I have a rifle which I need to re bed what is the best way of removing this material that's already in there

Dremel is your best friend...

I chip it out with an electric engraving pen. I posted pictures on here in the last year or so.

I use my mill

Dremel, or as gemby said, a mill if you have one.

+1 Best method

thank you

Acra glass and gel heat up with a heat gun and scrape it out, Dobson and marine text use a mill