The wood does not appear to be oil soaked. So if that is the case and the crack can be clamped to close it, I would choose Titebond II or Titebond III. I would also use compressed air to blow some glue as deep into the crack as possible... which is going to blow excess glue everywhere, so it should be done outside. Then pull the crack closed with the correct force. That means neither too loose, which results in a weak glue joint. Or too tight, which creates excess squeeze out, and results in a glue joint that is also weak due to starvation, i.e., insufficient glue left to produce a good bond. Proper clamping with wood glues is at least as important as the choice of glue. Wood glue may fill a crack, but is simply not a strong filler. For this particular repair, I would use rubber tubing or strips of old inner tube to pull the crack closed. The best wood glue on Earth will not produce a strong bond without good prep and technique. I have intentionally broken planed and well fitting black walnut samples glued with Titebond II, and found that if done correctly, the glue joint is at least as strong as my walnut samples. That's good enough for me. And it also provides a crack or split repair that is less visible than other adhesives such as epoxy.
Simply filling a crack with Titebond II will not produce a strong repair if the cracked wood surfaces cannot be pulled together. In that event, I would use a good clear epoxy to fill the crack. Epoxy is the best choice for filling a large open crack or void, but the repair will stick out like a sore thumb. I would also warm up the mixed epoxy to lower the viscosity in order to get it to seep down into the crack. Then remove the excess with alcohol or acetone, and let it cure completely before use. That should keep the crack from opening back up or getting any worse. And fortunately, the repair will be under the forend iron, so it will not be visible. In my opinion, the absolute best wood repairs on a gun should be both strong, and virtually invisible.
