I used to turn cylinder drums for a living. We never turned the inside more than in inch or so in from the rims, but if you must, a jam chuck sounds like a good idea as Mr. Mouse suggests. But first, I would turn the outside.
The setup that is best used depends on the quantity of shells you will be producing. If you will be doing multiples, then the time spent fabricating jigs for the various steps is worthwhile. Otherwise you cobble something up and hope for the best.
We used plywood discs on a heavy steel mandrel to hold the shells, with 5/16" machine bolts tapped in short angle iron brackets pointing outwards. 6 to 8 of these screwed around the perimeter of the disc acted as internal adjustable "chucks". Two per drum, one at each end.
A 6" depth for the 14" diameter is manageable enough where the above technique is probably overkill. Instead, just turn a big solid wood plug to jam the shell on with a friction fit. Think about making the plug a bit less than half the length of the drum shell, so you can turn both outside and half of the inside--then flipping it around and jamming it on another plug that fits the turned surfaces--either the inside, or the outside..
A long-and-strong gouge with a swept-back grind (Ellsworth grind) should work for both outside and inside.
If the O.D. and/or I.D. need to be sized accurately, I might use a Jamieson bar sliding along stop blocks attached to the front and back toolrests. Just a thought, never tried that in a real situation.
