If you groud up powder, it would break down the outside graphite coating and burn faster. I imagine if you ground it up fine enough, at some point it would detonate and destroy the rifle. It would have to do with the surface area exposed.
Not a fun experiment to do unless you had a precision grinding mill and an instrumented test barrel.
If you just load with a slow-burning powder, the first thing you'll notice is a bigger muzzle flash as unburnt powder exits the barrel. That doesn't hurt anything, but it's a loss of cartridge efficiency.
The XM 177's they issued us in SEA (10 inch .223 barrel) had that problem, it looked like you were firing a bazooka. They tried "fixing" it by adding a tube-type flash suppressor, but it still shot a flame about 4 feet.