speedloader for mid caps and low caps are essential... you don't want to hand load 120 rounds in each mag.
unless you realcap... then you're hardcore if you load by hand.
lowcaps and midcaps are essential for playing milsim.
There's little point in carrying 3 or 4 highcaps unless you like doing the chacha.
Sling is not necessary unless you need to go hands free... it becomes more necessary when you need to draw a pistol.
Slings are nice though cause you can just let the gun hang off your body while you walk... say back to respawn and give your arms a break.