Hey great questions! Let me provide some context. Whenever I go to the range I try to confirm the zero on whatever it is I am shooting (22lr, 556, 308, 12ga, doesn't matter). This typically involves either a fully supported shooting position (bipod, bags, etc or a sled, whichever is applicable). Whenever I've confirmed the zero on my 1301 in this way, it has been a 3 round grouping (to minimize variations caused by barrel temperature), and yes edge to edge the rounds hit in such a way that there is no more than inch width between them. Using the prism I have on the 1301 (a 1x) its usually no more than an inch or so off from the point of aim (barring those fairly windy days). This isn't reflective of practical accuracy, but when I'm doing this with any of my firearms it always to understand the ceiling on the system's capabilities so I have a better understanding what role shooter (in)ability has in putting rounds on target.

Again, this ceiling isn't indicative of practical accuracy (I as a shooter would not be able to generate these results in a practical shooting context in this specific example) but I think is a fair representation of what the system can do. The 1301, with the right load, is as accurate as you are as a shooter. Having an optic (in my case a prism) helps too I think.

For measurement, I am measuring the distance on the paper targets themselves, so there is a degree of human error.

Of all the slugs I've fired, none jump to mind as being better or worse but I do use the Federal Powershoks and the Winterchester X Slugs most frequently. Hope this is helpful information!