Well, there is an old method that stinks like heck and your wife may toss you out if you try doing it in the house but it certainly works well and is almost impossible to see once it is done and blended properly.
First you have find an old shed cow horn (not antlers) or hoof and melt it down in a pot. Then you have to very carefully get it into the crack. If the crack isn't open enough it needs to be enlarged so the melted material can be forced in. Then of course after it cools, which will be quite rapid, it needs to be finished off. You will also need to blend in some color if you can't find the right color horn/hoof. I suspect your butt is black.
If anyone thinks that horn/hoof glue isn't strong enough there is a written bit of history about a bush pilot that crashed in the North and broke his wooden propeller. He use glue he made up from melted moose hooves to repair the prop and flew out to safety.
I have tried this method on stock repairs and patching up the gaps on old fore end inserts where horn was used and it is as strong as most modern epoxies if done properly and looks good when bringing back an old bit of insert to fill in the holes.