Which is the best coating treatment option for making aluminium barrel and screw more resistant to galling?
1. thick teflon coating
2. anodizing
3. nitriding
If there are better treatment option than all above please just quote it?
Make sure that it isn't mating with other Al parts.
Or if it all has to be Al make sure that one alloy is much harder than the other.
And then add the best best coating that you can, the G-M stuff is very good.
You can electroplate silver onto the parts with some very rudimentary items such as a battery charger or welder and it has good galling resistance. Silver is less expensive to purchase in small quantities than other industrial metals such as nickel or chrome. Silver is soft and won't be as wear resistant so you may need to renew the costing regularly.