Any suggestions on reasonable 9x18 defensive rounds?  I'm not thinking of using my PA-63 as a primary carry gun or anything, but on the off chance I do have to carry it (e.g. if my Sig is down for maintenance) I'd like it to have something effective...

Check out underwood ammo.

I carry Hornady Critical Defense in my CZ-82.

9x18 mak FMJ has one of the lowest penetration in is class compared to other calibers

I wonder how far it would go in gel

Remember, it's wider surface area then 380.

Depending on your over penetration concerns, I wouldn't have a problem with fmj in a makarov.

I would carry FMJ before I loaded up some un-tested, under penetrating HP.

I would consider Hornady XTP loading in the MAK. But most tests in jell show it penetrating about 10".

How about Russian-made 95gr FMJ?  The last time I chrono'ed some garden-variety Brown Bear FMJ, it averaged nearly 1,100fps out of an East German Makarov, along with flawless reliability too.  Sure beats harsh language.

Hornady CD or XTP. It's not going to get to 12 inches reliably, but it's close. FMJ otherwise

I haven't seen anything that can expand and reach the 12" minimum. Stick with FMJ.

Like the .380, use ball or XTP.

Mine has hornady critical defense. I also have about 500 hand loads with an xtp that were compressed loads. They would be perfect but The group started to spread as I got the velocity up.

This is what I use, but in .380. Good stuff!

My main concern would be to make certain the round penetrates enough.

95gr XTP underpenetrates in bare gel:
https://m.youtube.com/watch?v=miSAAcqe8Wo

Id probably get the Underwood 95gr XTreme penetrator. Theres some hype about it but I would expect it to stay forward oriented and do at least 15in penetration in gel with or without denim. Or will penetrate less than the 115gr hardcast. Id probably use this bullet in .380 too.

Finally, I am not so sure the hardcast in the test above would be so bad.

https://underwoodammo.com/shop/9x18mm-9mm-makarov-95-grain-xtreme-penetrator/

That's my concern too ...

I have a CZ 82. It's a great little shooter. Great DA/SA trigger, sights you can see, and it's very accurate. I also stocked up a few years ago on those Czech 12-rd mags while they were still plentiful ...

But plinking at the range with it is one thing   -  for me, the problem has always been choice of SD ammo if I were to ever carry it. I'm not impressed whatsoever with the penetration (gel) tests I've seen with some of the *standard* JHP ammo.

I've never tried the BB hardcast stuff, but for a SD carry load, why shouldn't I just stick with 12+1 of military FMJ 'ball' ammo at mil-spec velocities?

Well, I usually carry a Glock 40 with Underwood  200gr XTP, hehe.

I think FMJ would do, maybe the shape of the hardcast would wound better. To me 22ij in gel isnt crazy penetration but it is over...

Three reasons, all of which are correct in theory, but may well be a distinction without a difference in reality.
1) A bullet with a meplat (flat nose) will tend to cut through tissue, while a jacketed round nose will tend to shove tissue out of the way. The bigger (wider) the meplat, the closer it is to a wadcutter shape and the more cutting it does. The limiting factor is reliability in feeding. The BB bullet doesn't have a real wide meplat.
2) The BB bullet is heavier. As Boris The Blade noted: "Heavy is good. Heavy is reliable." Heavy = more momentum, which = better / more reliable penetration, all else being equal.
3) The BB load is faster. Faster is also more goodly-er, all else being equal. When driving a solid bullet, we don't have to worry about going too fast (unlike with hollowpoints).
The downside (TANSTAAFL) is a possible reduction in feed reliability and an increase in recoil, slowing followup shots. You'll almost certainly get an increase in tissue damage. As to whether that increase is significant, it depends on your definition of "significant". You ain't gonna turn 9x18 into 9x19, but you can make it a little better.
Ultimately, what you hit is far more important than what you shoot.

Yes, and OP if you carry a JHP that does 10-11 inches in gel, and you hit the bad guy perfect, it may stop short of the heart of spine, which would not be good. If you use the hardcast you will almost certainly get the penetration you need.

I'll paraphrase what Col. Cooper said about the 9x18;...good round for the other side to use.

I would like an aftermarket barrel for my Glock 43 in the 9MAK. I have no idea why. I just like the round. Maybe one day we will see $4 a box MAK ammo again and it would be worth it... I certainly believe that with a little development, a load could be made to meet the FBI min standards. Maybe a 105-115gr HST going 1000fps. But no market for it I guess.

That ammo moving at 1100fps is certainly effective. My brother just got a Polish made/version of a Makarov. He's using Russian manufactured ammo and it's got about as much thump as my CZ-P01 with full power ball. He's using the Hornady XTP's rounds for carry. They certainly are accurate out of that little pistol!

Do remember that with the exception of the Vz 82, all the East Bloc pistols chambered in 9x18 Makarov are blow backs and all blow back pistols have much more felt recoil than a similar sized pistol with a Browning type action.

Not knocking the Mak cartridge here, but felt recoil alone is not a measure of how powerful any cartridge is, especially out of a blow back pistol.