Gunplay feels wrong

for me this is the first Halo MP experience and I have the feeling I don’t fully understand the gunplay I almost always need 1 full magazine to get the opponent down. Conversely, I have the feeling that after 3 to 5 hits I fall over.
I I play with m & k. I exhibited Aimassist.
Furthermore, I have the feeling that mouse acceleration is on. Aiming is inconsistent and a bit vague. Finally … sorry for my poor english