I will be getting a Glock 20 gen 4 soon and have been looking at various SD ammo in 10mm. What 10mm SD ammo would you use?

It depends on if you want full power (700 ft-lbf class) or lighter loads.

If full power, Buffalo Bore, Underwood or Double Tap.  Buffalo Bore does have a couple of low recoil/low flash intermediate power tactical loads (~600 ft-lbf)

Most "normal" 10mm loads are "light" 10mm loads (which still have more energy than your typical 9mm, 40 and 45 ACP loads); something like the Hornady XTP or Critical Duty loads clock in at >400 ft-lbf.

All of it.

Underwood.

If I was looking purely for a self-defense load against humans I'd probably go for something like the 175 grain Hornady Critical Duty or Winchester Silvertips.

I would suggest shooting a few mags of each to get an idea of how well you can shoot it quickly, one handed, with your weak hand, or whatever. At the end of the day, even with a relative canon, you've got to hit targets well for quick stops.



"Normal" 10mm with good bullets would probably be my choice, just because the recoil is more manageable.



 

I've used Double Tap and it was very accurate in my EAA Witness. I got out of 10mm before I heard of Underwood, but based on reputation I'd buy their stuff too.

If you want a steady supply of full power ammo for less money you'll want to start making your own.

I use Underwood

This.

I carry DoubleTap 165 grain Gold Dots.

Same here.

Federal 180gr Trophy Bonded JSP

The heaviest Gold Dot available.

For woods carry I use the heaviest XTP available.

Whatever is;

1. the most controllable
2. has the least amount of flash
3. has the best expansion or barrier penetration characteristics, depending on need.

Having the best round available doesn't mean a thing if you can't put it on target or if there is so much muzzle flip that it takes too long to get the sights back on target for a follow up shot.  You have to find what round works best with your handgun of choice.  I've got a RIA 1911 in 10mm that I'm still experimenting with.  Until I've found an acceptable round that meets the above criteria and that I'm comfortable with, it won't be used as an EDC.

Testing through my RIA so far;

Federal Premium Personal Defense 180gr Hydra-Shok's are controllable with very little flash.

Stay away from the Cor-Bon 150gr JHP's.  The ones that I ran through the RIA had a very large flash and sharp recoil.

Hornady Custom 180gr XTP's (#9126) worked well.

I just ordered various Underwood JHP's along with their Defenders and Penetrators so that I can test those.  I had good luck running the .357 Sig Defender through my Steyr M357-A1, so I'm hoping that their 10mm will be just as good in the RIA.

135 gr jhp Underwood. If you need something with a little less recoil,  try the 180 gr bonded hollowpiont. Not the full power load. The Delta Elite load. A tad slower but has less recoil.