A buddy is having cycling problems with his AR-15 with a 18" barrel w/rifle length gas system.  I took it out for him today and experienced the following:

Mil-Spec buffer and buffer spring
Magpul mag
5.56 62gr
would eject but not chamber the next round

Spikes T2 buffer and mil spec buffer spring
Magpul mag
5.56 62gr
ran reliably but would not lock the bolt following the last round

Spikes T2 buffer and mil spec buffer spring
Magpul mag
5.56 55gr
ran reliably but would not lock the bolt following the last round

Spikes T2 buffer and mil spec buffer spring
Thermold mag
5.56 62gr
ran reliably but would not lock the bolt following the last round

Mil Spec buffer and mil spec buffer spring
Thermold mag
5.56 62gr
ran reliably but would not lock the bolt following the last round

We have tried two lowers and two BCG as well with same results.

Any ideas to correct this?  I have not measured the gas how in the barrel but would it help to open it up a little?  Any opinions on what the hole size should be?

Thanks
Sam

Judging by the info you provided it sounds like it may be a buffer weight issue.

Try a heavier buffer than the carbine buffer(3oz) and Spikes T2(4oz). The H2(4.7oz), H3(5.6oz), or Spikes T3(5.4oz) buffers are similar in weight to a standard rifle buffer(5oz) commonly matched with rifle gas systems. I'd recommend trying a heavier buffer before anything else.

Sounds to me like a gas issue. I had the same problem with a build last year and it was because the gas block hadn't been properly installed and was canted, letting in just enough gas to cycle the bolt but not to hold open after the last round and occasionally not eject the spent casing. Straitened out the gas block and it's never had an issue since. I would check the gas system to see if its under gassed.

Sounds undergassed to me.

Does it have a pinned FSB or a low-profile gas block? If the latter, the first thing to check out would be whether or not that the gas block is centered over the gas port. If it is, then I'd measure the gas port (which if you don't have pin gauges, you can use the uncut ends of some drill bits. See which one fits the tightest, then convert the fraction into a decimal to get the port size). 

Also, what ammo were you using? Was it good, quality 5.56 pressure ammo? Weak ammo can cause cycling problems. 

As for gas port size. I'm no expert there, but I'd guess that your port should be at least .085". If it's smaller, I'd have a competent person open it up, going no more than .005" at a time (cut, assembly, test, etc.). I had an 18" barrel that had an undersized gas port, which I ended up opening up to .101" to get it to reliably cycle (this was for a wildcat caliber, not 5.56). It took two or three times of opening it up to get it right. We were only taking 3-5 thou off at a time. If you go slightly over, it's not big deal as you'll just be a little overgassed (which some prefer). 

What barrel?
Id check gas block alignment and gas port size.

.100 to 105 is the spec for SPR but I think it's a bit much.
Even the weakest ammo on frigid days will cycle with that spec though.

I like to run rifle spring and buffer or A5H2/3 buffer and rifle spring with an adjustable gas block with those barrels

Had already looked at the gas system and there was no issue with it.  Evidence of gas block on barrel showed the gas port was aligned well.

Thanks guys, I think I am going to pull it back apart and measure the gas portion the barrel.  I agree it seems undergassed, I could probably open it up a bit myself with a good drill press and some fixturing.  

Then next I would check the gas port size. It seems undergassed.

I know of gamers that run high 80'- low 90's for ports but they run light BCG's and such.

For standard stuff I'd want .095 to .100.
With the.104 port on my barrel a adjustable gas block really helps as I'm not arctic ninja commando.

Any updates OP? 

Did you measure the port? 

all of the bold indicate too low a bolt velocity.

More gas, or a lighter buffer.

You could use a lighter than 3 oz buffer. KAK makes a kit with aluminum buffer weights. You can even just take all of the weights out of the buffer and try that. Once it is broken in you could try to add the weights back. Always start with one round in the mag when range testing. If it does not hold bolt back with an empty  buffer then the gas port will need to be opened up. Adding a suppressor may fix the problem. Ig you have a friend with a suppressor you could try that to confirm the under gassed situation.

Try a GI mag.

.100+ is much too large.

My suspicion as always with modern builds is that the gas block is not aligned correctly to the gas port. A very common issue is seating the gas block against the handguard retaining shoulder on the barrel which usually results in misalignment.