OP, that crack in the stock of the rifle in the pic is nasty. The stock looks to have been refinished at some point so you don't have to be that careful
Get into it with a pick, like a thin metal scribe and scrape it out to get rid of the crud.
Take some Gorilla Glue and thin it down with water. Work it into the crack, apply some clamping pressure and leave it sit overnight. Use decent pads so the clamp jaw faces won't leave dents.
The Gorilla Glue will do its thing and foam up, filling the crack. Scrape the excess glue with a razor blade. SCRAPE it, don't cut it.
This should fill that crack and make it strong.
I've seen stocks just like you have come apart during firing. If your lucky all it will do is get sloppy. If you're unlucky you could be in for some serious, maybe personally disfiguring damage.
Your Call.
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