Spin drift in BPCR projectiles

[guninhand]

I chanced across a discussion of ballistic spin drift on the internet, and going by the formula given, the gyro stability factor (SG) for a typical BPCR boolit would be around 3.06. That is what I got for a BACO boolit of .458 inch diameter, 510 grains, 1.32 inches long, 18 twist barrel, 1240 fps muzzle speed, 85 deg F. and 29.92 atmospheric pressure.

Now put that SG into the formula to calculate spin drift and and you get over 52 inches of drift (left or right, depending on direction of barrel twist) at 900 meters.
So even in dead calm wind, my windage zero is off that much at 900 meters? Is this some fresh hell :???:or have I misunderstood the calculations?

I used this site http://www.ehow.com/how_7660060_calculate-spin-drift.html

and this http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

[Bullshop]

Have you ever seen a Buffington sight on a Trapdoor 45/70? The long range slider does not go straight up but has a built in slant to the left.
They knew all about it back then and built the compensation into the sight.

[guninhand]

This is a phenomenon that slipped by me till now.

Seems like a good reason to get a windage adjustable front sight; just crank off the spin drift value and use the tang windage as if spin drift never existed.

[Doc Highwall]

Bullshop, thanks for reminding me about the Buffington sight I forgot about that.

[montana_charlie]

So even in dead calm wind, my windage zero is off that much at 900 meters? Is this some fresh hell :???:or have I misunderstood the calculations?
Still, that only totals out to (roughly) five minutes of angle ... one 'line' on the windage scale of your Soule sight.
If there's any wind blowing at all, the 'drift' will get lost in the 'deflection'.

CM

[guninhand]

True, CM. But the aiming point bull alone is 48 inches diameter at 900m. It's certainly something to know for initial getting on paper.

[Don McDowell]

Get a mechanical at 800 in a zero wind condition and then let the rest of it take care of itself.

[Good Cheer]

My grandfather told us about the effect in long range shooting taught him by German officers before he was sent to France. With high RPM smallbore miltary ball it was a curve ball effect and included spin generated lift and well as drift.

[guninhand]

Get a mechanical at 800 in a zero wind condition and then let the rest of it take care of itself.

Good idea. Tks. Empirical beats theoretical any day.

[Don McDowell]

Biggest problem is going to be an absolute zero wind over the 800 yds, but you can get it close, and then things fall fairly well in line from there on out.

[EDG]

"Empirical beats theoretical any day."

In "THEORY", practice and theory are the same.

In "Practice" they are not. ;)

[Gunlaker]

I suspect that your numbers are pretty close. Zack over on the Shiloh site advised me that you'll see about 4 minutes of drift at 1000 yards. He's a man to pay attention to. If you've read the last issue of the Black Powder Cartridge News and seen the picture of the impacts on the spotting disc you'll know what I mean :-).

Chris.

[Don McDowell]

If you've read the last issue of the Black Powder Cartridge News and seen the picture of the impacts on the spotting disc you'll know what I mean :-).

Chris.

Didn't have to wait to see it in the magazine. Saw it happen live and in color.:-D

[Gunlaker]

It must have been something to see Don. That was some incredible shooting.

[Don McDowell]

There was another pin got shot out of the spotting disc, I believe on the 1000 yd line, and may have been by Jim Terry. I was in the pits pulling the target next to it. Shrapnel from that pin was flying everywhere.:-D
There were some of the country's top shooters at that match, and it's a great learning experience to there shooting with them.