I think the boys nailed it, bullet is too short compared to a 174 or 150 grain,
My 2 cents = try heavier bullets and observe if problems still exists, use a chronometer,
Then reload with 150grains, using larger diameter bullets to match the wear on your bore.
If you want speed of light varmint type shooting you could always try the classic and well proven 303-25 and there's other wildcat sizes out there too, etc, thats a big case driving a small bullet very very fast, 3240?
Or, a new barrel ...
but do not shorten it the way you're thinking, try everything else first.
Don't use boat tails on a worn barrel = keyhole issues.
and btw, welcome to the forum, for links to absolutely everything enfield try enfield resource.com