This question may have been ask before.
When does recoil start: in the chamber or when the bullet leaves the barrel?
What I'm trying to do is build a shooting rest with a remote trigger pull and maybe a shock absorber?

Thanks for any advise.
Mauser K98

Recoil begins when the bullet moves.

Why? I'd be curious what your purpose is. There are commercial benches available where you can secure the firearm and shoot a group w/ no movement whatever. This for grouping tests of guns. Not a lot of practical application.