okay, you never specify the M900 handguard. The #38s-EX will not fit over the M900a since the M900a is the same diameter as the
M4 double heat shield handguard. the handguard is too tall it will not clear the spaces left by the SWAM sleeve.
Also by getting the ARMS sleeve, you will not reinforce the forearm, since there is nothing for the rail to attach to the handguard.
If you want inforce the handguard by using a ARMS SWAM sleeve, you are looking at getting a KAC RAS handguard with rails at 12, 3, 6, and 9 o'clock position, and you will need to order an ARMS #38s-EX-Mod, this sleeve have rail slot build to the front of the gas block which lock into the top rail on the RAS and also uses three screw to interface with the slot on the top of the RAS. As far as fitting a sleeve over the M900, it will not do anything for your handguard, beside the rail will not fit. one thing you can do if you so set on getting a sleeve, you can get a #38s which will only poke beyond the receiver rail by about half an inch, you can grind the M900 handguard a bit so the rail will fit.
as far as using a SWAM and co-witness, it will do so as long as you got a Aimpoint mounted to a #22M68. if you have an EOTech, it is a good set up as it is, I would just buy a ARMS #40 BUIS and keep the current setup.
This is the MOD rail sleeve I am talking about.
[img]http://www.thermaldynamics.com/pictures/guns/M4RAS-2.JPG[/img]
[img]http://www.thermaldynamics.com/pictures/guns/M4RAS-3.jpg[/img]
Hope this helps.