RPM?,Weight? Velocity?

[NSP64]

Is there an optimum weight bullet for a given bore size and velocity?I mean can you shoot a heavier bullet at a slower speed,or a lighter one faster?I read somewhere there was a formula to figure out what bullet size(weight) was best for your bore.

[405]

Seriously, after reading the previous thread on " rpm threshold" my brain is numb! I've been shooting/reloading seriously for the past 40 years. I have a fairly strong backgound in physics. To apply your question to a formula would be nice but I haven't found one :) Experimentation within best guess guidelines is what most do. Of course there is a formula that will estimate minimum twist rate to stabilize a bullet of a given length. But, I've found that formula to also be just a "best guess". Aside from basic bore twist rate, velocity and bullet weight.... the acceleration of the bullet has a lot to do with how the bullet behaves. The acceleration rate affects bullet deformation and obturation and it affects how cleanly the bullet engages the rifling without attempting to smear or strip as it comes up to full rotational speed. The alloy/jacket material will also affect this. Larger diameter bullets of a given mass (weight) resist this acceleration up to "rotational speed" to a greater degree than smaller diameter bullets so are more prone to this rifling smear or strip effect. Bore surface friction per unit of mass is greater for smaller diameter bullets than for large. Faster burning less progressive powders try to accelerate bullets faster than slower more progressive powders. And so on and on. Wish there were a formula..... but then again it would take the challenge and experimentation and maybe most of the fun of discovery out of the shooting game.

[Ghugly]

I think some of the guys here were using carved, stone bullets while they waited for the discovery of lead. Given that, I think the best formula is a polite request for recommendations. If I had to choose between that and a carefully worked out formula by some renowned gun guru, my choice would be easy.

[Bass Ackward]

Oh very nicely written 405.

For therapy, go shoot a few rounds. :grin:

[Morgan Astorbilt]

Greenhill Formula
You're referring to the Greenhill Formula, which was developed after extensive trial and error. I've entered it below. It actually calculates the optimum bullet length for a given twist, not the weight. We schuetzen shooters swear by it. (Or maybe at it?):-?:-?

BL = 150 ÷ (T ÷ D²)
T = [150 ÷ (BL ÷ D)] x D

T = Rate of Twist BL = Bullet Length D = Diameter (Bullet) in Thousandths

[405]

Yep, that'd be the Greenhill formula. But as to the original post??? I suppose if I read it correctly it has a couple of questions. One may have to do with twist rate stabilization and the other has to do with practical application of bullet size/weight for a certain bore size? I'm not smart enough to figure out that part without the cumulative experience of the whole shooting community plus the work done by the ballisticians in the various labs. For example: if starting from scratch with say a 40 cal rifle designed for low pressure loads- the first place I'll look is in one of the several load manuals available. I'll see a range of bullet weights and powder types that were pressure and/or velocity tested. I'll usually start somewhere in the middle of the range that the lab tested for bullet weight and powder type-- depending upon my supplies on hand. But, generally, there is a range of bullet weights that seem to work best within any caliber. Also, there are types of powders that seem to work best with those bullet weights. Yes, generally the lighter bullets can be pushed out the muzzle faster than the heavier ones. Somewhere in all those variables there will be an acceptable combination of bullet weight, velocity, accuracy and pressure for the shooter and gun. Sometimes we hit it on the 1st or 2nd try - sometimes we never hit it.... but keep trying. I enjoy both the load bench time and the range time so that's not a bad thing either. If it were always easy I'd have given up on loading and shooting a long time ago.:mrgreen: