Because of my experiences, I have bought spares for things that have broken or gone missing, or that wear out. I'm not into fixing things that aren't broken, so I haven't worried about preemptively replacing recoil springs... then having to break them in again!
If you reload or shoot reloads, an extractor and maybe the parts behind that are nice to have. I had certain manufacturer's cases(AMER C) rupture in 9mm and lost extractors from my 3 different Kahr 9mm's... at an indoor range. I only found one, so I keep spares for all of my Kahrs now. The 9mm's extractor and pins/springs are all the same. The .380's are all smaller. The .40 and .45 extractors are marked(4 & two dimples) and different, but can use the front pin, springs, and rear pins of the 9mm. The EXCEPTION is the rear pin on the .45, which is bigger.
Slide lock springs, washers, and screws for all pistols. Some may be interchangeable. The P380's are smaller... screw is #5 TORX vs. #6 for the larger Kahrs.
Recoil springs for each, just outer for captured spring recoil assemblies.
I did get some side plate screws for my P9... no pin or side plate offered... and my P9 side plate did blow off when the case ruptured. I then culled the rounds from my re-manufactured 9mm ammo and tried those in my K9... NO side plate to blow off! The second round ruptured and blew the extractor into parts unknown... enough with the suspect ammo... destroyed the remaining 3 rounds!
I have lots of striker springs... bought the 5-lb 3-pack from Wolff, then bought the 10-pack and put them in all 7 Kahrs... one size does fit all. I put the old springs(all old springs) into small baggies and label them for "spares". I have new ones, too, but don't recall anyone having to replace one in their Kahrs.
I have spare Wolff magazine springs for all pistols, not just Kahrs, as well as recoil springs... some test kits with different rates.
If something flies off or breaks, Kahr will send you a new one, if you email them with a good picture and the gun serial number. I just don't want to be at the range and have something fly off or break, ending my range session! I had Three pistols with me once and two went down at the range, including my carry piece. I didn't have SD ammo with me for the PM45, but was able to trade a new box of my Speer 124-gr 9mm GDSB for the difference the LGS charged for the .45 GDSB... I paid 6 or 7 bucks and some change for the trade... instead of paying out $42.95 + tax for the 20-round box of .45 GDSB!! The range owner took pity on me!! My backup was to buy a box of the AAA TCJ 230-gr range ammo for $17+tax. I do get to rambling!
I think that's all of the spares. Reloaders or reload shooters may experience more overpressure events, and the extractor is likely to fly into parts unknown. I did have spares for events 2 and 3! And I did exchange the ones in the 3 9mm's and they worked in each other with actual firing... no discernible differences in those extractors. I did buy specific ones for each pistol to be sure.
Magazine parts... I have MANY magazines, and I have extra parts for changes I've made. Kahr will replace stuff, but if you're a one-mag kind of person... no "spare tire"... you might want to have some spare parts... followers for you .40 guys, especially, bases and locks and springs. The Magazine Maintenace Kit $15 from Kahr gives you a follower and a spring for a few more bucks than the spring alone... $12+.
JMHO based on my personal experiences. YMMV.
Wynn
USAF Retired '88, NRA Life Member. Wife USAF Retired '96
Avatar: Wynn re-enlists his wife Desiree, circa 1988 Loring AFB, ME. 42nd BMW, Heavy (SAC) B-52G's
Frédéric Bastiat’s essay, The Law: http://mises.org/books/thelaw.pdf
Thomas Jefferson said
“A government big enough to give you everything you want, is strong enough to take everything you have.”
and
"Peace is that brief glorious moment in history when everybody stands around reloading".