You got it figured as far as I know. I always thought that shotgun capacities were quoted for 2-3/4" shells and didn't even know 6 3" would fit my 870 security model. Does it flatten the spring? I am not sure about that. Some people say that modern coil springs don't lose energy from being stored compressed. If you are really jamming in that 6yh 3" shell, it is possible that inexpensive shot shells may mushroom and cause a jam.
I would refer the question to Remington. Actually, I checked its web site as I have that model and it says
Quote:
Magazine capacity is six 2 3/4" or 3" shells
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http://www.remington.com/products/fi...c-7-round.aspx
I feel better as I keep 6 2-3/4" in the tube and now I know that leaves some room.