For a bore guide for any caliber, take a fired case drill out the primer hole and push and glass a 10 inch section or aluminum arrow shaft in the primer hole (push it in to the shoulder) and you have an excellent bore guide for any caliber..I have a coffee can full of these...Take out the bolt and push the case into the chamber and your set with a perfect mating to the chamber and throat.

You can even cut a slot in the arrow shaft about half way and use that to oil a patch..I just put a towell or piece of whatever over the butt stock so the run off from pushing a wet patch into the shaft doesn't drip on the stock finish....

Cheap and efficient...