I have to say that I am aware of the issues with Buna-N and nitrile o-rings and their expansion when exposed to CO2 but I have been using a Buna-N o-ring as the end cap seal for a few days now with no expansion issues at all. I’ve probably gone through four or five CO2 capsules. I never leave full or partially full CO2 capsules in my guns so they only get used fo a short period before the cap is removed and a new cartridge is fitted. So I think it is entirely possible to use “standard” o-rings on the end cap.