Wont work, You won't actually want a riffled barrel for first strikes. They will spin on there own, look at the fin vain directions.
As far as the aftermarket barrels, lapco is one of the best. And I run a barrel adapter from a guy here to makes them for the MR5. My EMR5 uses the barrel adapter with a Lapco 14" Assault barrel and a Apex 2 adapter with Apex 2 tip. The adapter that the guy makes will allow the use of any standard spyder thread barrel. You can also use lapco adapters to adapt the spyder thread to A5 etc for more barrel choices if you use the MR5 adapter.