Lemme say I'm not an AR wizard, I have a working knowledge of these guys, but I'm not expert. I have a Colt SP1 and a range of upper. 7", 11", 16", and the original 16" A2 upper, as well as a Colt 6920 A4 upper.

So I went and shot it today, and it had a lots of issues with the 16" A2 upper and the 6920 upper (M16 bolt), It seemed like there were two issues, it would load the first round that I manually loaded, and the second round it appears the bolt wouldn't go back far enough and it would just close empty (without loading a round), or it would also not load the next round completely, ie it would start to load the next round but would stop short of going into full battery.. This happened on both 16" uppers.

Now swapping to the 14" colt A2, or the 11 CMMG A4 with an M42000, or the 7" CMMG A4, all functioned just fine..

I'm not particularly fond of the 16" uppers compared to the short ones, but I would like to know what the issues is. From what I can tell the lower, it has a Standard H buffer in it (it's not marked so I could be wrong). I prefer the short uppers so I don't want to modify the gun where as it would shoot the long uppers and not the shorts, or put any stress on the lower. I tried an H2 buffer and the same problem existed.

I suspect the buffer or the gas systems configuration in the two 16" uppers. Does anyone have any advice they can give me here.

Here is a video of the 11" with m42000 and the 7" upper.. I didn't take one of the 16" but they functioned badly..

https://youtu.be/u4MJvUk5MU8

https://youtu.be/vU1x5ngUXNg

Yes, I was using the same BC for all the uppers..  The receiver is a RR so there is no DIAS or LL here.  The same problem happens in SA and FA...  The 6920 upper I put on it works fine with my 6920 lower, but it's a different BC when it's on that lower..

I'll look at the bottom of the uppers.. I didn't think to look for that.  The long upper was probably the one that came with the gun before it was converted by John Norrell..

Thanks, I'll take a look..

You're right I suck at  AR machine gunning so far.. I've had it three days now and shot it once, I should have better skills by now :)..  But that's ok, until I hone them, I still have nun chuck skills, bow hunting skill, and computer hacking skills..  I just need to get the machine gun skills mastered at this point.

A SP1 is a early AR-15.  The upper tghat cane with a Colt SP1 should have been a A1 fixed carry handle upper (adjustable for windage only), without the elevation adjustment (A2 type carry handle upper receiver).

An unmarked buffer should be a standard carbine weight buffer,  Not a H buffer (unless someone changed it from a standard buffer to a H buffer by removing the end piece and inserting one heavy (tungsten) weight from a H or H2 buffer).

What are you using for ammo?  Could be it's a lttle underpowered.

Have you tried shooting with Lake City M193 or M855?

I have the pin adaptor.. The lower is the large pin.  The uppers are all mil spec except for the original.  

Also, everyone, I looked and the original upper doesn't have the auto relief cut, but the colt 6920 upper does (and doesn't function).  They both failed.  All the other uppers, the colt 14", and the CMMG 11 and 7", do and they function well.

Without the relief cut, an upper will not function in auto with a registered receiver - the auto sear is prevented from moving to catch the hammer by the metal of the upper.  Semi auto should be fine.

Most of the rest of full auto problems are associated with either under gassing (weak, cheap ammo, or too heavy a buffer), or over gassing (hot ammo and a bad spring or light buffer, or oversized gas port).  Add in bolt (carrier) bounce for additional troubleshooting fun.

Carbine receiver extensions and buffers are the bane of full auto.  Most registered receivers and DIAS and LL simply work with 20" uppers and rifle receiver extensions and buffer/spring combos.