I posted this in another thread, but I thought it would be good to post it here as well:
As Lurker and others have pointed out, these are the 3 metric o-rings that I think are the most likely substitutes for the #12 to get less bolt stick. If you plan on getting these o-rings, I recommend trying them in the order below from loosest to tightest. Product numbers from McMaster's website have been provided to hasten your purchase if you so choose to pursue this on your own.
Substitute #1: 1.6 x 9.1mm
This one is smaller in all dimensions (cord diameter, ID, and OD) than the #12. The OD is the smallest of the 3 and would actually be even smaller due to the additional stretching caused by having a slightly smaller ID than the #12. This would most likely be the loosest of the 3 and possibly more prone to leaking. But, if it seals, this bad boy should reap wondrous results.
Substitute #2: 1.5 x 9.5mm
If #1 leaks, then this o-ring is our middle man. It has the smallest cord diameter and largest ID of the 3. However, the OD still comes in a little bigger than #1 yet still a little smaller than #3. The larger ID and smaller cord diameter (compared to a #12) may give it a higher tendency to roll back and forth in the pocket and increase chances for spiral failure (though I think the problem is negligible).
Substitute #3: 1.9 x 8.8mm
This is the "fattest" and I would also dare say toughest member of the group. It has the largest cord diameter but the smallest ID of the 3. The OD is just over 2 tenths of a mm smaller than a #12, but the smaller ID will cause it to stretch in the bolt guide pocket and possibly decrease the OD just a little further. The larger cord diameter and tighter fit will make spiral failure almost nonexistent though I do wonder if it's sealing properties will be affected due to being stretched a little.
Everybody's guns will have small differences in dimensions and tolerances, so any or none of these could work for you. I plan on getting all 3 and just seeing how it goes.