Re: max effective barrel length for .243?
I've been running .243's in my tac rifle match guns for years and burned out more barrels than I can count. Mostly Bartleins with some Kriegers too. I've always built mine with a 7.5 twist and shot only 115 DTAC's. They've all been 26" MTU contours and velocities run around 3K. I could get more out of them but I don't step on them too hard so my brass will last longer. My load was usually Lapua brass, either CCI BR2's or Fed 210 M's, and H4350 powder. Some of my customers ran 4831 short cut and had good results with it too.
Those were heavy rifles for tactical/practical style competition so probably not a good way to go for a hunting rifle. I would recommend you have your smith call you and tell you what he gets for velocity loss after every inch. I bet you don't see a whole lot of difference down to 25" and I would stop him at 24". I would think you could get a 105 Berger close to 3100 fps from a 24 and it would be a little nicer in the field than a 27.
As a side note, barrel length is measured by the actual barrel itself as it lays on a table. Not from lugs or anything else, just the actual barrel. Depending on the receiver manufacturer and the lug thickness there's usually about one inch of barrel from the front of the lug going back into the receiver.