I can think of only two 'reasons' for a given barrel length, reasons as contrasted to 'fashion' considerations.

A given cartridge may need a minimum length to develop the desired performance at a given range, but after that, additional length is not anything other than there for looks or balance, and rarely for the latter, as I will pontificate on infra. My 28 pound, single shot, custom built and loaded 300 Ultra Mag uses a 30" tube plus a 2" brake. That barrel length is my 'load-specific' ideal length for the informal (but still hotly contested among us old farts) 1000 yd BR competition. For hunting and factory loads, you can take a 300 UM and chop off about 4" and the ballistics for that intended use works fine. The shorter length makes it much easier to carry. My Long Tom arrives at the bench in a wheeled case from a car parked as close to the bench as possible. Balance isn't an issue, as I'd no more shot it offhand than Osama would decide to convert to become a Baptist.

Now take a 470NE modern DR regulated for Federal factory loads, as many are. The rifle is expected to be used at under 100yds and, unless you are working with a PH who likes to see an elephant wounded by a client shooting from too far out or pot shot a buff at that range, you are going to be shooting at half that distnace or less. The 470 is designed to deliver its maximum punch at these shorter ranges and is loaded with a powder that will let it develop its full potential in about 24" to 26" of barrel. No need for a 28" set of tubes. Indeed, they are a pain in the butt since they will be far less handy in brush and other conditions where DRs are intended to be used. (You can even see this in bolt action DGR threads, WR bolt rifles often came with a 28" bbl was too long in heavy cover. Most custom DGRs I see now are much shorter, and I have seen a few under 20" for close in work.)

But ballistic considerations is not the end of the bbl length issue, especially on a DR. A DR should handle like a good shotgun, and balance and ease of handling are critical consideriations when you want to avoid, as said elsewhere on this site, becoming elephant toe jam. A DR with a set of 27 or 28" bbls may balance well but be hard to swing around and bring into use in heavy cover. With proper attention by the builder, even a set of 20" tubes handles just fine.

My Merkel 141 in 30-06 with scope (factory mounted and balanced) with a set of tubes just under 20" is perfectly balanced. It is sized and balanced to feel light and handy like a good 28 SXS shotgun. My hedavier Merkel 160 in 500NE with 23.6" tubes is larger, but it is just as well balanced and, while obviously a touch slower to swing because of the difference in weight and bbl length, handles very nicely.

Now, I tried a Merkel 140 in both a 375H&H and in 500. The former felt very nose heavy and not at all easy to use. Too much mass in the barrels. But, picking up the 500 sitting alongside, with the same 23.6" bbls, the lighter barrels (larger bore) made for a better balanced gun.

The answer? Other than fashion, I see no reason to go longer than you have to on barrel length, assuming the gun is balanced properly. Go as short as practcal for the load. If a manufacturer tries the 'one size fits all' method, and uses the same length for all chamberings, I'd make damn sure it balanced proprely before I laid down a dime.

Dave