There was a scene in some Movie where someone set up a person by leaving a Rifle out where the person would find it and pick it up in dim light, and, it had the Cyanoacrylate Liquid on the stock and forearm, anyway, guy picks up the Rifle, then can not put it down again, cops show up, yelling 'DROP THE WEAPON!!" and so on...not so good for him!
But, anyway, Cracks in Pistol Stocks would be one of two kinds usually -
From some hard blow or denting effect, or, from a differential shrinkage causing a failure at a weakest point along the grain, or, a combination of these two.
Often Pistol Stocks, when cracked, end up getting Gun Oil in the crack, and, as others have said, this would need to be got out of there using Acetone or MEK or other, prior to using any sort of adhesive.
But also see if there is any debris or irregularity related to the damage which is preventing the crack from closing fully, and, ammend that, first.
Cyanoacrylate is about the best thing for repairing cracks in thin Wood.
Also, if some gap filling is needed, one can come back and with the same Adhesive, add any sort of fine powder to the area needing filling, and, the Glue then will bind the powder particles and, with a few small applications like that, sprinkle in powder, add the tiny drops of the CYA, one can fill in some missing material, and, then sculpt things down to match the original surface, and, even the color, depending on the pigment or tint of the fine Powder material one elected.
Even plain Baking Soda or Cigarette Ash or superfine Sawdust work well for this.