Pistol powder is generally speaking much hotter/faster burning than rifle powder
Lets take two common powders
Winchester W231 for pistol....common 9m charge is around 4.5 grains
H4895 for 308....common 308 charge ~42 grains
You can definately stuff 42 grains of W231 into a 308 casing.
42grains of W231 in a 308 casing would cause an epic kaboom
I once witnessed the consequences of a similar reloading brain fart with an AR/5.56 cartridge....I was peppered with parts from several lanes away......thankfully the shooter didnt sustain much more than scratches......he was pretty clear headed after the fact, all things considered and revealed that he thought he may have accidentally loaded pistol powder into the batch he was shooting
The damage was epic, catastrophic bolt failure, cracked the barrel extension......the upper was toast.....
Had it happened to me, I would have gone right out and bought a lottery ticket.