The main ATF/NFA difference between a c&r transferable and a regular transferable Mg (both initially registered before the '86 ban) is that if you have a c&r licence ($30) the c&r gun can transfer directly to you across state lines on one form 4 ($200) with no dealer involvement and therefore no form 3 dealer tax ($200). If you're buying in state then it makes no difference, direct transfers all around!!
Physically the c&r guns are real pieces of history from their time periods. They generally don't have receivers manufactured in some small shop from tubes or plate, they are factory made and probably saw real wartime use. There are some non-c&r factory guns like the s&w 76, all the mac's (though 'factory' is a stretch in some of the later ones), colt m16, and quite a few more. Many of the normal Mg's were made and registered by small firms or even individuals making Mg's before the '86 ban, some with fantastic materials an workmanship, some not so much...
The main thing you absolutely have to have is that it was registered before the ban, with that and a non- working gun or even a bare receiver you can legally repair.
I think the main idea of the $200 tax was to end the trade in NFA items. When it was enacted in 1934 $200 was 20% of the average annual salary per (
http://www.paper-dragon.com/1939/priceguide.html) and a new Thompson could had in the mail for $200. The tax rate has never been raised so now we see it mainly as an inconvenience of time and a little cash on an item worth at least $3000. But in 1934 it almost killed the trade and a had great many irreplaceable guns destroyed.
I've heard that prior to the ban the ATF (part of the FBI at the time I think) went door to door through most of the country asking if you or anyone you knew had a Mg or other NFA device. Then you could 1- register it operational for $200
, 2- they'd deactivate it by brazing (they carried the setup with them) for a small fee and register it as a De-Wat for $5, or 3- they would confiscate and destroy it for free
. That's what I've heard from a few different people, but may not be 100% true, I don't know personally (not THAT old)
Sorry to get a little off topic, please ignore what you don't want