the jei mod says I can get certain materials from grinding guns. I put a gun in the top left slot in the grinder and nothing happens. Armor can be grinded but not guns.

Still having same issue on mob-drop guns. Armor will grind fine whether or not there is damage. I love this mod but so many guns drop from mobs that it would be nice to be able to do something with all the extras besides drop them in lava or otherwise destroy them.

Im still having the same issue where guns are un-grindable... Armor works flawless

As stupid as this is, you have to take the gun and try to fire it (yes, even if it doesn't have ammo) then you can put the gun in the grinder. It's one of the buggiest things about this mod, but fixes it on my server. Annoying but it does allow them to be put through the grinder. Assuming some sort of NBT issue.

Some guns have a durability tag. If it says Durability 1 / XX, it can't be put through the grinder.

Try supplying it with energy.

I am supplying it with energy it only grinds techguns' armour and doesn't grind any guns

Now that I've started up a survival world I've got the same issue. Some guns can be grinded, some can't. Sometimes even the same gun can't be grinded when it used to be able to be grinded earlier. So yeah, I have the same issue.

I am also experiencing this issue. Armour will grind but guns will not, regardless of the state of the gun being fully reloaded or empty.

Same issue here. With the new version 2.0.2.0_pre3.2

it happens when you trying to grind guns which drops from mobs. Guns which spawns in chests are works fine.
version 2.0.2.0_pre3.2 from curseforge