What brand of PDW has the best externals? Would a matrix PDW be just as good as the other brands that are more expensive? There are multiple matrix PDWs I could buy on craigslist for around $100 plus they come with attachments so unless it is extremely difficult to install a polarstar engine into one I would prefer to just use that. I am most concerned about external durability and quality, especially how well mags fit in. And if no one can help with the above issues what are some other rifle bodies I could use that use m4 mags and probably take the polarstar engine?