R-77 is at best comparable to 120A, and that missile got dumped within a couple of years of production yielding its place to the 120B, and then successive variants of the 120C.

The AMRAAM seeker was not 'bolted onto the SM-2ER'. The SM-6 seeker some form of derivative of the AMRAAM seeker, but it's not like you can take one right off the 120 and stick it onto an SM-6.

@Hpasp: Because PATRIOT uses link-16, in theory you could use an 'off board source' for MCUs, ie. receive position from datalink, then generate the missile MCU transmission. There may be reasons why you want to do something like this, but given that all the long-ranged PATRIOT missiles are SARH/TVM anyway ... what's the point