I think when Geologist had his .375 H&H Brno 602 shortened to 20", he only lost 50 fps from the 24" length, although I'm not sure what bullet weight he checked that against. Usually the heavier the bullet the less the actual drop in velocity is observed in a short barrel. The .375 certainly doesn't loose velocity at the rate observed when cutting the barrel length of a small bore, so even with the lighter for caliber bullets, the difference is unlikely to be very significant. If it does prove to be a problem, rechambering to a larger cartridge (.375 Ultra or .378 Weatherby) will easily produce long barrel H&H velocities from the short barrel.