Sorry, gents, I have to disagree with you- and the ATF HAS ruled on this- it is 21.
A shotgun and rifle are defined as "fired from the shoulder". And that pistol gripped Mossy is not a pistol, it is an "other". If you read the regs, it says 18 for rifles and shotguns- 21 for "other". BTW, that includes buying a bare receiver. It actually is listed on the FFLs bound book as "other".
I did NOT say it would make sense- and I did not write the stupid reg.