http://www.pkfirearms.com/node/12692 is the barrel I'm inquiring about. I'd like to make a sure lightweight kiss rifle using this barrel and it comes with pinned fsb but it looks like no handguard cap. Did DD leave enough space to put a round one on for Magpul sl handguards?

All you have to do is find one.

There was a gap between the shoulder of the .625 journal and the FSB on my barrel of same mfg and model. Just consider that carbines gas typically use round caps vs middies that have traditionally triangular caps. If you ever plan on trying the B5 keymod slim handguards, you will want the round type for sure. Also, the pins come out easily from the FSB on a new DD "part" barrel, but just make sure you hit from the correct side (the taper pins are larger on one end).