I happen to have a box of the shells in question: Gamebore 12ga Pure Gold, 2 1/2". That's a 30 gram (1 1/16 oz) load. The information on the box states that they're "suitable for use in guns proofed to a service pressure of 3 tons per square inch." That converts to just shy of 9,000 psi. I've shot them in light British 12's with 2 1/2" chambers. They're not pussycat loads, for sure! Whether you choose to shoot them in your Flues 12ga, I'd say, is up to you.