I think places like Midway sell the SA 6" barrels, but SA does not and will not sell the 6" slides. There is too much gunsmithing involved in making the conversion.
About the only solution is to send it to gunsmith like Jim Clark Jr. (there are a few others who also do the conversions with Caspian slides) who will convert it to a 6". But this is usually done for Bullseye competition shooting and is quite expensive.
After I got my SA Longslide, I sent the slide to Storm Lake Machine Co. in Tenn(?) and had them measure and fit to the slide a 6 1/2" comped barrel. Approx $250 for the barrel, link, pin, comp fitted to the slide. Feet were cut oversize for a gunsmith fit. Sent it back to SA to be fitted, so I wouldn't void the warranty. Custom shop did a fantastic job fitting the barrel. My only complaints were with Deb in the office. She almost doubled the quoted price then told me I didn't have to pay the new price, but " We have your gun" if I didn't.
Super job on the gun.My set-up is: stock main spring, EGW flat bottom firing pin stop, change firing pin spring fairly often, 20-24# recoil spring (depending on how hot my super's are). Tracy
have you called SA I dont see why not I was going to take my 6inch longslide trophy match and have them put a 5inch slide and barrell on it for IDPA, it is the same frame, but not sure about mil spec frame, call them