M193 vs M855 vs M855A1: Which 5.56 to Buy

These three names get used as if they describe the same thing with different numbers. They do not. They are three different projectiles built for three different requirements, and only two of them are things you can realistically walk in and buy.

The short version

M193 is a 55 grain lead-core full metal jacket bullet, the original 5.56 specification. It is cheap, widely available, and stabilises in essentially any barrel you will find on a modern AR.

M855 is the 62 grain load with the green painted tip, built around a hardened steel penetrator seated over a lead core. It was adopted to improve performance against light barriers and at longer range. It is heavier, it needs a faster twist rate, and it costs a little more.

M855A1 is the US Army's lead-free replacement for M855. It uses a copper core with a hardened steel penetrator and runs at substantially higher chamber pressure than standard commercial 5.56. It is a military contract load rather than a normal commercial product, and what reaches the civilian market is limited and inconsistent.

Twist rate is the part that actually matters

This is the practical decision point and the one most often skipped.

Bullet stabilisation depends on barrel twist rate, expressed as one turn in so many inches. A lower second number means a faster twist and more stabilisation.

• 1:12 was the original twist for 55 grain M193. It will stabilise M193 fine and will generally not stabilise 62 grain M855 reliably.
• 1:9 handles M193 and M855 comfortably and is a reasonable general purpose twist.
• 1:8 is the modern default on most quality barrels. It handles both of these and the heavier match bullets above them.
• 1:7 is the military twist. It handles everything discussed here and the heaviest common loads.

If you do not know your twist rate, check the barrel markings before buying a case of anything. Almost every AR barrel sold today is 1:8 or 1:7, so most people are fine, but older and budget barrels are where 1:12 still turns up.

What each one is actually good at

M193 fragments readily at close to moderate range from a standard length barrel and is the better general purpose choice for most shooters. It is the cheapest way to put rounds downrange and the easiest to find in bulk.

M855 trades some of that behaviour for better penetration of light barriers and slightly better retained energy at distance. The steel penetrator is what people focus on, but the more relevant difference for most users is simply that it is heavier and bucks wind a little better.

M855A1 outperforms both on the metrics the Army cares about, at a cost: the pressure it runs at is hard on rifles not built for it, and it accelerates wear on bolts and barrel throats. It is also not something you should plan a supply around.

The legal and range questions

M855 is not classified as armor piercing ammunition under the federal definition, which turns on projectiles constructed entirely from specified hard metals and usable in a handgun. M855 has a lead core, which puts it outside that definition.

That federal answer does not settle two other questions. Some states restrict ammunition types independently. And a great many ranges prohibit steel core or steel tipped ammunition outright, usually to protect steel targets and backstops and to reduce fire risk. A magnet check at the door is common.

If you shoot mostly at commercial indoor ranges, this alone is a good reason to buy M193 for practice.

What to buy

For most people building or feeding a general purpose AR-15: buy M193 for volume practice, confirm your twist rate, and keep a smaller quantity of a quality purpose-built defensive load rather than relying on surplus specification ball ammunition for that role.

Buy M855 if you specifically want the heavier bullet, your barrel is 1:9 or faster, and your range permits it.

Do not build a plan around M855A1.

5.56 worth buying

American

American Eagle 5.56mm 55gr FMJ Ammunition 150rds - XM193BK150

In stock

$104.99 at Palmetto State Armory+ Add to build

American

American Eagle 5.56 55gr Ammunition 120rds Mini-Can - - XM193LPC120

In stock

$47.97 at Palmetto State Armory+ Add to build

Aguila

Aguila - 5.56x45mm - M855 - 62 Grain - FMJ - 300

Out of stock

$164.80 at True Shot Ammo+ Add to build

A note on 5.56 versus .223

M193 and M855 are 5.56 NATO specification. A rifle chambered in 5.56 NATO can safely fire .223 Remington, but the reverse is not true: 5.56 runs at higher pressure and has a longer leade, and firing it in a .223-only chamber raises pressure further. Check your barrel marking. Most AR-15 barrels are 5.56 NATO or .223 Wylde, and both handle either cartridge.