Looks like it will be a Bar Action as the pin for the mainspring is visible in the forward part of the bar. Chamber length will, most probably be 2 1/2", or at least that is the length of cartridge it was intended for. Some I have seen of that era made for the 2 3/4" cartridge were marked as such on the forend lump of the barrels. I don't think 3" chambered 12 bore guns existed at that time. It could be a product of the Midland Gun Co. except for the proof marks dating to 1887 because the MGC. didn't come into being until 1889. Some of the features are not dissimilar to those made by W & C Scott of that period. It's not unlike th Model 67 made from 1885 to 1914, especially the strikers, hammers and fastening. I'll have another good look at the photos, but that is my leaning on this gun. Scott's made guns for the Trade. I still come across a good many guns that I find unlisted as to the name on the action. Quite a few were sold by small country hardware stores who put their name on or even by individuals. Lagopus.....
