I've got tons of 12g in the Sewell Silverbacks, never even close to an issue with fit. No extra jacketing on the 12g, tho to make it anywhere near a tight fit....
My mistake.....slight mistake.
I was most recently using the 90 degree variety, and the outer jacket goes onto those from the opposite end as compared to the straight variety (i.e. you have to put the jacket on, push it down the wire, install the plug, then feed the jacket back up and on there to thread it on).
I found the 12 awg to be a bit of finagling to get the jacket on there to get to its threads properly. Not bad, just a little more difficult than I expected.
I did that only for my speaker end, the other end is using a different style for the straight cxn. I've mentioned before, there is a different style that I prefer a little over the silverbacks.