Not that long ago (Xmas 2010) I was gifted a wonderful Mannlicher model and was in the same place you're in. The manual doesn't say a damn thing about how to do it, either.
Here's what I did, it works for me but is a bit clumsy.
Take the gun, make sure it is unloaded. Walk over to the vise, and make sure the gun is unloaded. Take a scrap of leather, your local hobby/craft place has some if you don't have any, wrap it over the lip on that metal end-cap.
Gently clamp the metal lip in the vise, while supporting the stock, this can be a three-handed job sometimes.
Gently pull rearwards on the stock, the metal cap will pop right off with little effort, if it slips out of the vise, that's OK, just clamp it in one tiny adjustment tighter (you don't want to crush it smaller here, that just makes it harder!)
Last time, I didn't have my mini-vise mounted, and I clamped it on anyway,l while dangling the gun downwards ... the weight was enough to wiggle it off. I may do it this way every time now, or look into attaching a weight to a set of vise-grips ... I've only tried it once but it was a two-hand job instead of three.
So far I haven't scratched the cap, damaged the wood, or done anything but scuff up the leather. One of these days I'll grind out the inside of the cap and/or trim it to attach to the stock ONLY. I have a spare cap and screw on-hand for when I screw it up, too.
http://www.midwayusa.com/product/77...er-10-22-international-number-1-international
http://www.midwayusa.com/product/624160/ruger-muzzle-cap-ruger-10-22-international
If you booger it up, the replacement parts aren't
too expensive, so don't panic.