More then likely the rim diameter or the rim thickness of the snapcaps. I bought a couple of sets of snapcaps from Dyson's at the Las Vegas show one year and none of my American guns would close on them. I carried them with me to the next years show and they refunded my money, said they'd had that problem with several Yanks.
In a similar vein many early Parker Bros. 28-gauges need their rim recesses recut to handle modern 28-gauge shells.
You need to go to our A.H. Fox Collectors Association, Inc. web site. We'd love to have you join too.
http://www.foxcollectors.com/
