Been pouring over some Sig patents for the their LMG, and thought I'd highlight some minor design changes from the production MX250 compared to the first prototype I saw in 2019. These are external changes of course, not much has been publicly disclosed about the internals so far 
The production model (left) now has a continuous length M-LOK interface on the underside, and the profile of the upper portion has been tweaked. The prodution XM250 has lost its side folding stock capability, due to the necessesity of a battery pack for the powered rail.
Aside to some minor profile tweaks here and there, it doesn't appear to have changed all that much. Of course there is the 'rail on a rail' to properly accomodate the XM157, without interferring with the feed cover. Kinda validates the FN Evolys' design in that department.
The SIG-LMG is understood to use a combined short-recoil and gas piston system of operation. With a bolt group housed inside the barrel extention, itself forming part of the barrel group that also moves during the firing cycle.
Here is @xmszeon great animation based on the patent information, to see how it works in motion.
https://twitter.com/xmszeon/status/1237036057377402881
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