Is there anything stopping a person from building out a M249 receiver into a pistol other than why?

Once a machinegun, always a machinegun

Semi receiver, sorry. Like those 80% receivers you see on GB. This one apparently showcased in 2018, but that gripod thing looks more like a VFG than a bipod to me. Never to be seen again..

m249 pistol by Peter Sakadinsky, on Flickr

The gun in the picture looks like one built by Machine Gun Armory (MGA), judging by the funky HK style trigger pack and the shitty looking welds.

Years ago they did build semi guns but I think what little market they had for these type of things have all but vanished once FN dropped their own M249S onto the market in 2016.

MGA does appear to still have a mostly operating website, so assuming they're still in business, you could ask them to build you one. However in my experience the company is kind of sketchy, and as mentioned before the weld quality looks like spilled beans.

This isn't something worth pursuing IMO unless you have a very specific reason to want a semi M249 in pistol form.

Those grip pods are legit on a saw. I much preferred it to the standard bipod that always flops out when you don’t and it to and catches onto everything.

maybe ok on a real saw or actual 249s but on a pistol I'd vote its a vertical grip and no go. And in any case the end of the brace is coming soon...