Had a Universal for a while, then sold it. My understanding is the Universals went through several different phases:
- When they first came out they were about 60% milspec and pretty decent shooters
- As time went on they had made some mods to the original design, moving farther away from milspec
- Last phase used two springs instead of one, that phase is dangerous. They had redesigned the back of the bolt, which allowed the hammer to possibly hit the firing pin when the bolt was out of battery. The only way to tell if it was dangerous was to test it--take a primer ONLY case, drop it in the chamber, pull the bolt about 1/4 turn out of battery and pull the trigger. If the primer goes BANG it's not safe to shoot. The other problem was that since they had made their own proprietary changes, parts were not USGI, so parts are hard to come by for later Universals.
Go to the M1 Carbine forum at jouster.com and do a search. They've got a lot more specifics there.