#6.5 English shot equates to #7.5 US shot.....correct?
According to my Eley Shooter's Diary, Brit 5's (220/oz) are very close in size to US 6's (225/oz.)
Brit 6's (270/oz) are slightly larger than US 7's (299/oz)
But if you take the time to count pellets--which I never do on anything smaller than US 6--you will find differences in size and pellet counts/oz. The Brits seem to do quite well killing driven pheasants with their 6 shot. But it's good to remember that those birds are typically shot as incomers, so not as much penetration is required for a lethal hit as pheasants the way we hunt them (walked up over dogs). I've had good luck on wild roosters with Gamebore Pure Gold 1 1/16 oz of Brit 6's as a first barrel load (287/oz per the Eley Diary. That's almost exactly the same pellet count as 1 1/4 oz us 6 (281 pellets), which is a classic pheasant load.
Per Eley, Brit 6.5's are 305/ oz, so larger than US 7.5 @350/oz.
When purchasing non-US shells marketed in the US, it's always good to know if the shot sizes are US or Brit/European.