that's actually pretty handy

I agree that the chart is a bit off.  Before I rebarreled my rolling block, it had a twist of 1 in 22  for 45-70.  It did not stabilize 500 grain bullets--it was best with 400 grainers.  I had a new barrel made with a twist of 1 in 20 for 45-100, and it does great with 500 grains, but I don't think it'd stabilize a 600 like the chart suggests.

Nonetheless, thank you for the link.

              3.5 X square root of MV X caliber squared
Twist =                        divided by
                                 Bullet Length
 
My bad, sorry I left it out!  This should be better

Old-Win wrote on Feb 5^th, 2011 at 4:02pm:


In case anybody else tried this unsuccessfully; X 3.5 is left out of the equation.  

Here is a twist program by Border Barrels (Scotland) that graphically shows what the subsonic range does to bullet stability. It uses  a stability factor of 1.5, the Don miller program uses 1.4 and the JMB program, I believe uses 1.3. With my guns and bullets (before the computer) I found that I could use a factor of 175 in the Greenhill Formula in the 1400 fps range.

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Frank

Bob,
Yes, I believe what you say about LR, I only ever tried it one time with cast bullets. I shot with the hi power guys, starting at 600. with my 32/40. I couldn't even get on the target. 

I built two 45/70's for silhouette but, my shoulder gave out shooting them BR before I could get into a match. That eventually turned into a torn long head tendon in my shoulder and I just can't take heavy recoil any more.


So, all my work has been done at 1 & 200 yards and that 175 figure is near the edge of stability but, works at 200 with a spitizer bullet (1.165 long, 208 gr x 15 twist). Slightly tipped bullets usually group well and I believe that at my velocity's that they are just entering the trans sonic range and that is what causes the tipping when it happens.

I've always subscribed to the theory that, it should be on the edge of tipping just after it goes through the target but, as we all know, it's hard to accomplish and we settle for some sort of a compromise to meet varying conditions and equipment.

But, experimenting is the only way that you'll know for sure what works and what doesn't. twist can very 1/2 inch from one barrel to the next from the same manufacture (that's all that Douglas will guarantee) and just that much can be the reason one gun shoots and another won't. 

Thats how I came on to the 175 number. Mine would shoot the 1.165 bullet and a friends gun wouln't (read key hole). Mine was 15 and his 16 twist. I shorten his mold by .050 and it shot as good as mine (both Douglas barrels both Ron Long molds).

Frank