Which Gun addon is best for DarkRP?

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M9K is a pretty good weapon pack and it's normally the one I recommend for everyone. It has a pretty decent arsenal of weapons to choose from if you combine all the packs. Another thing about M9K is it comes with a config addon you can get from the steam workshop to alter how the guns perform in game. M9K overall is just a really solid weapon pack for any server.

Another pack I can recommend is CW2. This pack combined with all the other official CW2 packs makes a pretty cool weapon system. I can say though, I have had some problems in the past with some of the optional CW2 packs (stated below). This weapon pack has tons of other singular weapons and smaller additional packs all over the workshop, if you enjoy the feel of CW2 but can't seem to find a specific gun, you could probably find a pack someone made with that gun in it.

Finally, the last weapon pack I can recommend to check out is FA:S2. It feels quite similar to CW2 but they have there small differences. The reason I suggest both of these packs is because I have used CW2 over FA:S2 and vice versa. It's something different.