Recently had three 30 cal semi auto rifles in for service. Of couse they all had to be cleaned from the muzzle end inviting crown damage without a guide. I cut off the back of a 260 case just past the web. Put it on the rod neck down before attaching the brush of jag. Stick brush into barrel followned by the cut off 260 case, which fits neatly into 30 cal bore and, walla, brush away with cheap instant bore guide held in place.

And for your 260, 6.5x55 etc drill out a magnum rimfire case ,leaving the rim then you can use a 22 cal cleaning rod.

I would suggest you have a piece of derlin plastic machined to fit over the rifle muzzle like a sock and have a hole drilled in the end to admit the cleaning rod.
A set up like this would not allow anything to touch the muzzle.
Also a slot could be machined on the side to allow this "plastic sock" to slide past any front sight base.
A few wraps of electrical tape would hold this muzzle end bore guide in place during rod work.