Weapon: M&P15, early example. 11,5 Daniel Defense barrel, so carbine gas system. Original BCG/Bolt (AR15 style). Carbine stock, normal length carbine buffer tube, Brownells CS recoil spring. Carbine weight buffer.

Runs perfectly, wet or dry, supressed/unsupressed, dirty or clean. No failures/issues during last 5k rounds (I swapped new DD barrel), super reliable. Ejection pattern is about perfect 3-clock, 2-clock with supressor.

Since original BCG, bolt included, starts to have 20k rounds through it, I bought BCM M16 BCG (so slightly heavier) and I get weird recoil sensation sometimes, maybe once per 50 rounds: After round is fire, I clearly feel BCG slamming forward, as if slightly delayed. It clearly separate from normal recoil.

Buffer tube is straight and round (measured that). There is about thumbnail size wear pattern inside buffer tube, on the bottom maybe 3/4 inch down the buffer tube, it could be caused by BCG cycling dry for 20k or so rounds. Not sure if this pattern is normal. There is wear at mouth of the buffer tube which would indicate carrier tilt.

I thought extra friction caused be increased surface material of M16 BCG (compared to AR15 BCG) might cause BCG to stick or something?

Any good ideas? What could cause the weird sensation of feeling bolt slamming forward.

Pull the trigger to the rear and hold it there with good pressure. Try to cycle the action using the charging handle without letting up on the trigger pressure.

Does the BCG stick to the rear when applying pressure to the trigger?

Have you tried putting back the original BCG to verify that the problem tracks the BCG?

Yes, original works perfectly...

Are you describing bolt bounce?      Have you tried an H buffer?

I checked the buffer - that was a good tip.

There are wear marks in the buffer which means the finish in the buffer tube has been scrubbed Off by the buffer itself, not BCG(s) as there no significant wear at bottom rear of the BCGs.

However there is wear on 10-11 clock at the back of the BGCs. Old BCG has been worn shiny and smooth, new one is not, but shows wear. Is this normal?

This is a particularly high round count rifle. As a military armorer it was not unusual for me to be replacing receiver extensions on weapons, particularly carbines, with similar usage. I guess what's more accurate is when I did replace them, they had fewer rounds. When barrels were shot out, they were rebuilt somewhere else and got new receiver extensions and other parts, though,  we never saw the particular weapon again. We would usually get another rebuild so I could see what was fresh on them.

Rifle length weapons were a little easier on receiver extensions. I think it was probably the longer buffer that helped.

High round count?

Not really, just old and has been in use for 15 years...

I just replace springs and other "wear items" regularly and keep accurate round count and maintence log.

Maybe its then a time for new buffer tube as part of annual overhaul...

So far the rifle has required few bolt rebuilds (gas rings, o-ring, extractor, spring), recoil spring, buffer and one Major overhaul with barrel, gasport and gas tube. Thats it...

All normal.

Nothing wrong with your tube -- no need to spend money you don't have to.

Can't tell you why the new carrier feels different -- perhaps new parkerizing needs to wear off to make travel smoother.  A heavy carrier is also going to travel differently.  In my experience I've never seen a carrier that needed replacing.  At 20,000 rounds I would have just replaced the bolt and maybe the carrier key.

If it still functions correctly there's no reason to spend money on different buffers.

20k rounds is a lot of usage at any age. It's good that you keep up with maintenance items. In all honesty, visible wear at that round count isn't really surprising. The new BCG is jamming because it isn't "worn in" with the receiver extension. It's  not that it should be, it's that the old one simply worked because they wore in together. I would venture a guess that, as long as the upper is still within spec, a new receiver extension and perhaps buffer will probably cure your issues.