Class 3 dealers are allowed to posses MGs for business reasons, not as a loophole to buy MGs.
Do some dealers buys post sample MGs for personal use, yes but it's a fraction of their business and sort of a perk of running a firearms business.
If you do intent to go that route just make sure you have some legit transactions if the ATF comes knocking. And probably wouldn't hurt to have a reason for why you have the MG, like you're testing something, showing to a potential buyer, etc...."they're fun to shoot" probably won't go over well. Also remember you will have yearly fees and in the end you will end up selling your MG for probably less than you paid for it as demand is low for post samples.
Other options would be to just get a FFL and buy pre may samples, the ATF seem to be less picky or so I've been told.
And of course transferable are pricey but do make good investments, Most have been doing 10% or more increase per year.