you can use multiple perks, overkill - two primary weapons, extra lethal - grenade or C4 or sticky bomb etc. you can have red dot and silencer, target finder with silencer and grenade launcher.
But you only have 10 slots:
1. Primary weapons
2. +2 attachments
3. Perk 1
4. Perk 2
5. Perk 3
6. secondary weapon
7. +1 attachment
8. Grenade slot
9. Flash bang slot (including 10.)
10. with another flash bang slot if wanted/needed
So thats 10 in all. But you can remove the lethal (grenade etc), remove flashbang 1+2, that will give you 3 extra slots to use on guns. So you can add more attachments, or more perks.
It's a good and flexible system once you play with it and know what works for you.

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