Smith & Wesson revolvers that are larger then the J-frame series have leaf mainsprings, and if properly adjusted the felt tension slightly decreases after the hammer has traveled through about ¾ of its travel. Older Colt double-action revolvers had a “V” mainspring that could stack if the two leaves came together before the hammer had reached full-cock. Again, careful adjustment would eliminate most of the felt stacking.
Later Colt’s, Taurus, Ruger and J-frame Smith & Wesson’s, as well as most other current revolvers have coil mainsprings, and the increased tension on the trigger stroke is progressive as the hammer travels backwards.
Objectionable stacking is usually caused when the coil or “V” springs are totally compressed – or almost so – before the hammer has fully completed it’s travel.