Hello. There are many mods, in my case it is MrCrayfish's Gun Mod, with non-standard item models. Is it possible to achieve the correct display of objects on the floor by changing the model using resource packs? Is there any guidance on this?

It's an issue with 1.12 and it cannot be fixed. It should not be an issue in 1.18. No idea what version you are using, but I doubt I can fix it from my end anyway.