Cupons ?

You get coupons by opening crates and you can not sell you m16. You can however sell you Deagle.

The desert eagle is arguably the best secondary gun in the game however if you only have an m16 and no other primary weapons I would recommend saving coupons up on a primary weapon. Which one depends on what type of player you are. If you like sniping I would recommend the PSG-RD (60 coupons i think). If you are a rifleman I would say go for ak-knife or m4a1-custom (ak-knife is 70 coupons I believe and more powerful but m4 is more accurate and only 50 coupons). Last if your more a run and gun or smg user I would say get the kriss which is in the same group as the ak-knife and is an excellent smg choice. Unfortunately, the guns good or bad factor is heavily dependent on the user's playing style and tactics so I can't really recommend one as much as tell you what works for me. You will have to just experiment with various guns and see which one suits you, you can try out guns without purchasing by playing elimination mode matches in alaska or crossroads.

get zp crates u will always get one or more just about and m4a1 custom all the way