I've posted this information before, but several years ago, I wondered the same things, having seen far too many guns with very obvious stock repairs. So I did my own little experiment by running some black walnut through the planer, and then cutting a bunch of pieces about 1 inch wide x 2 1/2 inches long. I then glued pieces together using at least 3-4 brands of epoxy including Brownells Acraglas, West System, and a 4:1 boat building epoxy. I also used a polyurethane Gorilla type glue, and a couple wood glues including Titebond II.
With a close fitting joint, the Titebond II gave me the most invisible repair. I then applied a coat of several stock/wood finishes I had on hand to see what effect that had on either accentuating or hiding a glue joint. I used Permalyn, Sherwin Williams Spar Varnish, Minwax Oil Modified Urethane, Tru-oil, Deft, Tung Oil, Watco Danish Oil, a clear gloss Polyurethane, and probably a couple other I can't recall off the top of my head. I was quite surprised at how much the choice of finish can make a glue repair more or less obvious. As I recall, the Watco Danish Oil, and the Tung Oil did very well, and the straight Polyurethane left the glue joints the most visible.
I then took a number of my nice little test pieces, and clamped them in a vise and smacked them with a shearing hammer blow across the glue joint. The Titebond II performed the best, and was actually stronger than the wood. However, with a crossgrain break like a broken wrist, I would still do as Damascus recommends and reinforce it with a brass or steel pin. And naturally, I would coat the pin and hole with good epoxy because wood glue will not give a good bond with the non-porous brass or steel pin. I will note that Titebond II was best with a close fitted joint, and epoxy has better strength when gaps may be present. But the goal should be to have no gaps which need to be filled with a thick epoxy.
I bought a bottle of Titebond III several months ago, but have not had a chance to try it on walnut to see if it is any better than Titebond II. The polyurethane glue gave a very strong and fairly invisible joint, but it was very messy due to the foaming squeeze-out which required some sanding to get off the surface so it didn't act as a sealer and inhibit the penetration of finish. The shelf life for unused amounts is very short as well. But I have read that polyurethane glue is a better choice on wood that may have traces of oil contamination, or for naturally oily wood such as ebony. Naturally, it is equally important to get a very closely fitted joint if you want a strong repair that is hard to detect. Correct clamping is a must, and a whole other subject. The goal is to close the joint completely without damaging the wood, and without too much squeeze-out which will starve the glue joint. All these little details make the difference between those excellent but expensive gun stock wood repair specialists who can make a shattered stock look as if it was never broken, and the mediocre to piss poor gray epoxy and baling wire Bubba-type repairs that are all too common.
I've never been a fan of faux grain because it is... well, faux.
