Right now getting ammo is very difficult if you play survival. 6 Iron and 5 Gunpowder for 30 shots in a M16 Mag seems like a lot. If empty magazines would be recycleable you could save at least 5 out of the 6 iron ingots. This could be handeled via an ammo refill table for each pack with guns. Also 5 Gunpowder are a bit expencive, maybe this could be reduced to gunpowder pieces (just like gold nuggets and ingots, maybe even same for iron)

Getting gunpowder and ammo will be much easier with future Flan's addon: https://www.youtube.com/watch?v=zlOjAbxSHjg
But IMHO guns right now is enough powerful for this cost if you shooting by single fire instead of bursts.

Base Flan's Mod has a toggleable option for a gunpowder crafting recipe (charcoal and glowstone dust) and as mentioned above, the Apocalypse addon will have a lot of gunpowder and iron available.

I do like the idea of being able to refill magazines though, similar to how Nerf weapons work