It's a real crapshoot to try and fill cracks. The stuff Minwax sells in a tube comes close sometimes but I doubt that it would withstand the shock if it's a high powered rifle. Most of them suck up stain like a sponge regardless of the color of the stain and the cracks will become "LOOK AT ME" areas.
An old trick is to use the sawdust from the actual wood mixed with PVA glue. I've had real good success with that on some things and not so good also.
RC's idea is very good, especially if you are going to completely fill the grain with your finish. If not you can end up with a big flat area amongst the grain. Sometimes you need to use a needle or something to break the surface tension and get the stuff into the bottom of the crack. If you don't, it will bridge over the crack and it will still be there when you sand it flush.
Be sure to give it plenty of tme to cure and become rock hard. If you try to sand it too soon it'll make a gummy mess.