There must be a reason that shot shells are only commercially availiable for .22, but I just wanted to ask anyway...
Can you load shot pellets into any old rifle case, say a .375 for example? How would you seal/crimp it, would a cork or paper cap work? maybe something from a muzzleloader?

...sorry, this was meant to go onto the reloading forum.

Express, you COULD do it. One way to seal it would be to load the case with powder. Stuff it partially full with kleenex, add the shot and then seat a proper caliber gascheck upside down in the neck of the cartridge. This would probably produce a round that would not feed thru the magazine and you would have to single load it.

My question is why in the world would you do such a thing? Performance would be horrible....and so would barrel leading! I think you are forgetting about your rifle's twist. Shotguns, of course are smoothbore. The rifle's twist is going to spin those pellets up like a turbine and when they exit the barrel they will be going OUTWARD about as fast as they go forward. Your "shot pattern" will amount to about 3 pellets within 3 ft circle when shot from 3 yards. Seriously.

There is nothing down this road for you but disappointment and a very fouled rifle barrel. I know because I've fiddled with similar crazy projects.

Not sure where you are going with this idea or why you think it necessary to even contemplate...but I bet we can think of a much better road for you to take.

Tell me WHY you want to do such a thing for starters.