#11 of what brand? They are different. I use Remington #11 on my Uberti's. Remington #10 are a little longer so should fit tighter but may require two hits to go bang? I have some #10 but haven't tried them on my Uberti.
So far I have been using Remington #10 on the revolvers (mostly Pietta, but I do have one Uberti and one Dart), and #11 on the sidelocks...nobody else' caps seem to fit well..either too tight or WAY too loose
Trick i found with nipples is find a cap that is too tight (mentioned above for needing two hammer falls to set off) then put the threads (tight but not tight enough to bother the nipple threads) in a drill and using a finger nail board ( not the wifes) and sand as the drill is turning the nipple just a bit at a time till the too tight caps fit snugly and almost bottom out on the cone mouth. Take care to maintain the angle of the nipple cone area. Then these caps will be dead nuts for that revolver. If i have the slightest problem with caps failing to ignite or so lose they fall off during recoil i give em this treatment.