Hello all,
Just got to shoot my ‘new’ Bushmaster XM-15E2S for the first time yesterday.  It is the first time I ever shot an AR-15.  Lucky me, it’s mine!  It’s an m4stlye preban.  Only modification is the hogue grip.  All else is original.  I’ve got 4 standard 30rd mags, and a red dot BSA that I have not tested yet.  
When I bought it a month ago I cleaned it thoroughly and lubed it lightly.  Everything seemed to be in order, in excellent condition and functioning properly, as far as I can tell, but I am a complete beginner.  
When I went to the range everything seemed to be fine.  Firing away, after about 30 rounds, I had my first failure to feed.  I had several after that every 5-20 rounds.  I probably shot 100 rounds total.  I was using Winchester White box 223 REM 55gr fmj 40 round value pack.  
What really concerns me, is that at the end, EVERY single round failed to extract.  I couldn’t figure out what the problem was.  It seemed a bit much for it to be just cheap ammo as the problem.  After I got the rifle home, I disassembled it to clean and lube and found that the Bolt carrier key was very loose.  
Is this what caused the problem?  
Any other suggestions?  
Any recommendations for decent ammo on the cheap end?  
Any possiblilty I may have damaged something by firing it with the Bolt Carrier Key loose?  If so, what should I double check?  Everything looks to be fine and in tact.
Is there any reason I should not disassemble my rifle at the range if I start to get more failures?
Is there perhaps an area I forgot to lube or didn’t lube well enough?
Can someone help me build a range in my basement so I don’t have to travel 30miles to the range? (just kidding)
All in all, shooting the gun was a great experience.  I expected that I would run into trouble at first.  Just got to get the kinks worked out.  Any advise would help. Thanks!

Isn't the XM-15E2S a Bushmaster model number?  Is this a Colt upper on a BM lower?  Not that it matters really, just odd.

Anyway, very high probability that it was the loose carrier key that was the problem.  Doubt it did any harm to your rifle, other than making it not function.  For peace of mind, should check the inside of the upper receiver, at the top where the charging handle sits, and see if the carrier key gouged the side of the receiver any.  Also check the end of the gas tube to make sure the key didn't hit it at a odd angle and deform it.

Ideally, you should buy new screws for the carrier key and apply the proper amount of torque and then stake them.  That said, most likely the screws can be reused, just need to re-torque and re-stake them.  Check out the military TM (linked on the front page) for specifics, can't remember them off the top of my head.

Wholesale Hunter has XM193 for pretty cheap right now, check out the ammo forum for info.

No problem with disassembling the rifle at the range as long as you don't loose anything.  

Doubt it was lube, rifle should run reliably dry at the round count you listed.

Torque on the allen-head key screws is 35lbs.

Sorry, yes, don't know why i said colt, it is a bushmaster.
Considering I don't have an inch pound torque wrench, should I take this somewhere to have it done?  Any ideas where in columbus ohio?  
I was also reading one of the tm manuals which states never to remove the flash hider.  THis is of course a different flash hider than mine.  I have removed mine several times and hand tightened it.  Should I do something different?  Should I not remove it when cleaning the rifle?  
Also, any advise on removing the handguards by myself?  My wife doesn't seem to want to help. lol.
Also wanted to know how many rounds I can safely fire in one trip to the range.  Do I need to let it cool down at any point?
THanks!

Then take a center punch and stake it per the TM.

You can tighten the screws down (firmly) with an Allen wrench and stake them. An option is to contact BFI and ask for a return slip.
I've never seen a warning against removing the flash hider in the 23&P, are you talking about the -10 manual? For operator level maintenance (cleaning) there is no reason to remove it. It needs to be tighter than "hand tight", especially if it's a 16" barrel.

There are several tools being sold that make job of removing the handguards easier.

Don't worry about breaking the gun. If you exceed 12-15 rounds per minute you'll be getting into cook off range.

You shouldn't be removing the flash hider everytime you clean, no need for it.  You can if you want, but there is no need to do it.  The -10 TM is the operator manual which gives instructions on use of the rifle and cleaning and is probably the TM you were referring to that said not to remove the flash hider.  The 23&P TM is the one that goes into more detail on assembly and disassembly of the rifle into its individual parts.  This TM was written for armorers, not for the average soldier, and will give instructions for removal of the flash hider as well as info on the carrier key.  Sorry I was not more specific in my previous post, I was tired and feeling lazy so I didn't link to it.  Here is the link to the TM you need:
[url]http://www.ar15.com/content/books/TM9-1005-319-23.pdf[/url]

I posted my method for removing handguards in this thread:
[url]http://www.ar15.com/forums/topic.html?b=3&f=66&t=176948[/url]

If the hanguards start to smoke or melt or the barrel turns red, it's probably a good idea to stop firing and let the barrel cool.  You can fire off a pretty good number of rounds before there would be any safety issues, like ten 30rnd mags as fast as you can.  That should get you pretty close to the cookoff threshold.  Doing this repeatedly though, would greatly reduce the lifespan of your barrel, causing the rifling to erode away from gas cutting, and as a result greatly reducing accuracy.  

Tweak's note about firing off a maximum of 12-15 rnds a minute to avoid cookoffs is somewhat misleading.  This means that you could fire off 12-15 rnds a minute for the rest of eternity and never get to cookoff barrel temps.  You can safety fire more than that a minute without cookoffs as long as you let the barrel cool down in-between strings.

Personally, the rule of thumb I use if I want to maximize the lifespan of a barrel, is to stop firing and let the barrel cool when it gets too hot to hold for more than a second.  The number of rounds it takes to get to this point is greatly influenced by the ambient temperature.  On 100F days, this may only be 5 rounds, on 0F days it may be more like 30 rnds.

The total # of rounds for a range trip will depend on how many rounds you brought and want to fire off.  Functioning may start to get a little finicky around the 500-1000 rnd mark, all depends on the rifle and how you clean and lube it.