Skizzums, the risk should be quite easy for you to see -- you're a handloader!
That blank round you're stuffing behind a bullet
is the primer and propellant charge. Just because it "fits" in the chamber doesn't mean it's correct. You know that .22LR has a SAAMI maximum pressure and every .22LR chambered
firearm running around in the USA is specifically spec'd to run within that industry standard.
However, when you stuff a nail gun blank in it, you are willfully stuffing an unknown primer/powder type/powder charge in to a firearm that is built only to cover/provide for SAAMI spec.
It's already "unsafe" just simply placing it in the chamber & lighting it off. It becomes more safe with every tenth of a grain of weight/resistance you place in front of it.
It's no different than you letting some non-gun person randomly pick a powder from your cache, randomly dump some unknown quantity of a powder in to a piece of primed brass, and then you topping it off with a bullet that is 35% of the weight of that normal round.
Why there's a risk here should be obvious to anyone that handloads.