The reason you don't really see much aftermarket support of the MRP is that LMT will not sell the special barrel extensions.
the other way to make a barrel for the, involves starting with a large diameter blank and profiling it to leave the chunk there in front of a standard barrel extension.
Lastly, you can sleeve up a standard barrel.
I do this kind of work, but I don't advertise it here as I am not a vendor here.
An email to me might get a good response though.