OK, I have the slick-side 604 (which will be a 602 when the NDS lower gets in).  I have the 653 build, with the 1/9 pencil barrel.  I also have an mid-length, fully Magpul-ed flat top build with a1/7 mid-length gas system pencil barrel (.625 under FSB).  I ordered a slip over XM177 flash hider (with attached grenade ring!) that hides some of the barrel.  Me thinks that if I put it on a mid-length barrel, it would stop just shy of the FSB.  If I put that barrel on the 653 build instead of the pencil, it might look pretty cool. I'll just turn the flat top build into a light weight 1/9 M4gery build.

I do want the full 16" of 1/7 barrel to maximize the MK262 MOD1 ammo I shoot, so I won't cut it down.  I'm not sure how good the slip over is, it just says black oxide finish (maybe some naval jelly?.  It doesn't have wrenc flats like the discontinued Bushmaster slip over does, but for $25 I though it was worth a try.  Worst case scenario, if I don't like it, I'll still have a 653 that the bayonet will fit on.

With that said, let's see your middy XM builds!

With that said, who has mid-length CAR15 handguards that aren't oval or M4ish?  Palmettos and Rock rivers are too big.

Here's mine

The barrel is a bit longer than it needed to be to get to the 16" mark but found that out after I got it back

PROPER!!!

That's what I am thinking of, but my PSA Middy handguards are too big.

Scratch the idea of taking my mid-length pencil barrel off my flat top and using it on the XM! Holy cr@pola! They must have epoxied the threads when the assembled it on the PSA premium upper!  IN an aluminum vise block, torqued as tight as possible (with hammer), I was unable to break loose the barrel nut.  The barrel kept turning in the aluminum.  It is Melonite, and very slick, and tough.  The aluminum 'smear" on the barrel wiped of with oil, and never left a mark.  I guess the melonite is as hard as they say.

Oh well, the slip over FH will go on the 1/9 carbine pencil barrel that is on my 653 build right now.