Since this is "Pretend" time . . . .
!st lesson - black powder explodes - smokeless powder burns.
Pistol cartridges utilize powders that are at or near the top of the burn rate chart (where the "faster powders" reside / / / /
You use the word "ignite" . . . . the pistol cartridge is self contained with the slug taper crimped into the case mouth.
So let's assume you load the cartridge in the barrel, base down with the primer facing down . . . .
Now I digress for a moment . . . as a former firefighter. we encountered ammunition "popping off" numerous times while fighting a structure fire - the ammunition is in an unconfined space, so when super heated, it will ignite the smokless powder to the point of igniting it, but because the cartridges are not confined in a barrel, when the pressure builds up enough that the slug will "pop" from the casing and danger is USUALLY minimal. Containers of smokeless powder in plastic containers will usually flare up and burn when it gets hot enough fo the container to melt. Black Powder IS A WHOLE DIFFERENT STORY. And some canned food stored in cupboards - when super heated, will explode as well. I saw that several times and actually got hit in my airlock by a kitchen cabinet door that was blown off.
So now roll the dice . . .
You fore the rifle and MAYBE the cartridge will clear the barrel before the explosion of the BP and the hot gasses have the chance to super heat it, causing it to cook off . . .
OE
The initial explosion COULD force the primer to compress it enough to strike the primer anvil, setting the cartridge off - you would then have the primer pressure plus the powder charge pressure - in the confines of the bore . . . now granted, the cartridge would be traveling down the bore - BUT - do we know if it will CLEAR the bored before the cartridge fires??? Maybe Yes . . . Maybe no . . . HOW LUCKY DO YOU FEEL TODAY?
IF the cartridge blows in the barrel, there very likely may be enough pressure ti ring the barrel, bulge the barrel or even fracture the barrel, my guess about half was or towards the muzzle . . . in which case you might earn the nickname "Stubby" for the rest of your life.
Now we're just "pretending" here . . . . DO NOT TRY THIS AT HOME.
Like Mother's always say . . . "Don;t put anything smaller than your elbow in your ear." And like my Dad always said . . . "You can't fix stupid." That's why my wife has told me for 51 years to not let my thoughts wander abd think about things that could get me in trouble!"