It is important that system nozzles are always fitted in couplings on guns/valves, or otherwise the O-rings will be flushed out, or water behind them will make the system nozzles very difficult to install.
If you need to replace the O-rings, here is a video guide to how it is done.
Note! When you disassemble parts with stainless steel thread, always remember to lubricate before reassembling. Watch the video tutorial How to lubricate stainless steel thread