The stuff from the range will not cast well for a m/l. I use it from my club for .357 bullets and its very brittle when cast, shatters like glass when it hits the backstop.
Look for some nice lead mate. Lead flashing, old pipes theres plenty out there. I weigh my balls when cast as theres a lot of variation, blow holes, cold shunts and the like I get around 8 grains variation over a batch.
But casting your own adds another dimension to your shooting, its fun and its free.
Air gun pellets would be good though, they are a nice soft lead, and dont worry about the other crud that goes in the pot it all floats to the top so you can scoop it off.
“If a cricketer, for instance, suddenly decided to go into a school and batter a lot of people to death with a cricket bat, which he could do very easily, I mean, are you going to ban cricket bats?” :- Prince Philip said after Dunblane