I am fairly certain that Colt M4's and 6920's have the same port size.
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They do, though he is right that a smaller port could be used.
I'm fine with my rifles not running .223.
Two things:
(1) The ST-T2 was designed for Spike's rifles that are tuned for that specific buffer... not yours.
(2) That buffer is way too heavy for the size of the gas port you're running. Try a regular spring and Carbine buffer.
+1
Send it back to DD and they'll get it fixed for you.