Jon;
Do realize those 1300 fps velocities were not actually measured, but calculated as in that era they simply did not have the means of measuring actual muzzle velocity.
However to get an average velocity of 1050 fps over 20 yds with British #6 (approx US #7) they were undoubtedly very close on their calculations. To kill pheasants at 50 yds with Brit #6's they certainly weren't shooting pip squeak loads. Also realize these were the loas the 96;1 ratio of gun weight to shot weight was worked out for & they shot a pile of shells on a driven shoot in those days.
I get a bang out of reading all the folks who can't handle a 2˝-1 (about 1175 fps) load from a 6 lb 16 gauge if they shoot more than 4-5 rounds a day.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra