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When doing the final sanding/polishing of the barrel before bluing; should it be done longitudinally or circumferentially? What grit size (400, 800, 1200) is adequate for the final pass? Any guidance would be greatly appreciated.

Longitudinally. Worn 600 grit wet-or-dry paper gives a semi-mirror finish. Go up or down from there according to your taste.

RAN

Usually you will get a blacker looking blue if the polish is circumferential.
Generally a circumferential final polish at 400 grit is going to be pretty good. I usually finish with a dull 400 grit belt. If polishing lengthwise, by hand, I'll use a 600 grit and finish with a well used piece of 600. A lengthwise polish seems to give a bluer blue and definitely looks different than a circumferential polish.
A very fine Cratex stick is another medium which produces a fine polish lengthwise.
When using a belt grinder and spinner to polish circumferentially, one has to be sure not to let the belt plug up on the edges. If it does it will stripe the barrel and make a good finish difficult to attain.
When polishing lengthwise, I like to back the sanding strip with a piece of semi-stiff rubber. An old recoil pad will work fairly well. If polishing up to a sight base, one has to use a block with a sharp edge to back up the cloth.
For a real smooth hand polish, I use a piece of smooth leather which is charged with 800 grit emery powder and well used.
I have used felt wheels on occasion but really prefer polishing by hand. I always say, "It takes me longer to screw something up by hand and I might get tired before I do!" GD

Thanks for the help folks. And graydog, the info on the color difference is great. I will go with the lengthwise as it will help to pick up the blues in the case colored reciver and the "peacock blue" of the switch barrel mechanism.