I have this Joseph Lang double rifle in this caliber, 1906 vintage. I slugged the barrels, they are .408.. Rifle is proofed for 60 grains cordite and 400 grain soft nose bullet (engraved on the barrels).

Recently I got hold of some boxes of late production original Kynoch (non-corrosive primer and non-cordite propellant).

Some googling suggest that this ammo is .410.. Is it safe to shoot the ammunition in my .408 barrels..?