SCOTUS will not take it. The machine gun issue has been addressed in US v. Stewart (2003)
And Stewart was a felon. Of course no Court is going to rule that felons should have machineguns! Get a client that's upstanding, a good citizen, etc., get him to file a Form 4 for a Colt M4 along with 200 other people, then see what you get.
To date, almost all of the gun cases that have gone close to SCOTUS are about violent felons breaking firearms laws. No matter what, the SCOTUS is not going to rule on the pro-gun side with a plaintiff like that. Gonzales v. Raich was a commerce clause case, so if the SC is going to use that, go the route of getting a law passed in Montana (they've tried) as spelling out that machineguns may be manufactured and sold within Montana free from 922(o) as long as they never leave the state. That creates a clear SCOTUS case.