Well, this is somewhat puzzling....
I don't know why a US airgunner would have British pellets and tune a gun for them — but he was such a pro, he might have done the unlikely.
The UK brand
Milbro has 13.3 grain pellets — well, they
had — because I think they're out of business:
Milbro Caledonian .22cal 13.30g I think a JSB domed 13.43gr is the closest you'll get with a quality dome.
Good luck on your search!
Matthias
Sources:
Find out about Milbro Caledonian .22 (5.5mm) from Airgun Pellet Database - The most comprehensive list of air gun pellet details and stats on the internet
airgunpelletdb.com
Just a quickie folks: Extorian's list says 13.3 grns for these - is that right?
www.airgunbbs.com
The following site is still selling(?) this pellet. With a declared weight (typo?!) of 13.00gr:
Milbro Caledonian .22 Pellets . Buy online at DefCon.co.za Home & Self-Defence Store.
defcon.co.za