The only way I see this as being legal is if the owner ships the firearms (or frames) to a MA FFL and they get transferred to the OP. Whether that's through a CA FFL or direct mailing, that would be determined by CA law and/or the receiving FFL. OP can fly out and assist grandma with the effort if she couldn't be bothered.
Good luck!
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The only potential PITA is the EOPS/AG list in MA might prevent the transfer to the OP if they're sent as full firearms.
Good luck!
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Why can't Grandma just call (or the new owner call) an FFL in CA and have him/her go to the house, sign a receipt listing the hardware, then they would ship them to the new owner's FFL in MA and the paperwork would be done and above board. It doesn't have to be complicated, does it? Notice that I'm not suggesting Grandma TAKE the hardware to the FFL as that would expose her to legal issues. An FFL, however, could just visit her home and take responsibility for the handguns and ship 'em for a fee. She would retain the list describing them along with serial numbers and, hell, use a cell phone to take photos, too. Easy as pie and above board.
Rome
The only potential PITA is the EOPS/AG list in MA might prevent the transfer to the OP if they're sent as full firearms.