I have no pic to show you, but I have a DD 12" tube over a 16" BM M4 barrel with the PRI gas block. I did this specifically to have the long sight radius on a carbine for accuracy at extended ranges. It worked out exactly as hoped, my 200yd group size (w/ irons) with this 16" bbl is the same as with my 20" bbl, something I am utterly incapable of doing with the standard carbine sight radius. I must give "props" to Capn_Crunch, his rifle was an inspiration for this project.
Sights: LMT rear, Troy front. I chose the LMT because the sights on this rifle aren't "back-up", they're "primary", and I wanted the A2 sight system to be a part of it. Then again, I'm pretty sure no magnified optics are ever going on this rifle, or I would have had to choose a folder.
Photoman, I know you said you're considering the YMH lightweight rail... I have one on a scoped 20" platform, and it's a good system, but may I suggest you look into a DD for your carbine. I know it's way more expensive, but there's definite advantages. First, WEIGHT. Second, better locking system to keep the rail in place and completely prevents any accidental handguard rotation. Third, tube shape - the DD isn't round, it's oval like an M4 handguard and is way more suited for gripping than the round tubes. As soon as I felt that M4 profile on the DD I was a believer.