On black Friday my father and I purchased a new Bushmaster XM-15, a Nikon Prostaff 3-9x40, and 2 50 round boxes of hornady ammo from carters country. Once we got it home, we took it apart and cleaned it. We shot our first box at 50yd and 100yd targets without a problem. After that we progressed to the 200yd targets and all hell broke loose! It would jam almost every shot. Can anybody explain this? Did the gun get to hot? to dirty? or was it a bad box of ammo? Ive read about taking out the bolt and carrier and letting them soak in mobil 1 motor oil overnight. Should I try this?

Without knowing any more details first I'd try a different mag.
Good Luck. Most of those factory mags are junk.

With AR's diagnosing problems with feeding start from the begining.

Magazines are key componenet thats overlooked alot These load the ammunition into the rifle so if there bad it throws everything off. Did you try differant magazines ? or more than one ? If not Try and get ahold of a MagPul P-mag they work great.

Next Bolt Carrier Group, do you have it lubed enough and the chanel it rides in ? With AR's it should bleed out every exit if done right the first couple of times. Your trying to get it into the pores of the metal so the more the better. You mentioned Mobil 1, that is a great place to start.

Is your Gas Tube and Index Key Clean ? Another often overlooked problem that can screw the whole system. If there not a good Q-tip can clean the surfaces quite well, and make sure the key is clean as well. If it is clean, is it properly torqued and staked ? Can you wiggle the key ? this can also cause problems and belive it or not it happens alot.

Next is ammo, but since it functioned good for the first time rounds it should not be that but its worth a checking if nothing else.

Please give us anymore info you can, if something doesnt make since or you don't understand let me know I'm trying to find pics to help explain.

Here is a picture of what a properly staked key should look like.

Thanks for all the help. I just lubed it up pretty good w/ some motor oil. The gas keys were staked correctly. I will look into getting some new magazines.

Save yourself time, heartbreak and [censored].

Buy a couple 20 and 30 round Magpul PMags. They're the best on the market and now US GI Issue (they have NSNs).

Use Slip 2000 lube or Militech. If that fails, Breakfree (used for 40 yrs on the M16, it's better than nothing).

Don't worry about the gas tube, if the carrier was properly staked, it's either ammo or mags in a new rifle or lack of lube.

ARs, like women, run better wet. And no, i didn't come up with that, but it's still true.