A good friend of mine is a USMC Veteran of OIF I and OIF III. He also fought in the intensive battle for Fallujah.
He said the fighting in Fallujah was the worst he's ever encountered, and possibly some of the worst fighting in Iraq throughout this whole war! We've been good friends for almost 2 years now, and he talks about his tours in Iraq all the time.
He told me their entry 'Stack' was typically composed of a 4 man entry team - followed by others. The first 4 men in were armed with:
1.) M-16, Remington 870 Pistol Grip (Breeching), and M-9.
2.) M-16, & M-9. (My buddy)
3.) M-249, & M-9.
4.) M-16, & M-9.
In his ENTIRE TIME over there, he's only had ***ONE*** malfunction with his M-16. He said they were in Fallujah, fighting through the city, door to door, street to street, killing just about everybody with a gun. He said he had fired between 400 to 500 rounds in a ~20 hour gun battle. He carried 14 M16 mags, plus 1 in the rifle (And 5 M-9 mags). After those ~20 hours of fighting, it was daylight again, the insurgents 'settled down' a while, so they all hunkered down in some shade behind two M1A1 Abrams tanks. He said everyone just quickly decided to resupply their ammo & water, quick chow down an MRE entre & take a quick nap. That was the ONE TIME that he didn't clean his rifle between firefights!
Shortly after noon, later that day, he said they were awoken by mortars walking in on them, so they all scrambled up, and moved out FAST. They spotted the mortar guys, they ran under a fence into a housing block. They surrounded the block, and started going door to door. He mentioned when they kicked a door in, he was 2nd through the door, a man on the far side of the room walked through the doorway with an AK. My buddy fired a burst, **BANG BANG** -CLICK- He had to transition to his M-9 pistol, and both he, and another guy in his squad (with an M-249) finished off the insurgent.
He had to take a spot further back in the 'stack' (switched w/ #4 in line) since they didn't have time to stop and fix his rifle - so he kept clearing rooms with his M-9. He said he'd never had a more naked 'oh shit' feeling like that in his entire life - and likely won't ever since!
He said it seemed like there was at least 1 insurgent or more per building they entered on that block. And he didn't have a working rifle! He said the only guns that went down (due to dirt / lack of maintenance) MORE than the M-16 was the M-249 SAW! He said the 249 in his squad repeatedly jammed, and wouldn't cycle... Called it a total P.O.S.! Most of the Civilians fled Fallujah before the Marines entered - so most of the 'insurgents' in the city were actually Syrians - not Iraqis.
After clearing those buildings on that block, some other guys provided security while they took a quick 20 minutes for everyone to strip down their guns, and clean them in the field. Both the M-249, and his M-16 jammed that day. Ever since then, the FIRST THING he did during down time was clean his rifle! THEN resupply ammo & water, and THEN eat something before getting rest! He NEVER had a single problem with his M-16 after he re-prioritized, but likewise, he NEVER trusted it 100% ever again, and was always weary of having another 'oh shit' moment! He also started carrying a minimum of 7 pistol mags plus one in the gun because you never know HOW LONG you might wind up relying on your secondary.
The poor bastard with the M-249 SAW kept having intermittent problems through the whole deployment though. For a while the SAW gunner also picked up and slung/carried a PPSH-41 Submachinegun and a few drums as a backup incase his SAW went down. After the battle of Fallujah, he was ordered to turn in the PPSH so they could crush it under the tracks of an Abrams.
On the positive side, he said the guy in his platoon lugging the FN M-240 7.62 MMG hardly EVER cleaned his gun (just squirt some oil in once in a while), and that sucker kept running through the whole deployment like a champ through countless rounds. Said it was a heavy bastard to carry, but it was one of the most dependable guns they used besides the mounted Browning M-2.