Editing Close Quarters Battle Receiver (section)

==Evolution==
* '''CQBR type 1''': Made from M4A1 uppers by shortening the barrels and opening gas ports, no longer produced, STD carbine buffer in use in host lowers at time of fielding.
* '''CQBR type 2''': NSWC assembled with contracted and /or overhauled parts complete with all accessories to current standards at time of build, will work with H, H2 or H3 balanced to ammo requirements:
** H works with [[5.56×45mm NATO#US Army|M193]] and [[5.56×45mm NATO#SS109/M855|M855]].
** H2 needed for [[5.56×45mm NATO#Mk 262|MK 262]], [[5.56×45mm NATO#US Navy & US Marine Corps|MK 255 R2LP]], or [[5.56×45mm NATO#US Navy & US Marine Corps|MK 311 R2LP]]; will still reliably feed and fire M193/M855.
** H3 may be needed for an extreme case, or when the gas port is eroded but the barrel still groups.
* '''CQBR type 2A''': NSWC assembled with contracted and/or overhauled parts, naked without any accessories or buffer. The buffer in the host carbine would be used or upgraded as necessary via the supply system same as type 2.
* '''CQBR type 3''': Colt-contracted upper, complete with all accessories. NO buffer supplied. The buffer in the host carbine would be used or upgraded as necessary via the supply system same as type 2.
* '''CQBR type 3A''': Colt-contracted upper, naked without any accessories or buffer supplied. The buffer in the host carbine would be used or upgraded as necessary via the supply system same as type 2.{{citation needed|date=June 2022}}
* '''MK18 type 1''': NSWC assembled carbine with contracted and/or overhauled parts complete with all accessories to current standards at time of build; will work with H or H2 and could have shipped with either. Units will balance buffer to ammo requirements:
** H okay with M193 and M855. (All shipboard MK 18s should have H buffers as this was the standard at the time of assembly.)
** H2 needed for MK 262, MK255 R2LP, or MK 311 R2LP; will still reliably run M193/M855.
** H3 may be needed for an extreme cases or when the gas port is eroded but the barrel still groups.
* '''MK18 type 2''': Colt-contracted carbine complete with all accessories (H buffer supplied). Units will balance buffer to ammo requirements same as Mk 18 type 1.
* '''MK18 MOD 0''' ('''MK18 MOD O'''): Designation given to the full rifle, consisting of CQBR upper receiver and M16A1 lower receiver
* '''MK18 MOD 1''': Similar to the changes found in the SOPMOD Block II kit, the MOD 1 features a gas block end without a front sight and a railed handguard, by [[Daniel Defense]], that extends all the way to the base of the flash hider to provide more accessory rail space for the user.
* '''MK18 [[M4 Carbine#Special Operations Peculiar Modification|URG-I]]''': The Upper Receiver Group - Improved (URG-I) features a gas block end without a front sight and a railed [[M-LOK]] handguard, by [[Geissele Automatics]]. The URG-I also replaces the MK18 MOD 1's heavier SOCOM barrel with a standard "government" profile cold hammer forged barrel with a longer gas system.<ref>{{Cite web|url=https://www.nsn-now.com/1005016713911|title=1005-01-671-3911, MODIFICATION KIT,GUN,WEAPON|website=www.nsn-now.com}}</ref> Primarily used by [[AFSOC]].<ref name="ReptileHouse">{{cite web|url=https://thereptilehouseblog.com/2021/01/20/in-the-wild-afsoc-urg-i-10-3-cqbr/|title="In The Wild: USAF SOF URG-I 10.3 CQBR"|date=January 20, 2021}}</ref>