Ted, are you sure your Silver Snipe isn't proofed any higher than a W&S? I recently acquired a fancier version of the same gun (BL-4/S56-E) in 20ga. Mine might well be later than yours. Date code says 1972. Anyhow, mine has the superior/magnum proofmark. Check the barrel flats on your Snipe. You should find PSF with either one or two stars in wheels over it. One is standard proof. If yours has two--which mine has--that's 17,600 psi, or about 4,000 psi higher than the standard CIP proof that Webley would have.
Nothing wrong with shooting a NID. As with most American classics, you need to drop down to the 16ga to get a weight similar to a Brit game gun. But you can also shoot the same shot charge you'd want to shoot in a Webley. And you'd have enough money left over to buy a Silver Snipe 12.
