Hi Keith, The Glock 43X is not recommended for +P+ ammunition, including the Underwood 90 grain, as it exceeds the firearm's pressure limits.
+P+ ammunition, which exceeds even the +P standards, is generally not recommended for use in firearms not specifically designed and rated for it.
Glock suggests using ammunition that is manufactured and meets SAAMI or industry specifications. Some +P ammunition meets these guidelines, +P+ normally does not.
Stay safe & have a great day!

Sir Keith,
While some shooters have used +P+ 9mm in Glock pistols without immediate issues, Glock does not officially endorse or recommend using +P+ or +P+ loads in the 43X. For safety and longevity, it's best to stick with standard-pressure 9mm ammunition unless your firearm's manual explicitly states otherwise. Glock's official documentation typically states that their firearms are rated for standard pressure ammunition. Many Glock models, including the 43X, are not officially rated for +P+ ammunition.
Safety considerations:
Shooting +P+ rounds in a firearm not rated for them can increase wear and tear, and potentially affect safety over time.
Hope this helps Keith! Please take care and stay safe 🙏

It’s not rated for +p+ ammunition.

It’s not rated for +p+ ammunition

Good evening to you!! The Glock 43x isn't rated for +P+ ammunition. Shooting such wound possibly damage the firearm and void the warranty. Hope this helps. Have a good rest of your weekend.
🇺🇸🇺🇸 'merica' 🇺🇸🇺🇸

No, it is not recommended to shoot +P+ ammunition (plus-plus) in a Glock 43X. While the Glock 43X can handle some +P ammunition (plus-p), +P+ ammo is generally not recommended for this firearm. +P+ ammunition is loaded to higher pressures than +P and can cause excessive wear and tear on the firearm, potentially leading to damage or malfunctions.
Here's a more detailed explanation:
+P vs. +P+:
+P ammunition is loaded to higher pressures than standard ammunition, but +P+ is even higher and can exceed the safe operating pressures for many firearms.
SAAMI Standards:
SAAMI (Small Arms Ammunition Manufacturers' Institute) sets standards for ammunition and firearms. +P ammunition may meet SAAMI standards, but +P+ often does not.
Glock's Recommendation:
Glock recommends using ammunition that meets SAAMI or other industry standards. They explicitly state that +P+ ammunition may not be safe for their firearms.

Hi Keith and welcome. +P is fine and +P+ can be shot in moderation. You won't get the full benefit you think. +P and +P+ = more powder and requires at least a 4-inch barrel for full powder burn for +P and slightly longer for +P+. You will get a marginal increase in velocity but not what you think since +P &+P+ perform best in longer barrels, more fire ball and muzzle blast due to unburnt powder. This will lead to lower speed follow up shots and quicker loss of night vision in a self-defense scenario in a 3.4-inch barrel. May or may not be the benefit you seek given the shorter barrel of the 43x. There are plenty of excellent 9mm ammunitions for best performance in a short barrel. Cheers, stay safe and enjoy!