The "events" mentioned above in this post is why things have to be carefully inspected and must be proved with an overload.
The best proof IMO is a standard load followed by a load of a double the service load and 2 patched balls.
I used piece of 2x8 with a block of 2" screwed to the end, this works best for hooked breeches with no tang. If this interferes with the tang it can be dispensed with or worked around as shown.
I use wood or lead blocks and "C" clamp the barrel to the board. NOTE. Swamped barrels should only be clamped at breech and muzzle to avoid BENDING the barrel.
CLAMP TIGHT. I usually clamp them vent up.
It may be easier to load off the board.
For flint guns a piece of green "cannon fuse" and some powder on the vent will do fine. I staple the fuse in place in this case is runs to small pile of powder with a second pile on the vent. If its windy I have to make adjustments for the priming trying to blow away.
This was a 1 1/8" straight in 58 caliber.
Dan