¿Qué vendors tienes y a qué nivel? Con Gunny a nivel 2 tienes una muy buena M4 aquí https://www.grayzonewarfare.net/threads/best-gunny-level-two-m4-build-gzw-v0-3-1-0.94/
I created one with Gunny lvl 3 last night based of this one. I changed just 1 or 2 attachments to win a bit weapon handling (6%) and losing 2% of control recoil control.
If you need help I can send you Gunny lvl 3 attachments.