I'm familiar with that Super-X brochure and have a few of them with different color covers. As I read the text, the breech pressure of the Super-X
progressive powder load is 3-3/4 tons. Also reading, that progressive load is safe in guns made for the old 3-1/2 dram load of
bulk powder - but no pressure is cited for that old load in the text shown here. It's likely that old 3-1/2 dram Bulk load is similar to the one I referenced earlier that gave 4.70 tons/sq inch with 1-1/4 ounce of shot according to DuPont's Ballistics Lab.
(COPIED) 4.70 tons/sq inch X 2240 long ton pounds X 1.10 rough conversion from lead crusher tons to transducer psi = ~ 11,580 psi which is on par with industry standard average max pressure for 2-3/4" 12-gauge.
Burrard's conversion
was not used in the snippet I copied.
Askins also wrote that progressive powder loads would generally produce higher velocity at standard service pressure, or (with a reduced powder charge) regular velocity with lower pressure. That was due to what he called the "barrel burning time" of progressive powders which in one test of DuPont's
Oval was nearly twice as long as that of the dense powder to which it was compared. It should also be noted that several of Dupont's progressive burning powders were utilized by Western in its earliest Super-X shells before the company switched to its own powders, and one of DuPont's in particular was remarkable in producing high velocity with relatively low pressure but it was very expensive to produce.
Again, I am very doubtful that Burrard's conversion has any real application to period American shotshell lead crusher pressures as expressed in tons/sq inch. On that line I strongly suggest that Drew (who I respect greatly for his work with composite barrels) include a note on future postings that his use of Burrard's calculation may not be universally accepted. Thank you Drew for your consideration.
frank