Hey folks, Looking to build a DMR for the first time as a way of practicing. The base model I am using is an old Echo1 Stag Arms M16: http://www.airsoftmegastore.com/953...15-m16a3-dmr-airsoft-aeg-rifle-jag-jp-14.aspx Specs are as listed: - 11.5-inch full metal quad rail - M120 spring w/ metal spring guide - 21-inch metal outer barrel - Inner Barrel: 509 mm - Muzzle Velocity: 400-420 FPS (w/ 0.20g BBs) - Gearbox: Full Metal Version 2 - Battery: 8.4V 1500 mAh Large Type Images to the stripped gun here: https://imgur.com/a/jpC63yh My questions are the following: Will any M4/M16 outer barrel fit into the upper receiver? The stock outer barrel has the iron sights molded on so I am looking to replace it. Minimal wobble would be best. What hop-up/bucking would you recommend for this gun? I haven't been able to find any builds around this gun so I want to make sure I purchase a unit that fits properly. Any recommendations for the gearbox? I think the M120 spring and metal spring guide should be fine, but should I do anything about the trigger? What length rail system would you recommend for the recommended outer barrel? Thank you all for your help!