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If you're talking about a U.S. MR556A1, then yes, you can put an MR556 upper on an AR15 lower, and you can put an AR15 upper on an MR556 lower. I think the European/Canadian MR223A1 you could not, but I think you can on the A3s, but I'm not super familiar with the non-U.S. configurations.

The only caveat is that the stock MR556 upper (really the BCG) has a firing pin safety that's not compatible with all AR fire control groups because of the height of the hammer needed to raise the FPS. Standard AR15 hammers WITHOUT the notch will work, aftermarket FCGs can be hit or miss, but there is a large thread here of compatible FCGs.

The FPS can also be "defeated" in a number of ways, both permanent and non-permanent, e.g., using a modified AR15 firing pin as a non-permanent solution, this will not only defeat the FPS, but also the FP return spring, at least hypothetically increasing the risk of slamfires. Mine has worked with no problems, but this is done at one's own risk.

Many small parts will interchange between the MR556A1 and the AR, so you can replace those stupid locking takedown and pivot pins on the MR with standard AR parts, add your preferred bolt release lever if you want something other than standard, etc.

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