On my weekly Airsoft related travels I came across a very interesting idea, an after market blow back system an M4. It fits into a standard stock tube but this does however mean that the battery has to be elsewhere, such as front wired AEG's with the battery in the front-end or for example a battery bag. This is a nice addition to the increasing realism market joining gas blow-back guns, Tokyo Marui's M4 Blow-back SOPMOD and G&G's M4 Blow-back. 

Check it out, the Tremors 3 system by Modify is being released within the next few days;