HK MR556 A4

The HK MR556 A4 is Heckler and Koch’s premium gas-piston 5.56 AR-15 rifle, built around a cold hammer forged cannon-grade steel barrel and a modern M-LOK handguard. With an adjustable two-way gas block and HK’s proven short-stroke gas piston system, the HK MR556 A4 is engineered for accuracy, durability, and easy maintenance. For the shooter who wants an AR-15 that runs cleaner and shoots tighter for longer, this rifle sets the standard.

What the HK MR556 A4 Delivers

The heart of this rifle is its 16.5 inch cold hammer forged cannon-grade steel barrel. HK ran a head-to-head 12,000 round test between a mil-spec chrome-lined barrel and their new cannon-grade steel barrel, and the results were decisive. After 12,000 rounds the mil-spec barrel’s groups had opened up 2.5 times larger, while the HK barrel showed no degradation in accuracy at all.

HK stands behind that barrel with a lifetime barrel warranty, so if you ever shoot it out, they will replace it for free. That kind of confidence from the manufacturer is exactly what you want backing a serious rifle you plan to shoot hard.

Features That Set the HK MR556 A4 Apart

• 16.5 inch cold hammer forged cannon-grade steel barrel backed by a lifetime warranty
• Short-stroke gas piston system that keeps the action cleaner and cooler than direct impingement
• Adjustable two-way gas block for precise tuning to ammunition and suppressor use
• Modern M-LOK handguard for lights, lasers, grips, and backup sights
• Robust steel construction built for long-term durability and genuinely hard use
• Proven HK engineering and quality control carried through the entire build

Specifications Woven Into the Build

The short-stroke gas piston system is the defining feature of the HK MR556 A4, redirecting gas away from the bolt carrier so the action stays cleaner and runs cooler than a traditional direct-impingement AR. That translates directly into more reliable cycling and less maintenance over long shooting sessions and dirty conditions.

The adjustable two-way gas block lets you tune the rifle to your ammunition and to suppressed use, while the M-LOK handguard gives you a modern, lightweight mounting platform. Together with the cannon-grade barrel, these features make the MR556 a genuine premium performer that earns its price.

Practical Use, Care, and Compatibility

Because it is built on the AR-15 platform, the HK MR556 A4 accepts standard AR magazines, optics, and M-LOK accessories, so outfitting it is simple and affordable. The piston system keeps fouling away from the carrier, but you should still clean the piston components and chamber regularly and keep the rifle properly lubricated for the best results.

Learn more about the maker at HK, and when you want to compare premium rifles, browse our full firearms collection to see how it measures up.

Clean-running, tough, and backed by a lifetime barrel warranty, this HK is a benchmark 5.56 rifle. Like every firearm, it ships to your local licensed dealer for transfer after your background check.