The carriage wheels are at the front of the carriage. They do not actually contact the slides unless the rear of the carriage is levered up a few millimetres, to assist with rolling the gun down to the firing position.
They are constructed of bronze.
On my model, the gap between the wheels and the slide would be about 0.3mm.
Today I attached the wheel brackets to the carriage cheeks (the sides of the carriage).
I had deliberately made them with a slightly large diameter, knowing that I would need to reduce the diameters after they had been fitted.
This is how I reduced the diameters…..
…on a belt sander, holding the oiled shaft in my fingers and using my thumbnail to hold the wheels in position. After a few seconds sanding, and being careful not to sand my fingers, I tried the wheels on the carriage, rolling it up and down the slide. That was repeated multiple times until the wheels were just clear of the metal slides.
The single axle will be replaced by more authentic appearing separate axles with dome heads and pins. The brackets will be let into rebates in the carriage cheeks, and tapered in their upper halves.