As someone new to handloading, I had already determined the right powder for loading my .348 with Jb's. Now my research is indicating a need to reconsider this choice because of the different requirements of the cast bullet . From what I am reading, a faster Powder burn rate may be required to speed pressure build-up and heat. This would result in softening the cast bullet base faster and sealing the bullet-bore gap before leading could occur.

My initial powder choice was Alliant ReLoder-19, a relatively slow powder that has proven superior in the .348 with Jb's.

Have I got my facts straight, and should I step up in powder burn-rate even when using gas checks?

BrushBuster
With cast rifle bullets I've always worked with fast to medium rifle powders. In the 243 with 100 grain cast bullets I used Rl-7. With the 45-70 I used IMR 4064 for heavy bullets and for the light weights I used Rl-7, again. With the IMR-4064 I got moderately high velocities I also had full to compacted case load of powder.With the lighter weight bullets I had maybe 80% loading density.
For my 375 Whelen I used one of the Accurate arms powder ?????AA2560????? I don't have the load notebook here. I used the data from Accurate Arms for the 35 Whelen, full charge loads.
I'd bet you could use one of the slower powders, by the case full and still get good accracy. Sort of like what the guys back in the 50s and 60s with all the surplus 4831 left over from WWII.
In my experience the diameter of the bullet will have more effect on sealing the bore then the powder used. With that said, Yes, the bullet will obdurate in the bore from the pressures. I believe that a proper fitting bullet will assure this.
Jim

Brush Buster--

Great handle, BTW......like a lot of other poured boolit cranks, I use all the wrong powders for the cast calibers I shoot. :-) My pet 45-70 load uses 3.0 grains of IMR-4198 to help light off 48.0 grains of WC-860 underneath a Lee 405 grain FN. WC-860 is a VERY slow ball powder developed for the 50 BMG. On the other hand, in the 9.3 x 62 I use 18.0 grains of Alliant 2400 with a 270 grain FN boolit. Both calibers do fine work, with better accuracy than my aging eyes can exploit without a scope sight.

I think the powder choice in cast boolit loading has a lot to do with caliber and individual firearm idiosyncracies--far more so than with the j-word projectiles. With cast boolits, you become a lot more than a component assembler--cast boolits are a "third dimension", lending considerable depth to the otherwise two-dimensional linear/vertical cartridge reloading equations.

Perhaps I missed it--but if you have caliber-specific questions, someone here has probably "cut trail" on your question previously. Speak right up.

well, I just used up the last of my Re7 and I wish I had a 25 pound keg of it! I just love this stuff with cast bullets in my 45-70 and I bet it would be good 348 fuel as well.

Jumptrap:
You didn't say if you had previously tried R-19 before finding eternal bliss with R-7? I would love to know the differences, because I have a big bucket of R-19 on the way.

I think your right, the two calibres probably have a lot in common.

My 356 Win. is similar in many ways and likes AA 2015 and RL7 as well as 20 gr of 2400. If I understand the first post correctly about softening the bullet base I have read that you can smear a thin grease on a bullets base and the duration g time in the barrels heat is not long enough to melt it. I think then that it would not soften lead appreceably either. Gianni.