CAUTION: The following post (or a page linked to) includes or discusses loading data not covered by currently published sources of tested data for this cartridge (QuickLOAD or Gordon's Reloading Tool data is not professionally tested). USE AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assumes any liability for any damage or injury resulting from the use of this information.
A chronograph will tell you the velocity, but that won't tell you the peak pressure value—only the average pressure taken over the whole length of the tube can be estimated from velocity, and average pressure isn't what poses a risk.
Don Miller's black powder estimator says 40 grains of Elephant FFG would give 1225 fps, so your 1200 fps number would seem to be within reason.
Reloader 10X and other rifle powders near its burn rate are way too slow for this cartridge. It's expansion rate is too fast when it fires, and the powders burn too slowly to make gas fast enough to do an efficient job of keeping up with how quickly the burning space is growing. When I try that velocity and powder combination with a 180-grain round-flat nose bullet in QuickLOAD, the ballistic efficiency is an abysmal 14.7%. Almost 54.2% of the powder is blown out of the muzzle unburned, and the peak pressure is less than 8,000 psi. Under those conditions, you can expect erratic performance, velocities, and lots of unburned powder lining the bore. This is, after all, a pistol cartridge.
The CIP thinks this is a 16,000 psi cartridge. If you don't want to shoot BP, try starting with 6 grains of 231/HP-38 under a 180-grain round-flat nose lead bullet and adjust it up to your targeted velocity. When it gets there, pressure should still be under 12,000 psi, according to both QuickLOAD and GRT. They can estimate pressure on the low side, but you still have another 4000 psi of headroom, and they won't be that far off.
