Indoor or outdoor? That doesn't look like something that would be caused by changing out the end cap. I've seen weird black magic happen with guns, where they shoot strangely for no logical reason... if you shoot enough, sometimes things just go strange. Also, what's the mechanical accuracy of the gun and ammo? The left is within specs for "range ammo" in terms of accuracy. 2-2.5 MOA is still basically factory ammo or an OK shooting gun-- that's not bad. (I don't buy that every DPMS someone puts M855 in shoots sub MOA, the internet has convinced some folks that everything shoots sub MOA or it's absolute trash)

It looks almost like your accuracy is the same (1" vertical shift, although the left is slightly more, but not wildly off). It almost looks like you made a bad windcall on 2 of the rounds if outdoor. If indoor, that's a really odd thing to have it open up horizontally like that. If indoor, you sure you weren't pulling the shots? How consistent are you normally? By consistent, meaning cheek weld, trigger pull, eye relief, everything. I've gotten to the point before taking the first shot I can tell if it's going to be a good group or not. There are days where I'm hunting deer and tracking a squirrels head at 200 as he bounces around, just to see how steady I can keep things. There's other days at 50 yards I'm wobbling all over and know it's going to be a more difficult shot. Sleep, food, caffeine, etc all play a role in my own personal shooting.

With harmonic changes, you usually see a similar group size (not same, but similar), but the location shifts. (Not always)

Another thing that used to plague me was this weird journey of getting to the range, and shooting like crap. Then I would get my groups down, and shoot well. Then start rushing shots and open back up again. Is there potentially some sort of behavior that is consistently different you can track without the can on there?