You don't need a license to make or own a suppressor. You do need to file an application to make or purchase one and pay a tax ($200). Only if you make them for sale or deal in them do you need a license. Nothing in this thread mentions doing anything illegal, at least under federal law.
Three comments, though.
First, why would anyone buy a .22-250 and then load subsonic rounds to use a suppressor?
Second, the sonic "crack" is not usually very noticeable from the firing line. As pointed out, it is quite noticeable and distinctive when down range or in the pits. The "crack" is exactly the same as the "sonic boom" experienced from jet aircraft and the cause is the same, though obviously less as the bullet is smaller than a jet plane.
Third, the Maxim silencer really was a silencer. When a Model 1903 rifle is fired with the Maxim silencer, the firing pin click can be heard. With no other significant sound, the sonic crack becomes much more noticeable.
FWIW, I have no idea what a Maxim silencer would cost to make, but it would be a lot more than the cans curently being made. If in doubt, look at the patent, which has been widely printed.