HI TV DOg!
I have currently seen the POF-USA design of the piston based M16,
www.pof-usa.com HK design may not be out yet!! Another company is ZM weapons ( or the LR300 series) but they want an arm and a leg for their system.
Anyway!, with a piston, the dirty gas will be only in the piston head area, just like the AUG, SIG550, AK, SAR21, SAR80.... the only downside is that when in extreme weather like hot and cold, there may not have enough gas to push the piston thus most of these rifle like FNC, SIG, AUG etc have a gas setting.
TO answer the question, I have gone thru 2000 rds of 5.56 on an AUG and the bolt and bolt carrier is spanking clean ! the only dirty part is the piston! wipe it clean and you're good to go for another 2000.... as for the M16 direct gas, the bolt gets so dirty pass 1000 rds! thus ini full auto fireing the rate of fire goes slower when the the carbon built up in the bolt and bolt carrier gets up...
In the US, a piston based M16 conversion is the way to do becasue we have all kinds of M16 parts we can get CHEAP and easy accessible.
As for CQB setup, I have tried several from Iron sights, to Eotech and AImpoint and also the ELcan or the ACOG TA-01NSN!
YEs! the best would be the EOtech ( I like the wide view and the hologram is just so easy and fast to aquire target) AImpoint M2 or would be by next choice! ACOG and ELcan type optics is too slow and the Magnification is just too big.. field of View is too narrow. Iron sights is good, but not as fast as the EOtech or dot sight ! but I guess no batteries needed and in SNAFU situation, all system is good to back up with an iron sights!
My M4 setup with a EOtech 551 Rev F with a ARMS backup iron sights !Knights industries RAS2 withM3 light and fwd front grip!
My ultimate CQB setup would be all the setup plus a piston based M16 upper!
http://i3.photobucket.com/albums/y66/davechng/Guns/m16-10.jpgDaveC!