The original .303 British cartridges were loaded with compressed black powder. They tried the BP loads with new-fangled jacketed bullets. I have a Brit Martini-Enfield dated 1893 on the left side of the action. It might have been one of those experimentals. Didn't work well, because the right side of the action was marked 1897 and it had a newer .303 barrel, burned out from Cordite and a lack of cleaning.

Apparently BP was also tried in Switzerland with their original 7.5 mm rifles and paper-patched bullets, either jacketed or very hard lead alloy. Those trials seem to have established smokeless powders as the proper path to follow. 

My understanding of both those early trials is that compressed slugs of BP were loaded into the cartridge cases before the cases were necked down to hold the jacketed bullets in place. You are starting out with a BN case.  You can probably get some sort of compression with a jacketed flat-based bullet, but not as much as the Brits & Swiss did 1888-1890.

I don't predict success, but I don't think you will hurt anything and you will have something to tell us about.  To me, nice rollers in 7x57 are worth some TLC. You might find one in lesser condition for your experiment.

Bottle-necked cases and BP have caused accuracy problems since the beginning of reloadable cases. The earliest Creedmoor cartridges were BN. After the first year or so, straight cartridges replaced them. The Spaniards gave up with their early 11 mm cases and replaced them with the Reformado.

The original load intended for those 7x57 rollers was a jacketed RN bullet about 170 grains. You might try a bullet similar to those, but a flat base for sure, using the techniques Val posted.

And tell us the results, please.

Black powder doesn’t like bottle neck cases, the smaller the bullet diameter, the less it likes them. It will hammer the shoulder of the case which will lessen the number of reloads before it separates at the shoulder, if you are lucky it will shoot the piece out of the barrel, or you’ll end up having to put a brush down to clear it
Gumpy

MartiniBelgian wrote on Oct 1^st, 2023 at 1:23pm:

It’s not fairyland, I know you just don’t like being corrected. Bottle necked cases do get hammered in the shoulder, they do separate there,  I've got several siting on my shelf that have done it. Some mine some other people’s, if you look at the design of bp bottle neck cases the shoulder is long and sloped for a reason, the 577/450 or the 450/400 2 3/8 are both good examples. Without actually trying it,  I would estimate five or six reloads in a 7x57 before he started to loose cases
Gumpy

I suggest you take good look at a fired 577-450 case, that is NOT a gently sloped shoulder.  The shoulder angle is at least as steep if not steeper than many smokeless cases.

MartiniBelgian wrote on Oct 3^rd, 2023 at 9:34am:


Really...mine are....and the ones I used to reload for a friend years ago, still have a couple here waiting to be collected 
Gumpy

I can tell you all that I have shot  compressed loads of BP in my 577/450 Martini and in both the 303 Brit and the 30 USA (30-40K).  No real problems arose. I had started with loads that filled the cases to the base of neck thru a 24" drop tube. 
Then ran a set where I filled the cases to the case mouth and then compressed down to the base of the neck.  
Again, no problems
Best to do this with Swiss powder which likes no or a little compression.
beltfed/arnie