Mini's are fine with faster burning rifle powders like 4198, Reloder 10x, Ramshot X-terminator/Accurate 2230 and 3031. I'm just naming a few off my head and not saying these are better than others in that burn range. My main load is 19 grains of X-terminator over a 55 grain bullet and it cycles 100%. 4064 is a good powder for 223 and although people say it is slow it is in the same range as other powders like BLC-2, 748 and 2520. At max it will be a compressed load but there is nothing inherently dangerous about that.
The only handgun powder I've tried to get my Mini to run on is 2400, which is didn't. The accuracy was fine but the spent shell wasn't even ejecting at max load. I worked it over what the 2004 Alliant manual said and there were no pressure signs but it never functioned consistently so I gave up. Makes a good low power practice round if you don't mind turning your gun into a pseudo bolt action rifle.
New gun, same ol' shot.