Here's a tough question. I have some ideas what's going on, but they're speculation based upon a very superficial understanding of internal ballistics.

Comparing two powders: BL-C2 and H-335. These powders rank side-by-side on burn rate charts-- #54 for BL-C2 and slower #55 for H-335 on the Varmint Hunters Magazine burn rate list.

Comparing ballistic data from Hodgdon Data Manual #26 (1994)-- looking at .222 Rem. Magnum:

In 40 gr. bullets, BL-C2 produces comparable velocity at LOWER pressure than H-335 on loads with the same 27.5 gr. charge weight in both powders. -- low end loads.

Then, same bullet, BL-C2 produces comparable velocities and significantly HIGHER pressures at the same 30.0 gr. charge weight as H-335. -- maximum loads.

Moving up the bullet weight to 55 gr.

BL-C2 at 24.5 gr. produces comparable velocity and LESS pressure than 24.0 gr. H-335. -- low end loads.

BL C2 at 26.5 gr. produces comparable velocity and LESS pressure than 26.0 gr. of H-335. -- maximum loads.

Seemingly, as the bullet weights get heavier, the working pressure in the BL-C2 becomes comparably less than H-335 for similar velocities. The BL-C2 takes about a half grain (0.5 gr.) to meet the velocity of the H-335 while producing less pressure.

I understand that nitro cellulose powder (smokeless) reacts under pressure to affect ignition and pressure development. I realize that heavier bullets create more internal ballistic pressure. But I'm stumped as to why BL-C2 should produce less pressures at heavier loads with a heavier bullet, less pressure at similar low end loads with lighter bullets, BUT more pressure with lighter bullets when the load/velocity is increased.

Feel free to get as technical as you wish. What I don't understand, I'll research!

Thanks!

Genghis,
The burn rate and pressure produced by any individual powder under varying loads is part of the reason why burn rate tables are not as useful as many think they are.
The rate of burn depends on load density and pressure/temperature ratios. The pressures generated are dependant on the rate of burn and the resistance to expansion. This rate is further complicated by any retardant placed on the powder and it's characteristics under pressure and temperature.
PaulS

OK, that opens that door. Thanks.

Now . . .

What sort of propellant do I want to look for to get a 40 gr. bullet going 4000 fps in a Rem. .223 Ack. Imp.??? A superficial understandign of "burn rate" suggests that I'd want a "bulkier" powder.

How would hihger velocity in a lighter bullet relate to load pressure as compared to a heavier bullet at a lower velocity?

Short of signin' on for a ballistics course at MIT, is there anyplace (online/library) I should look???

Take a look at the computer program called "QuikLoad"

At www.neconos.com they have a downloadable test version of Quickload.
This may already be enough to answer your questions.