After i played uncountable maps with low damage weapons i thought i make a tutorial for it!
Download the UGX-Weapons editor by treminaor
go to mods/YOURMOD/weapons/sp
drop any weaponfile (for example the "zombie_ppsh")
thats it for the basics!
Change the name
to change the name use this box and type in what you want as name
Change the damage
to change the damage look here
i think for a non upgraded weapon you should set the damage between 90(low) and 140(high)
change the clip size and reserve ammo
i don't know if it helped you