Your Mk. V should be just fine with Fiocchi or Hornady ammo so long as it hasn't been converted. If it has, you'll likely get light strikes since the cylinder won't have sufficient length to hold the case head against the recoil shield. If your revolver has been converted, your best bet to shoot it would be to hanload .45 ACP cases with moonclips or .45 Auto-Rim cases with .455 Mk. II loading data. A good bullet for handloading .455 Webley ammo is Remington's 255gr LRN. This bullet is reasonably close in weight to the original 262gr one and is of the proper .455" diameter. Also, the Remington bullet is plain swaged lead and can usually be found quite cheaply.
If you're handloading with .455 Webley cases, be aware that Fiocchi and Hornady brass use different size primers (one uses large pistol and the other uses small pistol but I don't remember which is which) so it would be best to either stick with one brand or the other or at the very least segregate the brass by brand.
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