Both are rated at 65,000 but the area of the base of a .270, is somewhat less, than the area of the base of a .300 Weatherby. Area increases significantly as diameter is increases, so the .300 Weatherby has a lot more "volume" of gas pushing against it's base, than the .270, although their pressure ratings are the same. To make an exaggerated comparison, a road bike bicycle tire holds air at 100 or so PSI, give or take. A tire on a semi holds air at around 100 PSI. The tire rubber on a semi truck tire has to be a great deal thicker to contain the same air pressure, as the bicycle tire. MM