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Hey, if your only option is to print the split parts I recommend friction welding them together. There are a couple of videos illustrating this on Youtube and it is easily done. Alternatively, sanding the surfaces to provide a good contact surface and connecting them with industrial glue will also do the trick if done correctly. It is very important to choose the correct glue.
Let me know if it helped you or I'll think of something else to facilitate the process.
edit: I split the parts in a way that pressure from your shoulder wouldn't tear the bond easily, given that you'd be pushing the buttstock into the corner where they join and thus against each other. So my only worry is any force that is perpendicular to those edges, but if they are joined properly and the bond is strong I believe there's no reason for concern.