The size of the syringe needle required for gun barrels would obviously depend on the weapon in question, and the scale it's being modelled in.
For example, a 20mm cannon barrel, in full size, would be approximately 35mm to 40mm in diameter, depending on weapon type. So, in 1/48th scale for example, this would be 0.73 to 0.83 mm. In most cases, you'd probably get away with a 1 mm outside diameter needle in this instance.