Something the GM of my first Traveller campaign taught me: "Higher Tech doesn't always mean 'sustainable' tech."
The reasoning:
The more complex something is, the higher the technical know-how and parts needed to keep something working, the harder that is to do as you go to lower tech worlds (or at least more expensive to get the parts shipped in). Batteries go out on your gauze weapon, or the contacts burn out on the battery in the ammo mag, or the magnetic field generating coils finally get corroded (yes, don't tell me that can't happen) etc. You can only carry so many spare parts, batteries only take so many charges, etc. Your TL-12 weapon now broken and you're stuck on a TL-8 world without it and either the parts cost a fortune (not much market for them) or have to be shipped in.
The reverse is also true. What tech level is the musket? What about the flintlock? What tech level are we today... and how many companies or even people have the know how to properly make parts for them, ammunition for them? Vacuum tube radios, how many people work on them and how hard is it to find the older ones for the older radios? This is technology maybe a century old and it's almost gone from our technology landscape limited to stocks of original spare parts.
The parts and materials you most easily find are the ones common to the local tech level. If, as Traveller/3I materials indicate, slug throwers almost identical to our modern day firearms are not just available but till being produced, then purchasing of spare parts, ammo, etc should be quite easy.
Think about this when automatically going for that TL-15 monstrosity that is cool because it's NOT that TL-8 Sub-machine gun.