A Finnish point of view in the matter of 9,3x66 (.370) vs. .375 H&H:

It was about the same time as Sako Model 75 was released, that Sako developed this then proprietary cartridge. It was the first cartridge from Sako since 7x33.

It was then said in Sako advertising that 9,3x66 equals or exceeds the .375 in energy. Ok. The comparison was made using .375 H&H loaded rather mild with 300 gr. Hdy SP and 9,3x66 with then new 250 gr. Nosler Ballistic Tip. This way the new cartridge showed "better" retained downrange velocity and energy than .375 in this setup. This is nowadays referred in Finland as "the most classic example of scam by manufacturer to market a new cartridge".

Actually the 9,3x66 differs from 9,3x62 by only a little (shoulder moved forward a bit and shorter neck - cartridge OAL is about the same, as dictated by magazine box) and the difference is more in pressure level, as the seated bullet eates away the space in cartridge. The actual difference that a handloader has using the same barrel length is 15 m/s (about 50 fps) in gained velocity over the more common 9,3x62 (with 286 gr. bullets). That actually makes no difference what so ever in the field as majority of the bullets are made for 9,3x62 anyway. The thing was that with the new cartridge there was no old rifles to worry about and the pressure level could be set a bit higher. They also used new high energy VihtaVuori N500 -series propellants that made it easier to reach new velocity levels.

IMHO the name is better now, as metrics are boring and lack the nostalgia...

If you want realistic magnum performance from 9,3 mm cartridges, you have to go to 9,3x64 Brenneke.