So an update to this seemingly never ending problem...
Patrolbase haven't responded yet, but it's been less than 24 hours so that's understandable.
I figured I'd take the inner barrel assembly out and make sure the hop is working, because my m16a1 had an issue where the wheel would just adjust but nothing would change and that led to bb's falling short and going everywhere. Thankfully, the hop is working and adjusting the wheel shows the nub. I'm going to replace the bucking tomorrow per suggestions (Ordered a G&G cold weather because they're quite good and affordable just to test with, that'll be here tomorrow) and hopefully get the fps to where it should be at, as well as fix the veering off shots. Fingers crossed.
Update 2:
Had a spare hop up bucking from when I replaced my m16a1 one, figured it couldn't hurt to try it in the 416 and it's firing straight every shot without veering off or dropping instantly. Guess I know what the issue was now! So alas, my saga with patrol base and their dodgy guns, and dodgy replaced buckings comes to (hopefully) an end.
Thanks again everybody for the help!