The problem with a Nelson is that it doesn't lend itself well to semi-auto conversions because of how stiff the cocking action is.
Heh. 'Stiff cocking.'
If you want an efficient pistol, a well-tuned Squall can get up into 30 shots or so...and Sheridans > Nelsons.
(I'm a Sheridan fan. Nelsons, while still perfectly good, just feel so...unfufilling to me.)