I'm actually going to try and order just one of the drum magazines. You need a license for the actual FN303, but I'm hoping I can get away with just the mag.
They're actually cheaper than mags for either the TPX or T9.1/T8.1 which I find ludicrous. If one can get the drum mag, you could do this to any marker.
Doing it to an Automag would be tough (ie. what the FN303 is based off). You can see from the breakdowns that the bolt is much longer than a regular 'mag. Doing it to an Autococker/mech blow back? Likely much easier.
Autococker - longer bolt (ie. have a custom bolt made or trim a regular bolt in a 1/2 blocked 'cocker)
Mech blowback - longer power tube or cut the body (ie. a Stingray)
The First Strike mags, according to what I've read from users, usually jam a round every 2 or 3 mags. Thankfully, it's usually the last one as the spring has the least tension on the round for that one. Revolver-style or stripper clip (ala SMG60/68) is where it's at I think. If I had spare cash I'd grab one of the SMG60's, bore out the clips, and you'd have a pretty cool marker than looks like a Sten. They usually go for around $250 so it's actually a fair price. SMG68 are way more (ie. $700 plus).
It's really too bad the
Destroyer never made it to market. It would have been perfect.