Best Ammo for a 10.5” AR Pistol: Why the 62-Grain REX Is a Top Choice

The 10.5” AR pistol has become popular among shooters for its compact size, maneuverability, and versatility. Whether you’re using it for home defense, as a truck gun, or range time, one major challenge remains—velocity loss. When running a barrel shorter than the standard 16” rifle length, the 5.56 round loses velocity, which can impact bullet fragmentation, expansion, and overall terminal effectiveness.

This issue is especially apparent with traditional 5.56 loads like M193, M855, and even heavier 77-grain rounds. These rounds depend on high velocity to perform as intended, and a shorter barrel often means compromised ballistic performance.

So, what’s the best ammo for a 10.5” AR pistol? Enter the HOP Munitions Solid Copper 62-Grain REX (Rapid EXpanding) round—a purpose-built solution designed to expand consistently at lower velocities, making it the ideal choice for short-barreled ARs.

Key Takeaways

• Velocity Matters – A 10.5” AR pistol loses velocity, reducing the effectiveness of traditional 5.56 rounds like M193, M855, and 77-grain HPBT.
• Standard 5.56 Struggles – Lighter FMJs may not fragment properly, while heavier HPBT rounds lack the velocity to expand as designed.
• 62-Grain REX Solves the Problem – Designed for short barrels, the HOP Munitions 62-Grain REX expands reliably at lower velocities, ensuring maximum stopping power.
• Solid Copper for Superior Performance – Unlike FMJ and OTM rounds, the REX delivers deep penetration and consistent expansion, making it perfect for home defense, hunting, and tactical use.
• Optimized Ballistics – With 2,870 fps muzzle velocity and 1,134 ft/lbs of energy, this round is purpose-built to excel where others fall short.

Why Traditional 5.56 Ammo Struggles in a 10.5” AR Pistol

The biggest drawback of using a 10.5” AR pistol is the loss of velocity due to the shorter barrel length. The 5.56x45mm cartridge was originally designed for 20-inch barrels, where it achieves optimal velocity, fragmentation, and penetration. As barrel length decreases, so does muzzle velocity and energy, which directly impacts how different types of 5.56 ammo perform.

Here’s how the most common 5.56 loads react in a 10.5” AR pistol

• M193 (55-Grain FMJ): This lightweight full metal jacket round relies on high velocity (above 2,700 fps) to fragment upon impact. In a 10.5” barrel, velocities typically drop below 2,600 fps, reducing its fragmentation reliability.
• M855 (62-Grain Green Tip): Known for its mild steel penetrator, M855 performs well in longer barrels, but in short barrels, its velocity loss results in poorer terminal ballistics, often passing through soft targets without significant damage.
• 77-Grain OTM (Open Tip Match): While heavier projectiles like 77-grain HPBT (Hollow Point Boat Tail) can offer better stability and accuracy, they lack the velocity in a 10.5” barrel to expand properly, limiting their effectiveness for defensive or hunting use.

The Problem: Terminal Ballistics at Lower Velocities

In a 16” rifle barrel, standard 5.56 rounds hit velocities where they can fragment, expand, or tumble, ensuring effective terminal performance. However, when you cut the barrel down to 10.5 inches, you’re looking at a significant loss of 300-400 fps, depending on the round. This means:

• Reduced fragmentation – FMJs like M193 are less effective.
• Limited expansion – Heavier rounds don’t expand as designed.
• Weaker terminal ballistics – Less velocity means less energy transfer.

So while you can still shoot M193, M855, and even 77-grain rounds out of a 10.5” AR pistol, they aren’t optimized for short-barrel performance.

That’s where the HOP Munitions 62-Grain REX comes in.

Why the HOP Munitions 62-Grain REX Is the Best Ammo for a 10.5” AR Pistol

If you’re running a 10.5” AR pistol, you need a round that’s designed to perform at lower velocities while still delivering reliable expansion and energy transfer. The HOP Munitions 62-Grain REX (Rapid EXpanding) round was engineered specifically to address these challenges, making it the ideal choice for home defense, hunting, and tactical use.

Here’s what makes the 62-Grain REX stand out:

Designed for Consistent Expansion

Unlike M193 or M855, which rely on fragmentation at high velocities, the 62-Grain REX uses a solid copper expanding projectile that reliably expands even at the lower velocities seen with short-barreled ARs. This ensures maximum tissue damage and energy transfer, even when fired from a 10.5” barrel.

The Perfect Middle Ground in Bullet Weight

At 62 grains, the REX sits between the lightweight M193 (55-grain FMJ) and the heavier 77-grain HPBT, offering the best of both worlds:

• Lighter than 77-grain rounds, meaning it maintains higher velocities.
• Heavier than 55-grain M193, providing better penetration and energy retention.
• Tuned for short-barrel performance, ensuring reliable function and terminal effectiveness.

Solid Copper Construction = Maximum Energy Transfer

Many traditional rounds use lead-core bullets, which can suffer from inconsistent expansion. The REX is a solid copper round, meaning it:

• Expands consistently at lower velocities.
• Transfers more energy into the target rather than passing through.
• Outperforms traditional FMJ and OTM loads in a 10.5” AR pistol.

Impressive Ballistic Performance

The 62-Grain REX isn’t just a gimmick—These specs allow the REX to maintain effective ballistics even from shorter barrels, something most traditional 5.56 loads struggle with.

• Muzzle Velocity: 2,870 fps
• Muzzle Energy: 1,134 ft/lbs
• Ballistic Coefficient: .300
• Sectional Density: .177

RELATED – HOP Munitions 62-Grain Solid Copper REX vs. M855 Green Tip and Mk318

Final Thoughts: Upgrade Your 10.5” AR Pistol with the 62-Grain REX

A 10.5” AR pistol loses velocity, which can compromise the performance of standard 5.56 rounds. While M193, M855, and 77-grain HPBT have their uses, they aren’t optimized for short barrels—leading to poor expansion, over-penetration, or reduced stopping power.

The HOP Munitions 62-Grain REX is engineered to expand reliably at lower velocities, maximizing energy transfer and terminal performance. Its solid copper construction ensures deep penetration and devastating impact, making it the best choice for home defense, hunting, and tactical applications.