What would that be for a .308? I understand that the rifle can't be more than 18 lbs, is that correct?
For F/TR, both U.S. and international, the weight limit is 8.25 kilograms. That translates to approx. 18.18 lbs.
Remember that weight is all inclusive - everything that comes with the gun as you would lift it straight up off the mat, including the bipod (and/or sling, if you prefer).
There are no limits on barrel length, other than practical. Too short and you'll be skirting the trans-sonic region @ 1000yds and long range accuracy may suffer compared to other options. Yeah, I know every tacticool rifle with a 24" barrel shoots 1/2 moa 'all day long' @ 1000yds... thats why we see so many of them in matches - not. Too long... and the barrel becomes somewhat 'whippy' for lack of a better term, and difficult to tune a load. And surprisingly hard to find a gun case to fit it in. Personally I don't really like the way longer barrels balance and behave in use, based on rounds fired from other people's guns.