On 24/05/2021 20:13, Murmansk wrote:
I have a circular saw jig which I made myself and it uses bits of
aluminium which are L shaped in cross section for the saw to run in.

I'd like to make a better one with longer "rails" and rather than
using the L shaped strips, which tend to sag, I'd like to use tubing
and I know you can get some things that run along tubing and probably
have ball bearing in, but I can't remember what they're called.

Can anyone tell me what these are called please so I can look them up
online

I would call them linear bearings. They can be fully circular or part,
so to fit round shafts or guide rails respectively.

so:
linear bearing pillow
linear guide rail

An idea:
https://www.youtube.com/watch?v=UB80HojMDyg