrel it's not much more, about 1 more click at 1000yrds and really nothing additional under that. This is because the real mover and shaker for spin drift is time of flight as the math of the matter raises ToF to the power of 1.83, nearly squaring the value. Because the adjustments are angular you also need the target to be really far away for the very small angles at play to mean much in the grand scheme of things.