burn rate & projectile weight

[redhawk41]

i have come up with this analogy to help clarify how powder burn rate and projectile weight interact.
picture a tee, baseball, and bat. the baseball is our low weight projectile, and the bat is our fast burning powder. the fast burning powder interacts well with the low weight projectile, as a quick smack is all it takes to send it flying. now replace the low weight projectile with a high weight projectile, in our example a bowling ball. now go ahead a give that sucker a smack. as we can see (by the broken bat) the fast burning powder does not interact well with the high weight projectile. in the case of the high weight projectile we need to accelerate it at a slower rate do to its greater momentum. the way i picture this is a shotputter. to shotput you have to get a good wind-up in order to launch the projectile with any effective velocity.

am i on the right track with this?

 

[rwilson452]

burnrate

Well, close anyway. it's really a matter of safe pressure. If you have a lighter projectile you can overcome the inertia easier. why the ball flies why the bat breaks with the bowling ball is it takes much more pressure to get it moving. if you swung the bat slower but could have a real long arc your bat wouldn't break on the bowling ball trying to get it moving. a push rather than a smack.


another reason for fast vs slow powders is barrel lenght. in a pistol you only have a few inches to get the job done where in a rifle you have a couple of feet to get it done so you lower the pressure but push longer thus get to a higher velocity.

 

[Tom Matiska]

Think in terms of volume instead of weight and you realize why a gun that could launch the bowling ball would probably take a faster powder.

"Slow" (relatively speaking) powders like H110 are good ways to accelerate a heavy 44 bullet down a long rifle barel. But the same powder is way too fast for the "faster" 243 Win.

The "faster" 243 starts with a larger case volume, and even though the lighter bullet accelerates faster, the small bore diameter requires a slower burn rate to maintain pressure as the bullet goes down the barrel.

That "slow" 243 powder in a smaller 44 Mag case wouldn't begin to maintain pressure as the "slower" 44 bullet traveled down the larger bore.

12 gauge loads may be even slower than the 44 mag, but the larger bore/smaller case has an appetite for even faster powders like Red Dot.

 

[redhawk41]

Tom. thank you, that makes perfect sense.
a larger bore diameter will require a larger volume of gas to generate the same pressure as a smaller diameter bore. a fast powder in a large bore will generate that volume of gas in a shorter time period to generate the desired pressure. a slow powder will generate the gas volume over a longer time period which will not generate as much pressure.
firearms fascinate me because they are simple, yet so complex.
and one more reason why load data cannot be "interpreted".