No trouble to straightening out .303 empties, friend. You can do it all at once with Unique or Red Dot and cornmeal or you can do it the easy way, in stages.
DO remember to put an O-ring (or a cheap pony-tail tie, $1 for 500 at your local Dollar Store) around the base of the casing on its first firing: minimizes STRETCH, works wonders with full rifle loads, too, makes your .303 brass last just as long as .30-06.
I annealed (torch to the front half of the casing, dump into cold water) a small lot of brass, dried them in the oven at low temp, primed them up and expanded the case-mouths first with an 8mm expander, then a 9mm expander, then finally flared the mouths with a taper punch until they just would fit into the chamber. Followed that up with 8 grains of Unique and one of those little soft-plastic .410 wads with a single 000 Buck ball. Looked awful but they blew out very nicely with no loss of brass. Some of them did not blow out all the way but a second firing took care of that. I was using
Canadian-made WW2 brass by Defence Industries. Likely you would find it easier to do with modern American brass.
For a serious COMBAT loading (yeah, right! With a MUSKET?) I find that 8 grains of Unique with TWO of the 000 Buck tends to plant the balls reasonably close together at about 85 yards with my Musket. Think a double-tap with a light load in a 9mm: I have no desire to stand in front of it. It breaks clays nicely.
You can also do a SHOT load for the Musket, in which case you can take small birds, Prairie Chicken and you can really drive the other guys at the Trap Club NUTS when you try a round with them. Try half an ounce of Number 5 for Chicken (Grouse), 7-1/2 for Trap. You can seal the front of the case with a cardboard wad and a bit of hard lawyer's wax. Isinglass would work better, but I don't know if you can still get it in a world terrified of "chemicals". I know we can't get it here any longer.
We can't all be fortunate enough to have our own Prison, but we CAN all have fun with a Musket. Personally, I'm looking forward to trying mine out with a Lee .395" round ball. Janice (twosteam) has been getting some decent accuracy with her 1850 smoothbore Musket (London commercial percussion based on a Bess barrel) and I can't let her outshoot me forever!
MUSKETS are pure FUN.
And the Lee-Enfield Musket was the LAST true Musket to be adopted for military Service. It is the END of a 600-year tradition.
Enjoy!