Any decent modern in-line doesn't need 30" of barrel for any reason, unless it has a poor ignition system. Many of the rifles made that state it has a 28" barrel, are also including the length of the breech plug.
I know shooters who are shooting 100grs. BY WEIGHT, that's 143grs volume, from custom rifles and using 209 primers, who's barrels are only 25". Long range modern in-line muzzleloader shooters don't use 30" barrels and they're shooting to 1,000yds accurately. Adding 2" overall to a barrel, doesn't make it a long range or better rifle or, burn the propellant better.
SPECIAL NOTE: DO NOT ATTEMPT SHOOTING MORE THAN 120grs VOLUME OF BH209 IN PRODUCTION RIFLES. FOLLOW THE MANUFACTURER'S INSTRUCTIONS.