Howdy Flinch. Not precisely what you are asking for, but maybe close enough? I don't have Magtech hulls, but I have old Rem-UMC and Winchester hulls. I don't have any of the Winchester at my current location, but I do have some of the Rem-UMC. My understanding is that the Magtechs are the same as the Winchester (might have heard that they are made on the same old Winchester machinery) and I believe the Rem-UMC are the same. Having said that, I have two loaded Rem-UMC with me at the moment and measured them as best as I could with my calipers. The overshot card is about 1/2" down from the mouth, so I had room to insert the caliper blades. These hulls are somewhere between 50 and 100 years old, loaded 50 times by me and unknown before I got them. So while they chamber okay, I expect the mouths are slightly out-of-round. On both, I measured between .765" and .773" by rotating the caliper around the inside. Hope that helps. Most people these days who shoot brass hulls use the Magtechs so it is likely that one of them will chime in. Coffinmaker comes to mind. Good luck.
BTW, I use no "special" tools or jig. I hand deprime mine by holding it over a trash can and hitting the punch (held with the same hand that is holding the hull) with a mallet, then push the hull down over the pistol primer on the table top to get it started in the primer pocket, then I seat the primer on my Lee LoadAll. Then dip in the powder and drop in a .125" Nitro 11 ga over powder card., then back to the press to add the wad (red plastic wad) and then drop the shot) and I use a thin 11ga overshot card, glued in with Elmers wood glue. A 10 ga overshot card would fit tighter, but I had a lot of the 11 ga and they work okay. The Nitro card makes a good seal under the plastic wad which is a little loose in the hull. I tried some fiber wads but the pattern is tighter with the plastic wad. If you were thinking of loading smokeless, I have some info on that but it is going to be more effort and probably require resizing after shooting.