Greg -
You've moved, I think, beyond semantics into pedantics. Most people - PhD-level archaeologists included - wouldn't make the distinction as to the size of the cask. The terms "cask" and "barrel" are, to them, fundamentally interchangeable (just like "keg" and "cask").
Second, I found the following support for iron-bound casks, some of which make no real reference to size:
Emphasis mine.
If "cask" were a measure (not to say you're claiming that; it is to illustrate the level of pedantry we can sink to), or if every cask aboard ship were of similar size, there would be no need to mark the head of the cask.
In 1699, Parliament issued victualling instructions for the Lord High Admiral as follows:
Again, emphasis mine.
Note that exact measure is unmentioned in the case of beer, and specified in the case of water. This is puzzling, and no substantiated reason for this exists. One might make the argument that beer, being much more important to the physical and mental health of the crew than water, was in such a constant state of flux that specification of amount was considered unneccesary by the authorities, but no supporting documentation for this can be found.
To branch out into pure conjecture, the etymology of the term "scuttlebutt" gives support(!) to the standard cask size being the butt, not the barrel or tun.
At the end of the day, it is unwise to take one extreme or the other. There is support for both forms of cask-binding, depending on the intended purpose of the cask.
From a practical, reenacting standpoint, we must refuse to let the best be the enemy of the good. I'd much rather have an iron-bound firkin in camp - even though there should be a wythe-bound hogshead - because it's better than an Igloo cooler. :)
Also, until you stand next to one, you don't really know how big the larger casks are. The biggest cask anyone not employed in the beverage alcohol industry has ever seen is the half-bbl (US measure) keg. A full bbl is twice the size of that, and is in reality even larger, because it's made of oak staves and not welded stainless steel sheets. I don't know about you, but I ain't gonna try and 1.) find a blinkin' hogshead; or 2.) try to fit it in my vehicle to take it to an event.
(1) William Mountaine: The Seaman's Vade-Mecum, and Defensive War by Sea: containing the Proportions of Rigging, Masts and Yards Weight of Anchors, Sizes and Weight of Cables and Cordage, List of the Navy. The Exercise of the Small Arms, Bayonet, Granadoes and Great-Guns, Duty of Officers, &c. also Shewing how to prepare a Merchant-Ship for a close Fight. W. and J. Mount & T. and T. Page, London, 1756
http://www.bruzelius.info/Nautica/Provisio...(1756)_p71.html
(2) House of Commons Journal Volume 12 - 12 January 1699
http://www.british-history.ac.uk/report.as...sp?compid=39699