TIPS - I just did such a repair and have done many in my prior life as a gunsmith (part-time, day job as an Engineer). One repair I just did on a broken Mauser stock you cannot see the repair unless you look close and know where to look. But of course this means the entire stock also needs to be refinished.
- Advise DO NOT use solid dowels, they squeeze the glue out and make it a DRY joint
- Recommend you use a HOLLOW rod, like a threaded rod used on lamps - if room - as these can be somewhat large(r) in diameter
- If no room for a threaded rod, get some graphite arrow shaft material, obtainable for FREE at most archery ranges
- Cut arrow shaft to length, and grind many holes along the SIDES of the shaft material so it looks like Swiss Cheese. This does a few things - it allows the epoxy to flow and completely fill the deep hole, but then the glue makes the chemical bond with the wood & shaft and then the shaft holes makes a mechanical bond with both the glue and wood
- Using such Swiss Cheese tubes glued in place and I've never had a repair weaken or re-open
- TiteBond III or a weeping/penetrating epoxy is suburb. When I use epoxy, I use 2 types, wetting out the hole 1st with a thin penetrating epoxy into the deep hole, so it flows/weeps into the tiniest cracks you can't see, then use the typical slow cure/setting epoxy
Where your break is at almost a complete 90-degree break, this
will need serious reinforcement, like the tube option mentioned above, but one tube on each side, the longest you can fit without crashing through the outside or into the action.
I would attempt this repair in 2-Steps! First, I would ascertain where you can safely drill holes into each side, ideally up to 3" long or so, but you might be limited in one side or the other. Then cut your tubes to length and see if you can 'dry fit' the tubes in place. If so, then glue one side in only, using the techniques above.
Then dry fit AGAIN, to ensure that the tubing stubs
as cured, still fit (even if loose) into the mating holes and effect a perfect alignment at the break. You may need to modify the receiving hole so as not to be pushing the stock off to one side or another. Then glue the other tubes in, as well as gluing the 2 faces of the break together. The stock will need to be refinished and the checkering recut.