Underwood 10mm For Sale

Jeff Cooper wanted a cartridge that split the difference between 9mm capacity and 45 ACP authority, and in 1983 the 10mm Auto arrived to do exactly that. The FBI adopted it after the 1986 Miami shootout, then almost immediately downloaded it because full-power recoil was more than most agents could shoot well. That watered-down version became the blueprint for the 40 S&W in 1990, and most factory 10mm sold today is still loaded closer to those reduced numbers than to what Cooper drew up.

Underwood is one of the few manufacturers that never went along with it. The 10mm loads on this page are built to the original velocity spec, and the catalog splits into two clearly different jobs. The 135 grain JHP is the light-and-fast end, trading bullet weight for velocity and violent energy transfer. The 200 and 220 grain hard cast loads are the opposite approach entirely, and they are the reason a Glock 20 rides on so many hips in bear country.

JHP or hard cast? They are not interchangeable

• 135 grain JHP: the lightest and fastest load here. Rapid expansion and energy dump, best suited to two-legged threats where over-penetration is a real concern.
• 200 and 220 grain hard cast: lead alloyed with antimony and tin to resist deforming, cast with a wide flat nose that cuts straight through rather than expanding. Every bit of energy goes into penetration, which is what you want against heavy bone and hide.
• Hard cast is a woods load, not a defense load. A non-expanding flat nose that punches through a black bear will punch through a home wall just as easily. Pick the bullet for the threat.
• Check your barrel before running hard cast. Glock and some other manufacturers advise against unjacketed lead in factory polygonal-rifled barrels because of fouling. If you are carrying a Glock 20 in the woods, an aftermarket conventionally rifled barrel is the common solution.
• Full-power loads work your pistol harder. Many shooters running Underwood-level 10mm through a stock gun step up to a heavier recoil spring.

More Underwood ammo

• Underwood Ammunition - the full catalog across every caliber Underwood loads
• Underwood 9mm Ammo - +P and +P+ defensive loads
• Underwood 380 ACP Ammo - monolithic and high-pressure pocket pistol loads

Related 10mm ammo

• 10mm Ammo - the full catalog across every brand and bullet type
• Bulk 10mm Ammo - range and practice ammo at the lowest cost per round
• 40 S&W Ammo - the cartridge the FBI's downloaded 10mm eventually became

Other 10mm brands

• Federal 10mm Ammo - Power-Shok, Fusion, and Syntech range loads
• Magtech 10mm Ammo - affordable 180 grain FMJ in bulk case counts

Reviews and buying guides

• Underwood Ammo Review - our full assessment of the brand
• Best 10mm Ammo for Bear Defense - where the heavy hard cast loads earn their keep
• Best 10mm Ammo
• Lead Flat Nose Explained - why a flat meplat penetrates the way it does
• Lead Solid Bullets Explained
• 10mm vs 40 S&W - the parent cartridge against its offspring
• 10mm vs 45 ACP
• 10mm vs 9mm
• 10mm vs 357 Magnum
• 10mm vs 44 Magnum - the real question for woods carry
• 460 Rowland vs 10mm

For muzzle velocity and energy across every 10mm load we carry, including the gap between full-power and downloaded factory ammunition, see our 10mm ballistics database.