How did u originally introduce it to gunfire?
Do u have access to any other gundogs that are not gunshy?
If u could go out with someone a few times a week with a pack of dogs, just let them free run/play in a field and fire a starting pistol in ur pocket or game bag to muffle it, idealy when ur dog is not to close and just having fun.
If it shows no shyness work closer and un muffle the pistol.
But u really don't want to rush it to quick
Joe Irving the famous trainer used to fire pistol a few times point blank into dummies before throwing them, his thinking was should be a gunshot smell on wot ever u have shot.
So even rubbing recently fired shot gun shells on dummies, might get dog used to smell also and associate the smell with dummies/retrieving ie something fun positive
As has been said u want it to be so busy/ focused on something positive it doesn't noticed the shots going off.
How u achieve this might depend on how confident the dog is.
Dummy launchers can be quite loud althou u do have the benefit of a retrieve, but may depend how much ur spaniel enjoys retrieving, not all are mad keen on it.
if its not a mad keen retriever it might not work as well.
If u go dummy launcher route might be best to get a mate to use the launcher from a distance initially and work closer with time, if the dog allows.
Personally I've never seen the point in the clay ground route, esp if dog is just walking to heel or sitting, in my opinion could do more harm than good.
But many seem to think it works.
Like all things in dog training usually a few different ways to achieve the same goal and a lot might depend on the individual dog.
Another route, althou not really normal gun dog training method, could probably adapt some clicker/treat system, rewarding everytime someone else fires a shot at distance.
