First off BR barrels are not slower because they are tighter.
I think that is a misconception stemming from the tight bore Palma rifles which are 30" and set up to only shoot 155s. They are the only barrels set up that way normally.
BR rifles use standard match barrels.
Now there are some barrels that will be faster due to the type rifling, which gives less pressure with a standard charge which means you can go to more powder and get more MV.
These include canted rifling types such as the Rock 5r, Broughton 5C and any other with this type rifling. It is widely known that most Broughton 5C barrels will run about 50-75 fps faster than standard rifling in same twists.
Also twist rate will be a factor. For example, most people use the old standby of a 1-10 in a 30 cal. That barrel will develop higher pressure and lower MV than a 1-11 or 12 twist with the same 300 win mag shooting 210s as the 1-10 spins the bullet faster and gives you a higher pressure. The 1-10 was the standard for heavy bullets at lower velocities. When you up the MV (such as a 3006 compared to a 300 Win or 300 WSM
) you can go to the slower twists with less RPM, and still shoot the same heavy bullet faster.
Seirra says that you need a 1-9 for their 240, yes if you shoot it at the lower velocities. Yet if you take a 300 Weatherby (no free bore either), 300 Ackley or 300 Rum for example, they will all easilty shoot the 240 and very accurately as they are much faster and do not need the faster twist to stabilize the bullet.