North American Arms Mini-Revolvers (often called derringers) are 5-shot, .22 rimfire, single action revolvers, which have a spur trigger design. They are reminiscent of late 19th century pocket revolvers, only much smaller and made completely of stainless steel. Mini-revolvers were developed by Freedom Arms in 1978, which stopped selling them in 1990 and then sold the design to North American Arms. Since then, North American Arms has further evolved the design, switching to a safety cylinder design, featuring notches machined halfway between the chambers. As a result, the revolver may be safely carried with all five chambers loaded by resting the hammer in a safety notch. Older NAA mini-revolvers can be retrofitted with this safety cylinder design upgrade at no cost to the owner simply by returning an older design NAA mini-revolver to North American Arms for an upgrade.