The 1.7" (3 pdr) Whitworth rifle is believed to have been a breech loading field gun. The projectile for this gun measured 1.68" across the flats, hence a 1.7" land diameter for the barrel. The 1.92" Whitworth rifle is a muzzleloading rifle. A picture of this gun is on page 208 of the first book. Much confusion exists when measuring Whitworth projectiles because of scattered, incomplete and contradictory information from the factory further confused by misprints of information. You may need an expert to measure and identify a Whitworth projectile. Hope this helps.
Ron