As a matter of fact, we shot trap today - only 3 of us showed up, Taylor, Hatchet Jack and myself. All used 20 bore flintlock singles with Taylor starting off with his 15 bore Manton SXS cap gun, then switching to the flint 20 bore after a while. He shoots better with his 46" cylinder bored 20 bore, than with the cylinder bored Manton. Exact opposite for me as I find no difference in shooting a caplock when standing beside the trap, compared to 16 yards with a modern shotgun - feels like cheating to me. 25 straight is no problem with caps, just like with as modern gun.
There are fellows out there who jug-choke smoothbores, so using a modern barrel is not necessary. Actually, it's not difficult to jug choke a barrel yourself. Merely read Ned Robert's CapLock book and just do it.