Was looking at the 507 to replace a Burris Fastfire that has started blinking every couple seconds....don't have any stores that carry them even close to me....would the bases made for a Fastfire fit a 507 ?
No. The Fastfire uses the same mounting footprint as the Vortex Venom or Viper, also more or less commonly known as the Docter footprint. Holosun uses the RMR/Trijicon footprint which is described by some as the most common industry standard, shared mainly by multiple obscure and fringe red dot manufacturers that few people have ever heard of - except for Trijicon and Holosun.
A thesaurus will show that a common synonym for the word "anarchy" is "red dot footprints". There are at least four different mount footprints that I know of shared by a few sights each and my knowledge is very limited so there might be more. Then Sig Sauer tells everyone to go screw themselves with their own footprint(s), I think.
There are some charts hidden on websites here and there showing which sights use which mounts, but even those will sometimes use a different term to describe the same footprint as someone else.
Here is a decent one:
https://www.egwguns.com/blog/2021/07/22/test-block-head/Fwiw, EGW seems to have some good resources to figure all of this out but you still need strong search-fu to find them.
