I was told from the guy I bought it from it did run. He said he ran a 20 round mag through it and it worked fine. I'm not sure how much he used it before that. When I got it, it had a M16a1 upper. I fired 20 rounds the ran perfectly. The next mag would not cycle. Some times there was one in the chamber with the hammer up, some times there was not one in the chamber. I figured it was a gas problem with the upper so I wasn't too worried.
Yesterday I put my LMT 10.5 " upper on it, and a new VLTOR collapsible stock. It rand maybe 3-5 round bursts then maybe one round, to three rounds at a time. I took it home again and replaced the entire FCG including the sear with almost new parts that I semi recently used in another gun that worked 100%. Took it back out today, same thing. I've now replaced every single part on the lower other than the bolt catch and mag release. The upper has no issues I've used it on other guns in auto recently. So now I'm thinking there is an issue with the lower. I'm really hoping I' wrong about that. Has anyone else dealt with this or have any suggestions?

More info:
I used mostly 223 Winchester Ranger ammo. It's always worked before. I also had some old PMC, and some Prvi. I do have some M855 to try still but all of the other ammo has run through this upper before.
I'm also using a standard carbine buffer not a "H" buffer.

I thought it was probably the sear engaging too soon, but I have no idea now that I'm using a sear that I know works.

The conversion was done by Neal Smith from Mentor, OH. What I read about him was that his conversions were top notch. That was one factor that made me want this gun.

What mags?  Are they known to be reliable in other guns?

Mostly gen two and three pmags. I did use one aluminum mag too. I have used all of them before with no issues.

Could just be bolt bounce. Could be the buffer spring has seen better days.  Do yourself a favor now and just go to the A5 buffer sys.

Yep, I'd try new springs and a heavier buffer.

First swap to a heavier buffer. If that doesn't solve your problem search for Dano's post on checking the timing.

Get a heavier buffer especially with a short barrel. My M16 will in no way shape or form run full auto with a carbine buffer, it needs bare min a H1 and cycles best and ejects best with a H2

I honestly didn't think that would make the difference. I have a H and H2. I'll try them both. I'll let you know how it works tomorrow.

+1 A5 buffer system is the best as long as you are not concerned about 1.5" additional OAL when stock is fully pushed in.

It's a brand new Vltor stock I put on it. It came with a buffer and spring so I don't think the spring is a problem.

When i bought mine i assumed everything was fine. When i finally got around to rebuilding it into a MK18 config i took the buffer tube off to find the cap of the buffer weight had come off and the weights had turned to powders, also the spring was not the correct length.

For the money you put into a transferable its worth IMO just completely rebuilding the rifle head to toe with brand new factory mil spec parts. I bet once you put all new parts and a havier buffer weight your issues will disappear. Mine runs flawlessly, thousands of rounds without a single jam.

Thus the A5. No more playing musical buffers every time you switch uppers.

Well that leaves the spring out then. Had you gone to their A5 system to start with. You might not be having this conversation now.  No one I know of has regretted going to the A5 on their M16. Even if it doesn't fix this problem . It will still work better with all the upper you use and reduce your rate of fire some.

I got it running. So it was the buffer spring. The new spring that came with the Vltor stock is a XP spring. I put a old spring it it and it ran fine. It worked well with standard buffer, but it also ran with a H and a H2. I left the H2 in it for now. That was truly the last part I suspected would cause a problem. I've seem worn springs cause issues but never this. Thanks all for the feedback.

I don't believe you. I require video evidence.

Glad you got it working. As peekau said pair some pics/videos if you get the chance

http://vid22.photobucket.com/albums/b318/bruh44/Mobile%20Uploads/0429151203_zpszel8ibqd.mp4

Here's my mother-in-law shooting it.

I took the advice and used the A5 system.   No test fire yet but I went with Sprinco green and A5H2.  Good so far?