Primers

[Chris Pope]

It would have been Winchester, rem or federal and definitely had brass not steel. Sorry but I just can't remember which one or the specs. The gun did not require a lot of muscle to open but it didn't fall open either. At the time I wasn't thinking primer issue but worried too much pressure was causing some kind of damage. However, based on your question I will now go back to my ammo closet and see if I can figure out exactly what i was using.
I have since used the Parker hunting with hand loads for hunting and super light factory loads at the skeet range- no problems...

[Kensal Rise]

As you've seen, primer selection and cartridge loads can effect the function of many old doubles. So can firing pin tip condition. If they've been roughed up by corrosive priming in the past, they can stick. The action's cocking speed is also a consideration. Because of the design of the Parker's action, it can pull the tumblers back to cocked position a bit slower than other designs. (For more details, please see my new book, Birth of The Boxlock) It can all add up.