Well after a long wait (have had the rifle for about 3 years), I finally got out to do some 200 yards shooting with my Bushmaster DCM rifle.  The rifle, after today's range visit, only has about 200 rounds through it.  Prior to today's visit, I had fired about 130 rounds through it with NO problems of any kind.

Well today, after about 15 rounds, the rifle started to short stroke.  The cases would eject, just barely as I could see them land a few inches from the rifle (I was prone) and the bolt failed to pick up a fresh round.  I tried the ole load one round at a time to see if the empty mag holds the bolt back but no luck, bolt just did'nt come back enough.

Tried different ammo and mags, no help.  Tried a totally different Bolt/Carrier group, no help.  Tried my DCM Bolt/Carrier group in another AR and it shot fine.  This leads me to believe the Bolt/Carrier is not the problem.  I checked gas rings, all ok.  Checked for signs of leaks around the key, no signs.  Again, swapped Bolt/Carriers w/ another AR and all worked fine but the DCM.  Bolt/Carrier and rifle were lightly lubed as I always do.

What am I missing??

ETA:  I noticed that when cooled down, the first two or so shots were fine, then the problem starts.  The temp was about 85 degrees, heat a factor??

Possible that oil put in the weapon 3 years ago turned into a solid somewhere in your gas system.  You might try using some type of solvent in the gas tube, or hold the bbl. upside down and fill the bbl. until solvent gets to the gas port in the bbl.  Or spray carb cleaner into the breech end of the gas tube with the thin plastic tube that comes with the can and see if it comes out the gas port into the bbl.

Quoted: What am I missing?? ETA:  I noticed that when cooled down, the first two or so shots were fine, then the problem starts.  The temp was about 85 degrees, heat a factor??

You said Bushmaster!!!!!
Around here, the just sends off the alarms to check the key screws to make sure that there tight (35 in lbs).  From there, check the front sight set screws to make sure that they are tight, and finish off the trouble shoot by giving the chamber a real good cleaning (making sure to get all the fouling out of the last chamber cut just before the throat). Helps to spin a 30 cal mop wet with solvent to confirm that you have all the fouling out.

To add, if you made any adjustment to the front sight tower to zero it out, this may have blocked the barrel/tower gas passage (barrel not indexed with the sight tower).  The easy way to confirm that the passage is not blocked is to just spray some CLP down the tube to make sure that is flows into the barrel bore (CLP will also loosen up any slug that may have hardened and blocked the gas tube/port passage while the rifle sat stored).

To sum it up, since your getting a few rounds to cycle with the rifle cold, then as the rifle heats up, it starts to short stroke, you have a leak in the gas system (metal stretching as it heats up and allowing the leak). Compound this problem by possible a fouled chamber that could be making the rounds act as if there over pressured (chamber side walls lubed, then the CLP burn out after the first few shots) and Viola; Short Stroke City.

Note: You use the term lubed, hopefully this means CLP, and that the all of the upper bearing surfaces, including the buffer, recoil spring, and receiver extension cavity have been given a light coat of lube. At 200 rounds, consider the rifle still new, and still breaking in (upper surfaces still mating).

Quoted: Gas tube roll pin installed?

Epic,  
I can't wait to see where this is going!!!

Thanks guys, yes, gas tube roll pin is in place.  Will double check all above.