Rifling Twist for Reduced Velocity Loads

Yes, the stability factor does change, but not radically. A 1.25" 30 caliber bullet is stabilized with a 12" twist at 2700 fps, and the same bullet down to 1600 fps is stabilized with a 10" twist, which most 30 cals have anyway. For slower speeds, you might need a 1 in 8" twist.

Going to need more information but in general the slower you shoot a bullet the faster the twist rate will need to be to stabilize the bullet. A bullet needs a certain angular velocity to be stable so the slower you shoot it the slower it spins.

There a several good stability calculators out there on the internet. I like the Berger one: https://bergerbullets.com/twist-rate-calculator/

Give it a try, plug in all the relevant data and it should predict if your particular bullet, twist rate and expected muzzle velocity will result in a stable bullet or not.

Actually the twist needs to be decreased as the velocity increases

There is a moving window of what works well at a certain twist rate. As you slow go lighter to stay in the window. Heavier and slower is not normally a good thing. That's why the 300 bo runs a very fast twist for 30 cal.

Dale Alan said:

Rifling Twist for Reduced Velocity Loads...Key word, reduced. Please don't derail the thread right from the start.

Click to expand...

Why are you shouting????

My experience with lower powered cast loads in rifles suggests that you should just try and see what your rifle will do. My 30-06 that likes 165 grain bullets in full power jacketed does just fine with the right charges at 118, 150 and 198 grains. All of these loads are sub 2000FPS. Fiddle around and see.

brewer12345 is absolutely correct. A cast/coated lead bullet is a different animal then jacketed bullets.

I've shot 90gr to 220gr lead bullets in 308w's & 30-06's using 1 in 10 twist, 1 in 11 twist, 1 in 12 twist, 1 in 14 twist. Pushing bullets 900fps to 2700fps+.

The only time I ever ran into the rate of twist being an issue is when alloy or bullet design couldn't withstand the rotational torques/stresses applied to it (high pressure loads/high velocity). There is not much that can be done about a poorly designed bullet. But a slower twist will cut the rpm's of a high velocity bullet down allowing for a couple more hundred feet per second from the same bullet.

Something to think about:
A lot of 9mm pistols use a 1 in 10 twist bbl. Reloaders are using 90gr to 150gr bullets in them. Those pistol bullets are a lot shorter then your rifle bullets. They are running those bullets in the 800fps to 1400fps+ range with accuracy.

Your plan of a 150 cast in 3030 is common and I'm using that bullet for low power 308 for my son. The starting unique load is very easy shooting.

Also shorter bullets will stabilize at a slower twist than longer pointer bullets of the same weight and caliber and velocity. The difference is not huge but might be enough to help if you close. A 150 gr round nose flat based 30/30 bullet will stabilize at a slower RPM than a spitzer boat tail 308 bullet of the same weight and caliber.

I believe this is your jam.

Cast is forgiving if you don't go too fast. I am fooling with 180 grain cast in 350 legend looking for a modestly powered load for short range critter whacking without a lot of noise or blast. The barrel that shot 147 grain FMJ at 2300FPS shot the 180 just as accurately with a milquetoast load of 7 grains of Unique (definitely under 1500FPS, guessing 1200 FPS or less with no chrony). I haven't tried, but I have heard of 200 grain cast being pushed near 2000 FPS in this cartridge with success. Try your bullet in your 30-30 and be prepared to fiddle with the variables a bit, but under 2000FPS it should be fairly easy to get accuracy.