500Nitro and drbd,
Yes, the .577 2 3/4" is a BPE, and is lower breach pressure than full blown .577 3" nitro, even when using Nitro for black cartridges. Converting 12 gauge shotgun to .577 3" nitro is not adequate, but 10 ga. 3.5" mag. is. Shotgun actions are much smaller than double rifle actions, when considering older ones made from mild steel only. They lacked the strength of modern high strength alloy steel actions. 12 gauge Conversions using older mild steel actions works well for .450 NE---.470 and similar, and .500/.416, but are borderline for .500 NE and simply too small for .577. The fellow who showed pictures of his 10 ga. conversion on this site, using a Spanish sxs boxlock, was "Blaine" from Washington state. A very skilled gunsmith friend in Montana, now deceased, converted three similar Spanish 10 ga. mag. to .577 3", and I converted four too, all having well fitting Greener crossbolts. At one time, I had access to a British boxlock sxs double rifle, .577 3" nitro; taking precision measurements of the action, finding it almost the exact same size as these big Spanish actions, so they are most suitable and very strong. Using such an action allows final rifle weight to be around 13 pounds, well balanced, or even more weight, if desired.