A method and system of providing a needle-less injector specifically configured to the needs of a user. The method includes assembling a needle-less injector by positioning an inner housing within the outer housing such that the inner housing is axially movable within the outer housing between a ready position and a firing position. The method further includes receiving a user selection of a delivery spring weight from a plurality of available delivery spring weights. Then a delivery spring having the selected spring weight is placed within the inner housing. The completed needle-less injector having a delivery spring with the selected weigh may then be delivered to the user. In alternative injector configurations, the user may also select one or more of a skin tensioning spring weight or a hammer length along with or instead of the delivery spring weight.