Thanks Busta, But the barrel was clean and dry as was the breech plug. Prior to loading the breech plug had been removed and cleaned with a wire and degreaser, barrel was swabbed with several clean patches with plug out. No oil is used thereafter, only a small amount of silicone grease on the plug of which I am certain none got in the flash hole.
I am meticulous about cleaning my guns and could clearly see light through the plug flash hole after reassembly. As I stated previously the bullet/sabot combination had been shot 75 times or more with not a single problem and they are a very tight fit.
So this is very perplexing as to what happened, my only thought at this time is a bad primer as I stated.
Another thought that crossed my mind was could the powder have soaked up enough moisture in 3 hours to delay ignition?
It was a very damp,rainy day, although the gun was not subjected to direct rain.
Could this be a problem with this powder that has not showed up until now? I hope NOT, as this is an excellent powder, accurate and clean.
Don't know if the conditions can be duplicated at the range, but will give it a try in rainy March.
Thanks Again for your thoughts, TC