I'm not going to argue with you, because quite frankly it's pointless. You're obviously a narrow minded fanboy who doesn't even give anything a second thought if it doesn't begin with "M4" or "AR." That said, I have gone out and dug up some numbers on the XM8 and the M4, and written up this little side by side comparison. That said, read it and weep. The M4/AR-15 is now officially obsolete. Now, on with the punishment:
These are the two setups the comparison is based on:
The XM8 default layout, including carry/handle optics suite, which contains IR laser and illuminator, and a reflex red dot sight.
The M4 is set up with RAS, and equipped to provide the same functionality as the XM8's default configuration. Consequently, the following devices are mounted: M68 CCO, AN/PEQ2, AN/PAQ4, backup Iron Sights.
Now, let us take them head to head:
Weight
XM8: 6.4lbs in current config
M4: 8.8blbs
Length
XM8 (Carbine version, stock extended): 33.3"
XM8 (Compact Carbine version, stock extended): 29.8"
XM8 (Rifle version, stock extended): 40.2"
M4, stock extended: 33.0"
Cost
XM8 Carbine version w/Optics: ~$1800
M4 with above accessories: ~$2539
Modularity
M4 is customisable only to the extent of mounting accessories onto any pre-mounted RAS rails. Weapon requires extensive work to be converted to any caliber other than 5.56mm NATO
XM8 has a common receiver which can accept 4 different barrel lengths, as well as being able to make use of the picatinny rails already found on the weapon. Weapon may be configured to fire different calibers by changing out the lower receiver, barrel and bolt groups. This can be done by the soldier, and the armorer is not required.
Barrel Exchange
M4: Not possible at user or unit level, takes longer than 10 minutes, no fixed headspacing, only 14.5" available without modification.
XM8: Exchange at user, unit level possible, takes less than 2 minutes, fixed headspacing, 10", 12.5", 14.5", 20" barrels available and can be mounted with no modifcation required.
Operating System
M4: Direct gas system, introduces fouling into receiver, does not work well with short barrels (See: IDF 11.5" barrel CAR-15), not removeable by operater, cannot work if water is introduced into system. Cartridge case is not fully supported by the bolt head. Bolt is a 7 lug rotating Stoner bolt.
XM8: "Pusher Rod Gas System" (HK's name for it), no carbon introduced into the receiver, allows for greater flexibility in terms of barrel lengths and caliber changes, no buffer or buffer tube required allowing for shorter length, weapon still functions with water in the system, bolt head fully spports casing. Bolt is a 6 lug rotating Stoner bolt.
Reliability and Service
M4 stoppage can be expected somewhere around 6,000 rounds with proper cleaning and lubrication. Average cleaning time for the operator is about 15 minutes, and the barrel, bolt head and extractor should be replaced after 8,000 rounds. Cook offs at about 210 rounds.
XM8 stoppage can be expected at 20,000 rounds with no cleaning or lubrication. Cleaning time is about 5 minutes, and all parts will still be functional at 20,000 rounds. Cook offs aren't a factor until 240 rounds, at which point it's only a minimal chance.
Safety
M4: Manual safety blocks trigger movement, disconnector safety. User has no protection from wayward gasses if catastrophic failure occurs, barrel will rupture if blockage occurs.
XM8: Manual safety blocks trigger movement, disctonnector safety, inertia firing pin. Barrel vents direct gasses away from the opreator in the case of a catastrophic failure. Barrel design assures against injury in the case of a barrel obstruction (including water).
Accessories and Sights
M4: RAS system adds 11 ounces of weight to weapon before accessories are even mounted. MIL STD 1913 rails do not retain boresight alignment. Only sports iron sights by default. Rails must be added to allow the use of optics
XM8: Weapon already has picatinny mounting points on the handguards, and receiver. This means nothing needs to be added to the weapon to allow for attatchments. MIL STD 1913 rails can be mounted to allow the use of legacy equipment not compatible with the picatinny system. Flush PCAP provides 100% boresight retention for optic device mounted on the receiver rail. Standard optics suit includes 1 red dot 1x sight, one IR laser, one IR illuminator, all powered by a single DL123 battery, with emergency sights molded into the suite, and etchings on the red dot lens to make it useable in daylight if the power fails. Optics suite mounts to weapon using aforementioned Flush PCAP rail.
Fire and Control
M4: No ambidextrous controls except for the charging handle. Charging handle cannot be used if the weapon is shouldered. Fires semi auto, fully auto, or 3 round burst (with burst memory) depending on configuration of the lower receiver. Sustained rate of fire is 50rpm for 210 rounds
XM8: Safety/fire selection, magazine release, bolt catch release button, charging handle, forward assist, sight actuation switch all ambidextrously controlled. Charging handle can be operated when rifle is shouldered. Fies full auto, semi auto, or 2 or 3 round bursts with no burst memory. Sustained rate of fire is 85rpm for 210 rounds.
Buttstock
M4: 4 position collapsable. Not exchangeable by operator.
XM8: Modular. 5 position standard collapsable stock, optional fixed "sniper" stock, butt cap or folding stock. Exchangeable by the operator without any tools.
BFAs and Supression
M4: Uses standard BFA which provides no safety at all. Uses standard QD supressor. US Standard bird cage flash hider.
XM8: Uses Safe BFA which can catch up to 3 live rounds without injury or damage if they're accidentaly loaded. Live round excluder magazine also available. Shorter (9", 12.5") barrels sport 4 prong flash hider, longer barrels sport standard US bird cage flash hider. QD sound supressors fit all varients.
Grenade Launchers
M4: Mounts M203 40mm grenade launcher which cannot load longer 40mm grenades, and requires rezeroing each time it is mounted.
XM8: Mounts XM320 40mm side loading grenade launcher, which can load any length 40mm grenade, and maintains zero upon removal.
Compatibility and Compliance
M4: Compatible with MILES and Land Warrior, NOT compliant with USSOCOM SCAR-L Draft dated 9th September 2003, not compliant with USAIC FCR Draft ORD specs.
XM8: Compliant with all of the above, MIL STD 1913 Rails provide compliance with SCAR-L Draft dated 9th September 2003. Compliant with 67 of 70 specifications under USAIC FCR Draft ORD.
So, in other words, can your rampant fanboyism, and try to be just a little objective in the future.