The original Magpul Masada design represents an amalgamation of several recent rifle designs, incorporating what is considered by its designers to be the best features of each in a single, lightweight, modular rifle platform.
Design features from the Armalite AR-18 (short-stroke gas system), the FN SCAR (upper receiver, charging handle location), the Heckler & Koch G36/XM8 (liberal use of polymer components), the M16/AR15 (trigger pack), and the M16 (barrel, fire control group) are clearly prevalent.
The rifle also includes several features developed by Magpul, such as a quick-change barrel/trunnion system, adjustable gas regulator, non-reciprocating charging handle, and storage compartments located in the stock and grip.
The rifle was originally named after the Siege of Masada. Magpul company literature about the rifle states that, "Magpul Industries is not Jewish owned or Israeli backed, however Magpul has always found the story of the Masada as a bold example of defiance." When production rights were signed with Bushmaster, the Masada name was dropped from the product.
I believe from what I read that at the start it was a collaborative effort between both (they are sister companies) but for actual manufacturing now its Bushmaster.
Also,This Is ACR?