I guess the cap not fitting on the nipple tells you right there thats where the problem lies. Just for giggles look again at the tin of primers CCI for some reason use the exact same colored tins for both their 10 and 11 primers and I have mixed them up before because I use the 10's for my first set of shots out of my cap and ball revolver and go with the 11's after.

That being said a little metal removal from the nipple can't hurt and may help quite a bit. My methode is a bit different though. I still very carefully put the nipple into a chuck but on a cordless drill and use some 150g wet/dry followed by some 400grit. Do just a little at a time and good luck.

Also RWS primers seem a bit looser as well you may want to try some of them.