I remember an old article penned by Rick Jamison when pressure measuring devices became available for the average shooter ( albeit, with fairly deep pockets). An Oehler, I believe, is what he used.

Any way, he had a brand name gunsmith screw on a top flight barrel chambered in .300 Winchester magnum, originally in a 26 inch length, then had it cut down to 24 inches by the same gunsmith and run a series of tests from both barrel lengths. The load was 75 grains of pre-extreme H-4831, Winchester brass, a CCI-250 primer, and a 180 grain Sierra bullet. The load had a chamber pressure of 62 000 psi. The muzzle pressure with the 26 inch tube was 12 000 psi. When the barrel was cut down to 24 inches the muzzle pressure was 15 000 psi. That is a whopping 25% increase in muzzle pressure. That is why shorter barrels kick harder and are much louder that longer tubes.

As for muzzle velocity, I cannot say. A cut down barrel with be slower that it was when it was longer, there is no denying that. How much is very calibre dependant. In a .270 Winchester, Ken Water's findings were about 50 fps per inch. Hornady claims in their manual the tiny, and low pressure 30-30 loses 35 fps per inch. In a .375 H&H I cannot say.

Food for thought. Good shooting.