Doc Drew, very nice summary! Thanks for posting.
Bill, I've used that same 3/4 oz 20ga load you're sending out for testing. I dropped down a bushing, using 13.8gr 20/28. Very reliable, even in cold weather. I haven't had any tested for pressure, but from the "book" numbers, it certainly looks as if you'd be under 7,000 psi with that load. And that's pretty darned low in 20ga. I use the load in a Damascus-barreled British 20ga, still 2 1/2" but reproofed at 3 tons. Those 3/4 oz 20's are GREAT to shoot in lightweight guns!
