I have not bought a High Power yet, but it's on my list. It looks like MecGar magazines are the prefered choice. I believe the new Springfield ships with those. They sell a 13 and a 15 round magazine. Is there any advantage to buying the 13 rounder? I'm a little confused. That gun was in production for quite a while. If there was room for 2 more rounds, why didn't they just make them in the 15 round version from the beginning?

MecGar magazines 100%

Mec Gar.

I have 13, 15, and 20 rounders that all work flawlessly.

This.....!

Browning mouse, trap magazines

Mec Gar

Mecgar mags are good to go but none of them handle 15 roounds.  14 is the best I have done and 13 rounders are my choice.  I am not trying to take on the cartells from hell and I think AIA can kill you with 13.  1 extra mag and holy cow.

IIRC the 15s have a slightly extended plastic basepad - I’m too lazy to pull one out to check. Mine have all worked fine.

This.   Plus, additional rounds in a mag adds weight and for CCW I prefer as light as possible. Maybe I'm stupid but if I can't avoid dying with 13 rounds maybe it's my time.

ETA: Mec-Gar mags are excellent.

All four of my 15s hold that much, and the 20 holds a +1.

37 rounds on-tap for a modest weight increase over just using 13 rounders is pretty sweet.

Mousetrap springs are the only that have given my brother and I problems. Got rid of those post haste.

One of the arguments for a spare mag is to have another chance in the event that the mag in the pistol fails. Properly maintained, it's a low probability, but magazines are the most likely failure point in the firearm. I get it, though: on rare occasion I find a spare magazine inconvenient and take my chances. Usually I have the spare.

Springfield ships with a 15 round MecGar.  So find the cheapest MecGars and stock up.  

Mec-Gar have been OEM for the Hi Power for some time.

I don't entirely trust the 15s.  I sometimes get stoppages in some guns with them as well as failures to lock the slide back.  Hit or miss.  The majority I've used do not have issues but some do.  It should be noted that anti-tilt features had to be removed to make the mag fit 15 rounds in the same length.  I've reverted back to the 13s for carry for the time being.  Never had issues fully loading them, though.  

I do have a 13 that I believe to be Mec-Gar that fits 14 rounds and has for the whole time I've had it (over a decade) and it was my grandfather's and father's before that, probably bought originally in the 1990s and has run reliably the whole time.  I would have thought maybe the spring was weak enough to cause issues but so far so good.  

My Mec-Gar 20s also hold an extra round.  I have noticed that you have to ensure the top cartridge is oriented properly, as it will want to sit at too shallow of an angle, which will cause it to fail to chamber the round.  But if it is, then they run fine.  They do also have an issue locking the slide back reliably if the recoil spring is worn.  Putting in a new spring fixed the issue.  Mine were brand new in the wrapper when I got them, labeled for Federal Ordnance in El Monte, CA, which I thought was funny.

I have KRD 15 and 17 rounders that run great, but it can be tought to seat the last round in the magazine.  These are the ones with extended base plates to increase capacity.

I also have FM (Argie) 13s which run great as well as a Mec-Gar 10, which runs well and I only use now for specialty cartridges, like snake shot, or range use.

Echoing bigstick61, I run 18.5 lb Wolff recoil springs in my HiPowers.  Never an issue with Mec-Gar 13 round magazines but the slide will rarely lock back with the 15’s.

Huh?!

I have a 24xxx serial numbered gun and it's been dead reliable with the original 15 rd mag and the 5 or 6 others.  Never, ever a bobble.  The gun is dead stock, dead reliable and very close to being my most accurate centerfire autoloader.  I carry it frequently.

Someday l need to do a head-to-head shootout with my RIA Tac ll 10mm to determine which is most accurate semiauto.