A guerini or beretta is a modern "through bolted" gun with a hollow void in the stock. Their stocks can be purchased for $600-$1200 and are 98% inletted. 4 hours of shop time to use lantern soot to do final inletting is all that is required. Same for a CZ rifle, I've actually done final fitting for one in the bush with primitive tools by lantern light over the course of a few hours. These are mass produced stocks and mass produced guns.
If it is a hand built gun, for example a vintage 98 mauser sporting rifle, or a garden variety $2000 vintage british 12 bore boxlock shotgun, that was all hand fitting. They are slight in design, using the best possible materials, reliant on good wood integrity for their useful life. If one of those breaks at the wrist or head, a repair is unlikely to be salvagable. Particularly if the stock splinters and you have a hand full of small fragments. Sure it can be glued/pinned/epoxied, then checkering recut to conceal the sh&t-show, then refinished, and pawned off on 95% of buyers, but it won't likely hold.
Why do gun sellers try to convince people that such repairs as above are "Good as new, heck better than ever!" you might ask? Because its a $2000 ruined gun that needs a $5000 new stock. Thus, the gun is worth negative $3000 plus negative $500-$800 for having been restocked. ($2000 original value minus $5000 new stock minus $800 depreciation for being restocked = negative -$3800 loss to the owner)
There is one particular seller on GI and GB located in Kansas that has 100+ guns for sale at a given time. Nearly 100% of the advertised guns have such latent defects and he earns his livelihood acquiring zero-value guns, tarting them up with unstable repairs, and selling them to the masses for 10% under the value they should be if never damaged.