Can anybody enlighten me on the legality of a combustion airsoft rifle? I was thinking about trying to build one, but don't want the Feds knocking my door down.

I think you would run into the issue of propelling the projectile through explosion which runs right into the legal definition of a firearm. Or you might run into the destructive device issue. Both can run into regulation issues.

To clarify Simunitions, Simunition weapons are classified as firearms and are regulated under all rules governing the use of firearms. There are no exceptions to Simunitions in their regard to regulations as they all propel the projectile through the use of gunpowder. General Dynamics is the parent company of the brand and strictly limits its availability to certified law enforcement and military training units only. The restrictions of Simunitions to civilians has nothing to do with federal regulations, it is done strictly by the company.

Bottom line, it would probably not be worth the headache to do it.

-Stay safe