When Mystical Mechanics is installed alongside TechGuns, the icons of TechGuns weapons and armor with 3D models are turned into missing textures. The items retain their 3D model texture, it is only inventory icons that disappear. Items without 3D models retain their icons, as do blocks.
This occurs with just TechGuns and Mystical Mechanics installed, and with both on their current release versions.

I also reported this to Mystical Mechanics.

yeah ok, adding a label doesn't mean it's not a problem with your mod.

MM doesn't contain any kind of coremodding that would interfere with your ASM.
It doesn't contain any coremodding at all.

Very likely Mystical Mechanics contains some kind of core modding that breaks Techguns ASM.

it does not

Very unfortunate