It is very unlikely that it was proofed with 3/4 ounce loads regardless in which country it was manufactured or proofed. In my 65MM 28 gauge guns of very light weight, I use thinwall 2 3/4" plastic cases such as AA and load them with between 1/2 and 5/8 ounces of shot with slightly reduced powder charges and a filler. In heavier 65MM 28 gauge guns such as Parkers, I use standard 3/4 ounce target loads, either factory or reloaded. What make is your 28, Hal?