I vaguely remember reading an older article on this, i believe back in the late 1980's.
In general ( remember i have slept once or twice since then), I _think_ they determined that for _any_ caliber, a barrel whose length was more than 100 times the width of the cartridge in question was losing more than it was gaining.
( the above is also reffered to as another defination of "caliber" of large cannon, like the IOWA class BB's 16 iinch, caliber 50 cannons)
We've learned alot since then, and we all know there's people out there who will put WAAAAYYY to much effort into something, simply to prove it's possible, no matter whether it's PRACTICAL or not.
So i guess i'd say this Jon, you could setup a special purpose, specific use rifle with special loads, and haul it in pieces to be assembled onsite with a 50 inch barrel if you wanted,but....personally, under most circumstances i'd say 24-26 is more than enough, and i'd be fine with 22 damn near always
take the above with two grains of salt, a lime and tequila, then post response in the morning :P