Ptg offers a drop in that uses ai mk1 mags. I picked up one for my a3g, it didn't quite drop in though. On a regular spr it would have, but since the a3g has a bedded bdl floorplate it was kinda tight. I ended up slightly relieving some metal off of the ptg. It now fits like a glove, has a rugged trigger guard and a beefy skirt to protect the mag. Mags are cheaper then aics mags, only 5 rds, and have room for long coal. I competent smith could fit it, thread the barrel, and mount a nv bridge on the stock. If u like the fn platform then the next best option is the model that they make with their tbm and threaded barrel. Still have to get a nv bridge installed or just run a clip on. I think mcree makes a chassis for spr's, that gets u a folder, box mag, and ready for nv bridge. U would have to still get the action milled to take aics mags and threaded for suppresor.