Because there's a LOT more .30-30 data available, for a wider variety of bullets.
When I had my last .303, I compared the water capacity of fired Winchester-brand brass in both cases, as I recall with 170-grain Hornady Interlocks seated to the cannelure. Capacity was IDENTICAL, down to a tenth of a grain. And if case capacity is identical, then basically pressures and velocities are too, given the same bullets and powders.
This is true regardless of case shape, no matter what some claim. No pressure lab anywhere has been able to find differences in shoulder angle, case length, etc. Pressures and velocities may be more consistent due to case shape, but not average pressure and velocity. And the .30-30 and .303 are very similar in case shape.
Thanks for this, John.
