A lot of it is fast target acquisition and situational awareness (player spawns, populated areas, strats, etc). Unfortunately because each gun has different grip points imo that will affect your muscle memory. For example when I use the asval I tend to do very well. The moment I swap to an m4 or tapko sks it takes much longer for me to get on target. I did the same with the cx8 as I was leveling up spectre. I was so used to it that I became very proficient with it. The mpx seems to be a lot like the asval so i adjusted quickly. This is just my opinion though. May 1st is wipe day so it may be the best time to test things out. What server region are you on and when do you usually play?

Magtube! It's on the expensive side but it lets you change the angle of each piece so you can adapt it to every gun

I got mine with all the upgrades (carbon fiber frame, carbon adjustable stock, 2 point sling, and pro straps). So currently it would be around $230. You can easily get it down to $160 with no carbon frame and no adjustable stock. I would recommend keeping the 2 point strap and the controllers pro straps. Also it gets discounted often