It's not just the carrier key that you need to be concerned about. There are other issues, such as a lack of parkerizing under the front sight bas block, a front sight that isn't "F" marked, the lack of HPT/MPI of the barrel and/or bolt carrier group, amongst other things
You can however get an excellent AR for the price of a Bushmaster or DPMS. Bravo Company USA makes excellent uppers and lowers (and complete rifles and carbines). They are a top tier gun, at an affordable price. One place to get them is G&R Tactical (
http://www.gandrtactical.com/).
Bushmaster could easily correct the problems with their guns, but they choose not to. An informed consumer will not support a company that takes short cuts to save a buck, at least in my opinion.