Md. Code Regs. 29.03.01.07 - Magazine Capacity
A. Except as provided in §B of this regulation, a person may not manufacture, sell, offer for sale, purchase, receive, or transfer a detachable magazine that has a capacity of more than ten rounds of ammunition for a firearm.
There's lots of conversation about using MagGuts "upgrades" on the forums but I don't see anything related to the legality. Is this a "manufacture" activity or not? BGOS, I know.
IANAL but it would seem that an out of state purchase and installation, before returning to MD would not be covered under this law.