I should have added in my previous post, the one-size-fits-most bore guides I've used have a tapered end designed to fit in to most chambers, and utilize a threaded "cross-bolt" to hold the guide in place. It's a threaded piece, maybe 1/4" in diameter, that screws in to the bore guide, perpindicular to the guide itself. When you insert the guide, you slide the cross-bolt in to the cutout in your stock for the bolt handle. In theory, this keeps the guide from backing out of the action as you draw your cleaning rod back. In practice, it doesn't work this well and the tapered end pulls back some and doesn't seal like it should.
All the "tailored fit" guides I've used have an O-ring seal around the end that goes in the chamber. This results in a tight fit and, in my experience, they stay put. In fact I have to really get a good grip on 'em sometimes to pull them back out when I'm done.
But I guess any kind of bore guide will keep your cleaning rod aligned and keep you from damaging the chamber or throat with your pointy jags and such.