Satuday I took my new build out to shoot. (DPMS 11.5" w/5" FH chromelined all the way.) and my 16" RRA m4 chromeline upper(720rds down the barrel). I shot Guat, 62gr wolfpoly, and some adcom. After minor tuning to my 4xscope I had the DPMS grouping at least .5"  better with each load From 50-75yds. Any experience?

Quoted: Satuday I took my new build out to shoot. (DPMS 11.5" w/5" FH chromelined all the way.) and my 16" RRA m4 chromeline upper(720rds down the barrel). I shot Guat, 62gr wolfpoly, and some adcom. After minor tuning to my 4xscope I had the DPMS grouping at least .5"  better with each load From 50-75yds. Any experience?

Barrel length has little to do with accuracy potential.  A longer barrel is not necessarily more accurate than a shorter one, even of the same design.  Longer barrels just generally create more velocity than a shorter one, with the same ammo.

Barrel *quality* makes more of a difference here - the quality of the blank, materials used, rifling design, and throat/chamber design and finish.

It may be that your short barrel just liked that ammo you were using today.  It may be that your longer barrel just pushed the bullets you were using faster than they like to be pushed.  That being said - NONE of those ammo types you listed are known for grouping at all, anyway.  Guat surplus, while very hot, grouped horribly for me, in comparison to all other surplus/M193 I tested.  Russian Wolf always groups pretty poorly as well.

Which, if any of your barrels were free floated?

Not unusual....shorter barrels are stiffer, therefore deflect less & have higher accuracy potential than longer ones, other factors held constant. its a trade-off thing....and there is alwayas a point of diminishing returns to each feature.

Hotgun

Non Free floating. Fal you are right about  them not being known for accuracy. I might have a new best toy now. At least for 150yds under(11.5 dpms w/5" FH). Thanks for the insight FAL.