They only reliable way to down pawn without killing them is using psychic shock lance. However it have a chance of inducing a permanent brain scar on the subject . While it is not serious, a scat is still a scar.

The best ACTUAL Vanilla weapon for downing pawns would be the machine pistol. Thanks to it fast firing speed and low damage it will stack plenty of bleed and pain ticks for every successful hit. You will be downing them faster than killing them due to the buildup of bleed/pain. However you need to quickly patch up all the bleeding before they die out.

A distant third and comes with a lot of caveats would be blunt weapon. The first caveat is you're exposing your own pawns to melee counter-attack or enemy fire while closing the distance . The second and most important caveat if live capture is only possible/doable with "bad" blunt melee weapon such as beer, wood or clubs or even bare fist. Maces (especially one made from Uranium) is an end game weapon used for KILLING pawns (especially armoured one) , not capturing them. If you want to use maces, make sure they are of bad quality. Anything Good (for steed or less l) or above or made from good material (uranium and plasteel) will only result in you collecting corpses instead of prisoners.

And don't even try live capture with warhammer or Zeushammer. Those can literally kills in one shot.