Larry Vickers posted up a nifty eight minute video about some of the copies of US SOCOM firearms available.
Unfortatly he only covers some of the ones made by High Caliber Sales(HCS). I purchased a HCS MK12MOD1 upper and had to return it due to the multiple issues it had. Also they have started using Bolt Carrier Groups that are not staked, cheaper non-Colt receiver, etc. I can not recommend High Caliber Sales due to my bad experience with them.
The supply of MK12 parts is drying up. If you want one, it will be better to get it sooner rather than later. Items like the correct handguards (for both the MK12MOD0 and the MK12MOD1) are getting much harder to source.