I've done a bunch of work on LR308's, including several 16" versions. The DPMS carbine buffer is already toting 2 tungsten weights. Although Cav_sct is right about the use of standard tungsten weights to accomplish the goal, the DPMS carbine buffer only has 2. The only way to make the stock DPMS carbine buffer heavier is to add depleted uranium ingots
I found the best way to deal with the overgassed systems or reduce the recoil pulse on the rifles I worked on is to install an adjustable gas block. I've used JP's on several and really like them. On the 16 inch barrels you want to use the stainless JP model. It will erode less quickly than the aluminum one on a short/carbine/high pressure gas system.
I have also used the Tubbs on LR308's, and if the rifle isn't too overgassed, it works just fine to reduce the cycle velocity. The latest version will close on with the pivot pin in as bushwack noted - it took a little wiggling, but will go.
Honestly, I think the best approach is to
decrease the weight of the reciprocating asembly (carrier, buffer, etc) and use an adjustable gas block. The stock assembly is a
lot of mass moving back and forth and adds a lot to the recoil. To illustrate... I recently installed a custom made muzzle brake on mine. With the gas system in use, it reduced the recoil to a
mild 30-30 or 7.62x39 recoil. With the gas system shut off (I used a JP on mine too), it "recoils" like a mild 223, and I mean that literally. The bulk of the "recoil" on mine is now caused from that huge carrier cycling.