RE: AR-15 ? Bolt won't stay open after last RND
 

most likely it is some sort of feed error from 1 or more mechanical errors occuring. I would guess it is a weak spring in your boltcatch on the side of your reciever. Personaly I would replace the bolt catch spring and it would be smart to replace the boltcatch button itself. Next check your bolt, examine the fins and look to see if there is any excessive wear around the portion that hooks onto the boltcatch tab. I suspect it is either of these 2 things. The bullet not firing on the 2nd shot is a classic case of a bad firing pin and/or bad ammunition. If it isn't extracting the bullet then examine the extractor and it'sspring.
What I would do is borrow a friends bolt, shoot a few rounds to see if it is in the bolt. If same thing occurs. Use your bolt in your friends gun and check. Generaly with 2 guns you can find any problem...

RE: AR-15 ? Bolt won't stay open after last RND
 

here are some referances for parts:

http://www.del-ton.com/AR_15_Lower_Parts_Standard_s/85.htm

http://www.del-ton.com/AR_15_Upper_Parts_A2_Flat_Top_Parts_s/93.htm

RE: AR-15 ? Bolt won't stay open after last RND
 

I don't know if you've done this already so please disregard if you have but make sure to completely field strip the weapon and clean it throughly.

After it's completely clean (including gas port) check it for function (feeding, extracting, locking open, etc...). If it functions fine then try different types of ammo. If you've been feeding it a steady diet of dirty ammo this might be your problem.

RE: AR-15 ? Bolt won't stay open after last RND
 

Then lube it until its dripping wet with CLP- ARs run better wet.;)

RE: AR-15 ? Bolt won't stay open after last RND
 

Yeah, do that 1st ofcourse :D

If you can get ahold of some Slick2000 gun lubeDO IT. Use after a thurough cleaning with CLP ofcourse...

RE: AR-15 ? Bolt won't stay open after last RND
 

i just remembered one of my bushmaster every so often the bolt would not stay open. did not happen alot so i never worried and i clean my guns every time i shoot. that crap is corrosive

RE: AR-15 ? Bolt won't stay open after last RND
 

"if you want peace, prepare for the lord"

RE: AR-15 ? Bolt won't stay open after last RND
 

My first inclination is also a magazine problem. Make sure that you try a different BRAND of magazine, I've two "cheapo" M14 mags, neither of which feed properly, but rifle works just fine with GI mags. I haven't found an AR magazine (GI) yet that didn't function properly in either of my Colt or Rock River rifles.

Otherwise, could be a number of issues, some which have been addressed already. Aside from magazine issues, the most common offender that I'veseenis a poorly assembled rifle, particularly a "kit" or "bargain" rifle. Rarely have I seen bolt hold-open problems or failures to fire from name-brand rifles.

RE: AR-15 ? Bolt won't stay open after last RND
 

And the problems with those can usually be fixed of you pull the FSB, carefully realign the fsb with the gas port, (if the fsb is misaligned and pinned the the barrel, your SOL unless you use sleeve and bearing retainer loctite to resecure the FSB) and make sure the gas tube is carefully aligned with the gas key- you might have to retorque the barrel nut to get everything aligned properly.
It doesn't take a great deal of skill to index and assemble a barrel/gastube/fsb correctly, but it does take a bit of time. Some of the bargain basement uppers are put together by the unknowledgeable bubba in his garage.