It look's like you're well on your way. I'm just starting to stretch my legs alittle after a long period of load development for some rifles. I dont think you can tell a whole lot from one group but I would say it shows promise. I've just recently been geting some .25 - .5 MOA groups at 212 yds fairly consistantly when my bore is clean with a good load. My best group at 427 yds (max distance at my range) is 1.8", about .4 MOA. I'm hoping for sub .5 MOA groups to 600 yds with these factory Senderos. I think if you keep your rifle in good order, torqed rpoperly and bedded well, with a good scope that is mounted securly and really concentrate on some good consistant load work You should be able to tighten it up. In any case, you're doing a lot better than the average shooter who is getting 1 MOA on a good day.
On the case that chambered a little harder than the rest, it very well could have thrown that one shot off. I have seen it in my experiences. Once I had a 300 RUM case that had a good size dent in it just below the shoulder. It sent that bullet about 5" away from the other 3 bullets in that group that measured 2.5" @ 427 yds. Another time, I had a few 300 RUM case that were really swollen from some over pressure loads. When body sizing then they got stuck in the die and it took a of force on the handle to get them out. I beleive I bent the rims down a little on those cases and they are a little hard to chamber as result of tight headspace. They always throw fliers. Check all your brass and make sure it's unifrom. I use the irregulars for fouling shots.