A 'dissy' is a dissapator, with rifle length handguards and sight radius but a 16" barrel; the name supposedly because the full coverage of most of the barrel dissipates barrel heat without a chance of toasting your hands.
Mine has a gas block about 6.5" from the muzzle, or approximately the same distance as the gas block to the muzzle of a 20". My pinned 16.5" has the gas block about 7.5" from the muzzle (sorry for the 'abouts' but the FH makes it challenging to measure perzactly).
So, unless I'm missing something, the claim to fame of systems such as this one is the same dwell time for the bullet downbore from the gas port, tho' it does place the gas port closer to the chamber and, I suspect, somewhat higher pressures.
I got a tentative zero on the dissy today, with mixed results. A decent group at 50 yards, off the paper at 100, and beating the living hell out of a 10"x18" clanger at 200 yards (this with 65 year old eyes and XS irons, and green tip NATO). I put it on a Government Carbine lower because that has an SSA in it, tho' the carbine buffer remained as well. The carrier is ramped and not full circle, so it is a bit lighter than standard.
It was pleasant enough shooting, tho' I experienced no epiphanies; we'll see how it all works out. Function was 100%, tho' it required a full stroke to chamber the first round; it does have M4 feedramps. It is about 7lbs, so it's heavier than an A1 or a Government Carbine.
Moon

