I wouldn’t load it like a muzzleloading rifle, I’m not sure but I would be concerned about the pressures you would build in the chamber. That adapter would make it a muzzleloading shotgun. Which IS LEGAL but the regs are very specific about bore size and shot size for muzzleloading shotguns during muzzleloading seasons. I’m not sure the 20guage would be legal but I’m sure the 12 would be. Anytime anyone decides to buy a muzzleloader tag in WA they need to closely read the regs, there is a lot that goes into a legal muzzleloader in WA.