I heard some rumors about power armor that would let people do that, but as far as I know it's still in prototype and they were having problems powering it.
You're probably thinking of the HULC lower body exoskeleton. One idea was to install a pintle mount to the frame that could hold a heavy weapon but I suspect that's just a "yeah, you
could do it ...
BUT ..." And the problem isn't powering it per se. It's keeping it powered in the field for 8-24 hours without everyone needing to carry so many batteries that you cancel the load carriage provided by the exoskeleton.
As Shadowrun doesn't really deal with equipment weight, such items are a bit of an edge case, mechanically. Such a mount would count as a mobile tripod (SR5, p. 433) and it would have its own carrying capacity (SR5, p. 420) and hmm.. That's about it.