I have owned, repaired and fired Enfields since 1963 (first one cost $12 in surplus shop) and I have never heard of an oval chamber. I have checked some of the chambers on the 30-odd Enfields I own and never found one in that shape. Obviously, I need to re-check all mine with careful attention to chamber shape.

Fictions get told around so often that many assume they are truths when they are really misconceptions or simply maliciousness on the part of the teller. "Ignore ignorance" was one of my Mother's favorite truisms.