2150fps sounds too hot to me.

That's standard velocity for the 350gr out of the 450ne.

Most properly regulating NFB loads for 450bpe rifles I've seen cruise at the mid to high 1800's for straight case guns and up to the high 1900's for the 3-1/4" bottleneck guns.

My magnum 450, which uses 3-1/2" bottleneck cases and a 5-1/2 dram charge with the 350grsn bullet, regulates at 2025fps, and that's with a NFB load of Varget and dacron.

On page 168 of Graeme Wright's book, Shooting the British Double Rifle, the listed MV of Kynoch's .450NFB load is 1750fps with a 365gr bullet.

I would be very hesitant to consider the use of ammunition reported to run 350gr bullets at 2150fps in a 450 BPE rifle.

When you're working with a BPE double rifle, you're looking for the regulation load for the gun, not the max load for the cartridge components.

Are you looking to create a load to build a homebuilt DR around, are you shopping for an old BPE DR and want to know how far you can push it, or are you just curious?

With modern components, there are plenty of examples of guys pushing much higher than 'vintage' velocities out of modern barrel steel on modern actions in such cartridge types as the various Sharps cartridges and the .45/70...

--Tinker