Question: would it be possible to cut the rims off some fired .22 magnum and slip the LR cases into those, as a spacer? Assuming the firing pin gets enough purchase.
Of course buying the correct ammunition is the best and easiest way, I'm just thinking along 'no other option' lines.
OK, don't take this the wrong way, but this is starting to get silly.
No, you can't do this. The dimensions just ain't gonna work, not to mention all the time and effort you'd put into it trying to make a lost cause work..."no other option" aside.
HOWEVER...let's back up a bit and discuss "birdshot" in .22 caliber in the first place.
Long, long ago, in a childhood long gone, my first rifle was a Marlin 783 bolt action .22 WMR. I could not even BEGIN to count the thousands of rounds I put through that gun.
Yes, I tried .22 WMR shot shells through it, as well. 'Cause, you know...who wouldn't, right?
They worked fine, functionally speaking. Pull the trigger, it goes "BANG!", chambered and ejected great.
HOWEVER...they were somewhat less than effective. At any real distance (meaning more than a very, VERY few feet from the muzzle, they simply weren't ethical to use against critters. If you were ridiculously close, you'd probably kill small critters. Like the chipmunks all along the hillside at my cousin's house. But you'd have to be really good to sneak up close enough to be that effective.
What is more likely to happen is you'd injure the little buggers and they'd run off. Perhaps to die later, perhaps to live on, only with tiny little beads of "birdshot" in them which were really not much bigger than large grains of sand. They are #12 shot size, which is 0.05 inches in diameter.
Shooting shot through a rifled barrel also produces really bad spread patterns vs. distance. This means shot shells are really much better suited for smooth bore rather than rifled barrels.
Shooting shot shells through pistol barrels? Even more dismal performance.
The typical proclaimed use for shot shells is as a defensive "snake round".
BUT...here's the deal with snakes:
If you have the time to shoot a snake "defensively", you have time to walk away. Or use a walking stick to discourage them while you walk away.
Snakes aren't going to "stalk" you as you actually represent more of a danger to the snake due to your size. Snake bites are typically the result of "stumbling upon" one.
So, for snakes, dress appropriately such that snake bites are less likely to be effective, be observant, and carry a walking stick.
Now, if you're going to intentionally shoot snakes for some reason (varmints, venomous snakes on your property, whatever), don't screw around with it. Head shoot them with a proper bullet or shotgun shell and be done with it.
ALL THAT SAID:
Shooting is FUN. And certainly shooting different types of ammunition is also fun. So, by all means, experiment. Buy snakeshot rounds and shoot them, see what kinds of patterns you get at various distances, shoot different types of targets at different ranges (paper, soda cans, etc.). Do it safely (wear PPE, use the correct ammo for the gun, etc.).