Hearing that this 8 bore Pinfire rifle had a 1 in 12" rifling twist really got my attention. I would have expected to see something more like a 1 in 36" twist rate found in many 12 gauge slug barrels.
I became interested in rifling twist rates at a very young age. The first deer rifle I used was my Dad's .244 Remington, which fell out of favor compared to the .243 Winchester, due to its' 1 in 12" twist that supposedly wouldn't stabilize 100 gr. bullets. My Dad found that it would stabilize 100 gr. Nosler Partitions to around 1 MOA, because the Noslers were more blunt than most 100 gr. .243 spitzers. That was plenty good for deer, but the same rifle would shoot tiny one-hole clusters with 75 gr. Sierra hollow points.
But then, I'd also heard about the problems of over-stabilizing some bullets such as .17 caliber bullets with thin jackets at very high velocity. Upon doing a little research, it seems that a 1 in 12" twist in an 8 bore rifle will most likely work just fine. There are several handy online rifling twist rate calculators such as this one where you can crunch some numbers:
https://bergerbullets.com/twist-rate-calculator/I played around with this calculator using some wild guesses as to bullet weight, bullet length, and ballistic coefficient of some imaginary lead round nosed .840" diameter bullet at roughly 1000 feet per second. I used very low ballistic coefficient numbers including some low B.C.'s similar to what you'd find with a lead round ball, and ended up with several different stabilization factors ranging from 40 to 55, which the calculator says will be stable. That is quite a bit higher than what are considered more optimum mimimum numbers like 1.5 to 1.7. But from what I read, over-stabilization isn't near as much a problem as under-stabilization. A very extreme high bullet rpm will cause more yaw or drift at extreme ranges, but that sure won't be a problem in an 8 bore. It could create more instability if there are casting voids that produce an unbalanced bullet, but that can happen even with a normal twist rate.
One other thing I wonder about concerns what I've read in the past about naked lead round balls or bullets "stripping" the rifling and leading the bore at higher lead bullet velocities. If that is actually true, it might create the need to use a slightly harder bullet alloy, a different bullet lube, or even paper patching. But it isn't likely that a very old 8 bore Pinfire is going to be pushed to any magnum velocities. BTW, this is a cool gun in amazing condition. It will be interesting to see results of actual shooting.
