I guess the mod works fine, since you say that there is no problem with other weapons. So there might be something wrong with the M16A1+M203. You should check, if its files have been downloaded correctly. The Steam Workshop integration in Killing Floor sometimes fails to download subscribed items. I wrote a little guide about those problems back in the day:
https://steamcommunity.com/sharedfiles/filedetails/?id=351654492

I hope that helps!

Hey dude thank you I'll be trying to follow the guide, this has been clearing my mind a little bit and now I understand some more about how all this thing works. Quite a pain in the ass but is ok I'll let you know if I can make the mod work or not.

I tested using SP but I didn´t get the mod to work but I found something in the mod´s description that I don´t understand what it is. It is this: "Code for issuance in early wave: M16m203AssaultRifle.MutM16M203" I know this probably doesn´t have anything to do with your mod but if you can help me I would really appreciate it because I don´t understand anything.

I can't offer you any support for ServerPerks. The code you are refering to is the pickupname. It helps to spawn weapons via console commands or to add an unknown/unsupported weapon to ServerPerks/WeaponsWorkshop.

Here is a guide for unsupported weapons in the WeaponsWorkshop:
https://steamcommunity.com/workshop/filedetails/discussion/147258902/864971765341835331/

I have tested doing what you said but it doesn't appear to work. One thing I can say is that the "Code for issuance in early wave: M16m203AssaultRifle.MutM16M203" isn't the pickup code, the pickup code is "Code for SP: M16m203AssaultRifle.M16m203Pickup" but I still tried using the pickup code in the advanced options of your mod (which is really good btw) and the "code for issuance in early wave" too but nothing seems to work. Maybe steam isn't downlaoding the mod correctly or the mod is outdated or something else. I will try redownloading it again but I don't know if it will work. Thanks for helping a new player that doesn't know nothing like me.

So... I tried downloading the mod again and when you start downloading it says that the mod is 4.2MB which is also what says in the description of the mod's page on the workshop but it is kept at 0 bytes, after you wait a little bit the mod reduces to 2.1MB and downloads instantly with no problem