dellet
Gold $$ Contributor
What you do is file a couple flats on each end of your barrel so you can put one end in the vice and a pipe wrench on the other and can have a variable twist barrel.
If you have a single purpose in mind, pick the best bullet for that purpose. Then the cartridge, then the chamber. The twist will generally sort itself out once those are known, but if you don’t know the bullet and velocity, you can’t nail down a twist. I think the 30 caliber probably has the widest spread of any bullet, 1/5 in the Blackout to 1-20 in the 30 carbine. There are hundreds of different bullets available for the caliber, many specialty for certain cartridges.
Thats the lead in for the fastest twist for a cast bullet answer.
Velocity and time of flight vs alloy and tensile strength.
A wad cutter design will fly farther faster than a VLD design. Lead melts at 625f, gets soft such earlier. So how long the bullet, specifically the tip is exposed to temps above say 500 will and the tensile strength will factor in to what RPM the tip of the bullet starts to bend.
There is a 130 grain cast bullet designed for the Blackout that was more or less a VLD design, nicknamed the crayola of death because thats what a loaded cartridge looked like and powder coated it showed up in various colors. It didn’t have much success, poor accuracy that was finally attributed to the nose slumping in flight. I don’t recall any baffle strikes. Any way, when you do the math. 2000 fps in a 1/7 it’s 200,000 rpm. Thats pretty tough on a cast bullet with a needle thin nose.
I cheated the accuracy contest and shot it in a 1/5 as a sub load. Groups at 100 yards were about 1.5”. The 1/7 and 1/8 twists above 1500 fps produced groups that were huge with a lot of holes out of round. It was initially thought that part of the problem was the nose being bent while feeding. When I posted my results as being cycled through an AR, it pretty well confirmed it was the tensile strength of the nose, the heat generated in flight and excessive rpm. The problem was not knowing when The deformation occurred. In the barrel, in flight and how far. The result were clouded by different alloys people were using, barrel temp and powders. Then with cast bullets the type of lube can make a difference in friction and therefore heat.
Contact that with 265 grain cast bullet at 1450 fps in a 1/5, spinning well over 200,000 RPM was fine. The difference was the 265 grain is a dual diameter round nose bullet. .308” for around .600” stepping down to .300” diameter with a round nose and about a 180” metplat. About as close as you can get to a 1.4” wad cutter.
For hunting more weight for a slow bullet is always better, really even for a fast bullet to a point. I never will understand the push for lighter and lighter bullets to shoot slower and slower, for larger and larger game. But that’s me. Matching the bullet to the job tho, there is a place for well designed light subs. Seems silly to use a 50 caliber 700 grain bullet to shoot a rabbit in your backyard garden, then again 29 grains from a 22CB lacks a certain amount of macho. So an 86 grain 30 caliber is a pretty good choice.
