For the 405's I like to put just a slight crimp on them , nothing hard , just a bump , it seems to help with my groups ann also lets the round slid into the chamber without hanging up on the case mouth .
As for the gas check you'll still have to bell the case mouth some , unless you have the GC's I'd not worry about them , I run my cast from ww's up to around 1800 fps with just a bare base and have no leading problems .
stimpy