Need some input from the knowledge base here.

In the last two 3 gun matches I've had a rifle run perfect all day until it didn't.    5 stages in and suddenly I fire two rounds and double feed, drop the mag, clear the double feed, grab a new mag off the belt, charge, fire one round, double feed....drop the mag, clear, re-insert mag, charge, one round, double feed, repeat, switch back to original mag, repeat another DF.   Gave up the stage at that point.

The first time it happened I tore the rifle down, inspected everything, checked the mags etc, found nothing,  Chalked it up to operator error when loading rounds into the mags and not getting everything seated right.   put it back together, ran it at the range and life seemed good.    Now it's screwed me in two matches and while, knowing me, its probably still operator error I don't know what else to look for and I've lost confidence in it running when the timer is going.

I know 99.5% of the time its a mag problem and not a rifle problem but, when it does it, changing mags doesn't seem to make a difference.  

I've fired hundreds of rounds at home and in practice and never been able to replicate the problem, and like I said, it ran perfect through the first 5 stages.  

The only other thing I can think of is BCG since that's the only other thing that comes into contact with the rounds but no clue why that would make any difference.

The particulars:

Gen 3 30 round P-Mags with TTI extensions
Rainier Arms forged upper without forward assist
Rainier Arms 18" Select stainless barrel in .223 Wylde
Forged Lower, milled from 80%.
AR-Gold trigger group
Rubber City Armory light weight BCG
Seekins adjustable gas block
Standard weight rifle length spring and buffer

thanks in advance for any insights

My rifle started doing the same thing, first with one mag, then with others. The gas system was starting to plug up to just slightly short stroke at times. Problem is made a bit worse by the adjustable gas block which is easier to build up carbon in.

Now that is an interesting thought.

I did notice that this rifle dirties up the exterior of the barrel around the gas block way more than any other of my rifles like the gas block has a significant amount of blow by. I'll check for leaks and make sure everything is right with that.

If the rifle short strokes enough to keep from ejecting spent brass, it should be too short to pick up a new round from the mag tho, shouldn't it?

Re check tune on adjustable gas block.

Are you shooting reloads or factory?

How often do you clean rifle?

Extractor? Extractor spring?

How old is buffer spring?