Ok guys,
I've done my homework and still need a few view points here.
My setup:
Sig PM400 AR Pistol 11.5" Carbine
Swapped factory parts for:
KAK SB15 Dedicated Pistol Buffer Tube
Tubbs Flatwire enhanced buffer spring (not trimmed)
Have tried CAR, Spikes ST-T1, Spikes ST-T2, and have ordered but not yet received or tested BCM H3 buffer.
Upgraded my extractor/ejector springs to Tubbs in all my BCG's.
Commonly using 55gr FMJBT. (Wolf Gold, PMC, Federal 193 etc..)
On a hot day in the desert I was running the Sig 11.5" With One round in the few different mags I used, the bolt would lock to the rear. Assuming its not under gassed.
Spent casing would just sit in the upper disconnected from the bolt laying there. More commonly, Empty casing not ejected jamming up against the next round trying to be fed into chamber.
Read up on Dwell Time and that is why I purchased the BCM H3 buffer to test out in with the Tubbs Spring.
Seen online about trimming down the Tubbs spring. Skeptical because I have the Tubbs Spring in my 7.5" combined with ST-T2 and it's running great. Also have the Tubbs in my 16" middie and 18" middie never had to trim a thing with those.
Wondering if its a combination of the KAK Buffer Tube I bought with the Tubbs spring.
Always assumed with shorter barrels, slowing things down was a good thing. At this point my tests will be Tubbs spring with H3 buffer and changing back to the standard MIL CAR spring w/ different buffer weights to see whats best.
Like Tubbs because of the snappiness, enhanced life of spring, and the less Spong sound inside the tube. Tubbs springs are working great in my other 3 rifles.
I have read across internet travels to apply the O-Ring to the extractor spring which I have done now but have not yet tested. I find it hard to believe that the enhanced Tubb Extractor+Ejector springs aren't enough on their own. So I threw an O-Ring on their anyway.

To me it sounds like an extraction/ejection problem. I have a 11.5 and a 10.5, never messed with heavy buffers. Try getting all new ejector and extractor parts, or a complete bolt assembly, you could always use a spare bolt anyways.
Your spent case is comeing out of the chamber, but not being ejected out of the receiver, right? And your bolt locks back?

Bolt locks back no prob.

The extractor/ejector springs are upgraded Tubbs enhanced. They seem to work fine in all my other BCG's and I switched out the particular BCG that was failing in the 11.5" and swapped in the Nickel Boron that runs great in my 7.5" with no luck. I did recently add an O-Ring around the spring to see if that will help next range trip.
The particular BCG I am using in that 11.5" is a Titanium Nitrided BCG from Parallax Tactical. Great BCG with upgraded Tubbs extractor and ejector enhanced springs. There is nothing visually wrong with the Extractor either. Doesn't have that many rounds through her.

Sometimes it looks like the empty casing doesn't make it out of the chamber completely, but It could also be visually confusing me because maybe its sticking to the bolt and jamming back towards the chamber after failing to eject.
The next live round does want to go up so I know the bolt is coming far enough back ( i think)

Like the other guy said it sounds like ejector problems.  Just cause you upgraded doesn't mean you already had the solution.  Try a different bolt with the oring installed and see if it still does it.  If so it might just be a tight chamber or need some serious cleaning.

Thanks again,

Last night did a thorough cleaning and installed the O-Rings on all BCG's. We'll see if that does the trick.

Next range visit:
Testing GI MIL spec buffer spring that came with the rifle and standard CAR buffer. Will interchange buffers if needed after the spring swap.

Also, can't wait to test out the H3 from BCM.