Thank you for comments - to be seen, yet, if that was OP's only issue. I had come up with (I thought) an issue that was out of order, but had nothing to do with the symptoms displayed.
A CGN thread some years ago about a BSA m1917 conversion - new-to-rifle CGN owner was getting 4" or so groups at 25 yards - had tried multiple bullets and loads - no change. Discovered rear base was not matching to curvature to the rear bridge - that scope base had larger radius curve than the receiver - so likely could "rock" - so required epoxy bedding to stabilize - a good solution, except problems persisted. Turned out that bore slugged to .282" groove diameter, not the .277" that one would expect for a 270 Win. The CGN owner found that out by slugging that barrel, before he mailed that rifle to me - and I found same diameter when I slugged it here. Still sitting on shelf waiting for "inspiration" for a solution to that ... Is some Nosler Partitions in .277" - here for that BSA 270 Win - about the only exposed lead base bullets I could find in .277" - maybe those will swell up enough to fill that barrel size - to be tried, at some point. In my mind, unless it is capable of at least 1.5 to 2 MOA at 400 yards, is kind of pointless to have a 270 Win for hunting?
I am not adverse to a 270-7mm - I do not have similar - about 10 or 12 years experience taking deer with 7x57 - is only the 7 mm that I have - but that BSA rifle would also need the throat to be reamed, because, as is, can not chamber 270 Win brass with 7mm bullets in the neck - throat is too skinny - from cerrosafe cast, appears to be a "perfectly dimensioned" 270 Win chamber.
https://www.canadiangunnutz.com/forum/showthread.php/1863303-VERY-strange-BSA-problem
