(sorry if wrong forum, I figured it has to do with optics so it should go here)

So I recently purchased an M&P15OR (16" carbine flattop), mounted a Burris Fullfield II 3-9x40 Ballistic Plex scope on top of a Burris AR PEPR mount. During this past weekend I took my rifle and pistol with me while visiting an old high school buddy up in Albuquerque we decided to hit up the range. He's in the AF and has had at least some rifle training before, so he was probably a better shot than I was to be honest.

At first, I got on paper at 25yrds (scope was almost dead accurate at this point), then moved to 50 yards and then took it out to 100 yards. While at 100 yards, I couldn't tell what the issue was with the groupings that were offset from bullseye (up and left by 2MOA each direction). No matter what scope adjustments, the shots always pulled up and left equally.

Since we were only using the sandbag rifle rests, we weren't sure if the rifle's accuracy issues were with the heavy pull trigger (which if I've read correctly is 7-8lbs). Or, if it was with the ammo we were using (Federal Bulk Pack 55gr .223 - Walmart packs). Or, if it was with the fact that I believe my friend was changing the scope adjustments thereby altering the adjustments I was just making and altering them more than I remember (As the range had a 10min live fire, then 10 min cease fire we would alternate rounds between the rifle and my pistol). Or if it was improper length of pull (which from my quick and dirty measurements, the stock should be out another 0.5-1.5 inches as I've got long arms).

I'm hoping to get to the range again this weekend as a solo shooter to adjust it myself only, but does anyone have any tips or tricks to fully figuring out if it is the ammo, trigger pull or something else?

I ordered a ton of ammo from my friend who owns Bullet Boy Ammo, and we did a loading of all bullet weights to try and find which bullet my rifle prefers (1:9 twist), between 55gr, 55gr VMax, 68gr BTHP Hornady or 75gr BTHP Hornady). He is using the match loads that he's built-up for those heavier bullets, so most likely the accuracy will increase when I get some better quality, non-plinking ammo.

So, fine tune it all with ammo and form or upgrade the trigger (which won't be for several months along with a free float rail).