Where it comes from
Bullcap is not a Dunhill Group number. It is a later catalogue and artisan name for a squat bulldog whose crown is flatter, more “capped,” than a tall English bulldog. American and European lists used it for a pocket pipe with the beads still cut. There is no single inventor to name honestly. It is a proportion inside the bulldog family.
The specification
401/24 is the bullcap on this site, and it has a card: 118 mm long, 56 g, chamber 18 × 32 mm, bowl 41 mm high, 52 mm outside. Smooth, bead lines cut clean, from the 2024 Facebook archive. That chamber is a millimetre narrower than the 20 mm holes on 381/23 and 370/23. The length is the shortest measured pipe in the cabinet.
Next to a bullmoose
A bullmoose is heavier and thicker in the shank. A bullcap is the short, capped cousin. If you want the bulldog’s beads in a smaller pocket, 401/24 is the plate. If you want mass, look at 420/24.