I have an LR308 AP4 that has been very reliable so far (except when suppressed).  When suppressed, it would fail to eject about 2/3s of the time.  I tried adding an Armalite buffer spring (didn't help), a CAR Heavy buffer (didn't help), and a brand new extractor and spring (didn't help).  It still only had issues when suppressed, however.  A few weeks ago I went to the range and at some point blasted off 5 quick rounds two different times (we rented a bay, much fun was had) with no problems (unsuppressed).  Then I went to the lease and had a FTE on the first round (unsuppressed).  I attributed it to the Magpul magazine being able to be inserted too far (see my other thread) and fixed that problem with the mag tab adjustment.  Went the lease the next weekend and it jammed again (unsuppressed).  Went to the range today with a DPMS mag and it jammed so I just shot with no rounds in the magazine (unsuppressed).  

Is my gas system dirty from shooting suppressed?  It is well lubed and relatively clean.  I'm shooting Federal GMM 168 gr and I have a few hundred rounds through it without problems (other than this one).  If so, how do I clean it?

Thanks.

NOTE: GUN IS FAILING UNSUPPRESSED NOW.  I FAILED TO MENTION THIS IN THE ORIGINAL POST.

Gas system is self cleaning.  As you know suppressed causes more gas likely issue is not a heavy enough buffer/spring combo.  Talk to Slash about one of his buffers.  heavybuffers.com  Ought to solve it.

A suppressor will dramatically increase the chamber pressure and can cause the extractor claw to skip over the case rim, leaving the empty case in the chamber. This is the classic Vietnam jam that earned the M-16 its reputation for poor reliability. Since the mod for extractor spring and heavier spring/buffer did not help, your best remedy is to install an adjustable gas block or adjustable gas tube.

Now I see that I did not state well enough that the gun is failing UNsuppressed now.  This is a new problem, either unrelated to the suppressor, or related in that the suppressor has fouled the gas system or worn something out on the gun.

OK, for some diagnosis of the problem, we need some more information.
Are you using the factory A-frame gas block? If not , what are you using?
What is your current receiver extension, buffer, and buffer spring combination?
What extractor and extractor spring are you using?
When you fire without a magazine in place, do you get the same malfunction?
When you fire with an empty magazine in place, does the bolt lock back on the bolt catch?
Can you manually lock the bolt back?
What position is the empty case in when you have the malfunction?
Is the bolt carrier key loose on the bolt carrier?
Does the bolt pick up a new round  when it malfunctions?

Karl

The buffer is most certainly a 308 buffer (shorter).  
The spring is slightly longer than the DPMS spring when extended (1 cm).
The spring has only one flat end, and I did not have that end toward the buffer so the end of the spring was actually digging into the buffer.  I switched it.
The DPMS spring is flat on both ends.
I'll verify the round position in the chamber next time I shoot it unsuppressed.
I wish I had brought both springs and both buffers to the range with me, but I will be sure to do that next time.

I don't think I have a serious issue in sizing or anything because, as I mentioned in the first post, the gun has been flawless when unsuppressed since I bought it.  It only started jamming when unsuppressed a few weeks ago.  The time before I shot 5 quick rounds two different times (unsuppressed) with no problems.  Next shot at the lease was a jam (unsuppressed), and it has been jamming almost every time since.  When I shoot with no rounds in the magazine the round stays in the chamber so it is not magazine related.  

I just cycled it manually and it was fine as long as I let the bolt carrier come in hard.  Once I rode it in and it didn't fully seat.  (baby is sleeping, wife complaining  ;) )

Check the ejector and spring. Sounds like the case is staying on the bolt face too long and hitting the back of the ejection port at a shallow angle. Inspect the cases that malfunctioned for a half-moon cut/scratch on the lower half of the brass. Let us know what you find.