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Do CZ75 parts fit? How much aftermarket fun can you have with the Witness? I had a 10mm Witness once, and wanted only a flatter trigger.
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CZ75 parts do not fit, frame size is much different. Witnesses were/are made with small and large frame size. Most common in the US is the Large frame (which can handle .45acp and 10mm as well as 9mm/40sw/38super), the small frame is close to CZ75 size, grips fit the small frame with some work and mags might work in the small frame.
Aftermarket it somewhat scarce, but there are a few specialist shops that sell parts / upgrades. For the large frame guns, MecGar makes mags that work great. Wolff spring kits abound, Hennings and a few others make go-fast parts such as upgraded guiderods and firing pins and such. Aftermarket grip options are thin... Tanfoglio aren't well known for consistency in frame spec year to year and as a result grips may drop in fine, or require fitting. Haven't looked for grips for the compact Witness, but I sprung for a set of Hennings grips for my full size. Nice thin aluminum grips (took about 0.2" off the width of the grip... nice) but spendy.
Flat triggers do exist... they're for SAO conversions. Looking at doing one in my full size 10mm Witness at some point.
Still pondering a compact 10mm version...