I made my DMR out of a Hk416, your best bet would be an m14 its What I see Most people using as a DMR. But you could make one out of any M16 if you want. My personal recomendation would be an M16A4 all you need to do is add a scope or acog for a basic platform or for a more advanced version add the scope/acog, and long tight bore berrel which also might mean a new cylinder and a silencer to hide the extra berrel.
this is kindof a thought here, just wondering, ive heard in airsoft, since the guns arent as accurate as real steel, some people measure accuracy by how many rounds you can put on a target, meaning you have a higher chance of hitting them due to the higher number of BB's aimed at them. with that in mind, wouldnt a good airsoft DMR be something like an M249 or RPK with a scope? feel free to call me a retard, but it does make sence in a way :P
2 reasons i see this as a bad idea 1st is cost, most saws cost in the area of 800 to 1000$ 2nd is safety most DMR setups in my area shoot over 400 fps whch has the requirement to be pined to sEmi auto which isnt possible with most support guns.
Also one can get some amazing accuracy out of AEGs with the right parts and ammunition, no its nowhere near the same as realsteel but there are setups that can pull off some amazing shots outside average airsoft engagement distances.