This will soon be available at Echigoya for those who want a stronger recoil effect on their Tokyo Marui New Gen M4 SOPMOD... "Today a prototype of 401 grams Heavy Weight recoil unit made of solid brass (machined by Tirador of ARS) for Tokyo Marui m4A1 sopmod was just tested in the fully upgraded Tokyo Marui NexGen M4A1 and the recoil and kick was so hard and crisp.