That's been an issue with the Weatherby up until this test. Groups were pretty open with 180 gns, different OAL's, couple different powders. Some would show promise, so I'd load 10 and get a different result...frustrating. I think the issue was that I was going too long on OAL. In this test I went with several of the manual suggestions and in the case of the last loads the bullets at 3.33 wouldn't fit all three rifles, so shortened up and finally got the 4 shots into a good group.
The Marlin clearly didn't like the last two 180 grain bullets. Not only were the groups spread out but also many inches low on the target which surprised me. I think the XL7 is probably a 165/168 rifle. I also noticed another box of 165 grain bullets sitting on the shelf once I had completed the 48 rounds. Next time
As to the 4th shot flyer, could have been a few issues - 32 degrees hot, the barrel not cool enough, my damaged finger which was trobbing, perhaps some flinch on my part, etc. At least that's my thoughts. Once the hand heals I may retry the shots with same loads, etc.
Anyway, part of the fun and challenge of finding the right load for each rifle.