Factory = 1:9.25" twist, 22"
Krieger = 1:12" twist, 23"
Caliber _ 0.243 Inches
Bullet Weight _ 75 Grains
Bullet Length _ 0.942 Inches
Barrel Twist _ 12 Inches/turn
muzzle velocity _ 3620 fps
Temperature _ 68 degrees Fahrenheit (59 is standard)
Pressure _ 29.92 inches of mercury (29.92 is standard
Remember, I know just enough about this stuff to be dangerous. I quit reloading in the ‘80s because I would rather be hunting than messing around with the minutiae of cartridge loads, and factory ammo became just about as good as my reloads. As long as it goes BOOM, groups in an area I can cover with a fingertip, and explodes only after penetrating fur, hide, and the first rib, then it is “the bomb” and I’m good with it
. Having said that, I still like to keep up so:
I see what you are saying but something is wrong either here, or with my original calculation. Before selecting the barrel, I calculated that the 75 grain Hornady HP should stabilize from a twist as slow as 1:14.1. This was based on bullet length, velocity, bullet diameter AND Specific Gravity. I went with a 1:12 based on advice and the possibility that atmospheric conditions may be enough to destabilize this bullet from a 1:14" twist.
I found the stability factor from the 1:12" twist barrel should be 1.6 (if memory serves me and damned if I remember how I calculated that. Dan Lilja recommend no less than around 1.5). You are showing 1.15. One, or both of us, is getting something wrong. One error on your spreadsheet above is bullet length. The Hornady 75 grain HP is a measured 0.87”. Using this calculation with this bullet length, the stability factor would be 1.47. Also remember that Greenhill's formula was for flat based bullets and most formula's today have been modified to account for boat tail designs. I’m wondering if the underlying calculations of the spreadsheet above include a modification for boat tail designs? Remember this Hornady bullet is of the flat base design (as is the Remington 80 grain CoreLokt I also shoot in this gun) so Greenhill’s formula, only modified for higher velocity would be most appropriate. Since the 58 grain V-Max is a boat tail design it would be closer in length to the bullets I shoot but I don’t have any to measure.
Contrary to most who like the splatter factor of the new varmint bullets, I prefer the old hollow point designs in varmint bullets as I dislike “cratering” and prefer to preserve the pelts of the predators I hunt. Hence, I shoot only flat based bullets, which lend themselves well to slower twist barrels.
Finally, if this spreadsheet is correct, and this bullet has a stability factor of 1.15, then this makes the spreadsheet and/or the whole Stability Factor (SF) argument suspect, because, this gun shoots these bullets VERY well at only 200' above sea level and in all humidity and temperature ranges.
From the Shilen website:
- 8" Special for VLD bullets over 100 gr.
- 10" For bullets up to 120 gr. and VLD under 100 gr.
- 12" for bullets up to 85gr.
- 13"* for bullets up to 75gr.
- 14"* for bullets up to 70gr.
- 15"* Special for bullets up to 70gr.
Shilen confirms why this tube is handling the 80 grain CoreLokts well. Maybe we are getting too technical here?
When trying to find the very best barrel to replace my sorry factory tube, I was able to talk with Dan Lilja at length about barrel selection. He also suggested, based on my bullet and cartridge selection, to use a 1:12” twist to get that very small increase in accuracy this twist rate would gain. (The only reason I didn’t purchase a barrel from Dan was that he was so far behind and I needed it a little quicker. My next barrel will come from Dan).
Something to consider.