A Mare’s Leg is a lever-action rifle cut down so it can be carried in a (large) custom holster and held and fired with one hand.
For purpose of Really Wild West (a weird western setting hack for the Starfinder Roleplaying Game), any longarm can be cut down to be a Mare’s Leg, with an hour’s work and a successful Engineering check (DC 10 +1.5x the item level of the longarm). This causes it to become a small arm. You can also buy a Mare’s Leg version of a rifle anywhere you could buy a longarm 2 levels higher than the item level of the desired Mare’s Leg (though you may have to wait a day as one is custom built), at a cost of 120% of the longarm base price.
Base damage does not change, but Weapon Specialization applies as a small arm. A Mare’s Leg’ range increment is cut in half, attacks and any skill check to use trick attack with it suffer a -2 penalty, and any other penalty that applies to attack rolls with it also apply to trick attack skill checks. It has a Strength minimum equal to 10 + ½ its item level, and for every point of Strength you are below this minimum you take a -1 penalty to attack rolls (if you use a Mare’s Leg in two hands you increase your effective Strength by 5 for meeting this minimum).
Design Note: A Mare’s Leg is not an ideal weapon choice for many characters. It’s not a strict damage upgrade for characters most likely to be able to overcome the attack penalty and Strength minimum, and places penalties on the main use of character who can use it as a strict upgrade (operatives). It’d be very useful for some specific character builds, such as a high Strength envoy, drone mechanic, mystic, or technomancer… all of whom don’t have a lot of synergy from having a high Strength.
All of this is intentional. The Mare’s Leg is not a common item (it likely never existed in the real Old West), but it’s cool looking and iconic. It’s an interesting piece of gear to carry as a high-power back-up, or to add a tweak to otherwise quirky character designs, and that’s what it’s designed for.
Also, Really Wild West only goes through 10th level, so if you use these rules in a typical Starfinder game, it may have unexpected consequences when entering higher-level campaigns.
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