One of the reasons that the AR platform is awesome is because of the huge variety of loads available to suit almost any situation.
It is also probably best to get this out of the way… Anyone or anything you shoot with your AR regardless of bullet will have a really bad day. So this is about finding something “the best” or as close to it as possible. There are many really good choices, many of which have been named in this thread, and once you get to that top tier you are really splitting hairs (handgun self-defense ammo is similar in this regard).
Your barrel will play a role in your decision as well. The twist rate will determine how heavy of a round you can shoot. 1/7, 1/8, 1/9, and 1/12 are common twist rates with the fastest rate (1/7) capable of stabilizing the heaviest rounds (up to 77gr). Your length of barrel will play a factor as well because the length will determine the velocity (more out of a 20", less out of a 10") and some ammo is more velocity dependent than others.
With any choice you make here… buy a small quantity (maybe one or two boxes) and test to make sure your rifle “likes it”. Once that is confirmed, then buy whatever quantity you like, zero your rifle to it.
Wounding Mechanisms
There are 3 types of wounding mechanisms generally. They each have pros and cons.
- Expansion. Pretty self explanatory, things like hollow points and soft points
- Fragmentation. the projectile breaks apart into one or more pieces that each create a wound channel
- Tumbling. Rounds meant to tumble are the least consistent and most velocity dependent.
Most common military rounds (such as M193 or M855) are meant to tumble because of outdated geneva (hague?) conventions, but we have much better choices available as civilians. Some of the US military rounds like mk262 and mk318 use (I think) OTM-style bullets.
I will mention Temporary Stretch Cavity, because you may hear/see it when researching but I personally dont consider it an important consideration because any rifle round at home defense distances will likely be over the threshold (YMMV). TSC, is the temporary tissue expansion from when something traveling at rifle speeds hit a fluid object. Most tissue snaps back to its original shape, but will tear/shear from the expansion. Some organs are less elastic (like the liver) and suffer way more damage from TSC than other organs. The oft-quoted velocity that a round must be going in order to achieve TSC is (i believe) 2100fps, but that is dependent on a lot of variables.
Barrier Blind
I will also mention a term you’ll see a lot which is important (IMO), which is “barrier blind”. This is sometimes confused with armor piercing or over-penetration. What it means is that when the projectile goes through something does it perform the same as if it did not go through something. Meaning it was “blind” to the barrier. Usually, this comes into play if you have to shoot through something like a window, auto glass, auto door (sheet metal), drywall, etc.
For example, something like a hollow point bullet may hit a barrier (hog shoulder, auto glass) and deform such that when it passes through the barrier it no longer expands and then subsequently over-penetrates instead of expanding properly and penetrating to its designed depth.
Many (all?) fragmenting rounds are not barrier blind. Hollow point, soft points may be barrier blind if they are bonded or jacketed.
Ammunition choices
This article over at ar15com is a fantastic reference, although it is beginning to show its age as the last update was (I think) 2016. Skip to the .223/556 section for a good list of options, and there are good references and explanations scattered throughout the document.
Some projectiles are available in different loadings from different manufacturers. Usually what is important is the projectile. For example, one of the most common is the Sierra Match-King (69gr & 77gr) which is frequently listed as SMK and is available from a lot of manufacturers including Federal, Black Hills, Gorilla, Fiocchi, etc. The Sierra Tipped Match-King is one of the best bullets available right now for defense and hunting and is frequently listed as TMK.
The below ammunition choices are thoroughly my opinion. But hopefully this will steer you towards a shortlist of ammo to do further research on. There are tons of gel-tests and online reviews of pretty much everything I’ve listed below.
If you are running a short barrel (aka 10-13"-ish) rifle and are concerned about over-penetration, I would point you to one of the 77gr Sierra Tipped Match King loadings by Black Hills or Corbon (likely others as well). This is a fragmenting round and is very highly regarded in the hunting community. Review of the Corbon loading here. This round does not do as well at higher velocities from a longer barrel and is NOT barrier blind. This round is generally considered to be pretty accurate, although the SMK is generally considered more accurate but doesn’t perform as well as the TMK.
If you care about having a barrier blind round, I highly recommend the Speer Gold Dot .223 series which is a bonded soft point available in 55gr, 62gr, or 75gr (64gr is no longer made, but you may see old stock, let me know if you do!). If you have a slower twist barrel use the lighter 55/62gr options and if you have a faster twist barrel you can use the heavier 75gr. All are considered pretty accurate. One of the perks of the 3 weight options is that you can pick a load that matches your training/range ammo. Federal makes the Fusion 64gr marketed as hunting ammo which uses the same projectile at a slightly higher velocity which is just as effective and some say more accurate, but I find is usually significantly more expensive. The Speer Gold Dots are usually available from a lot of ammo vendors at a pretty reasonable price. Reviews can be found here and here.
There are lots of other excellent rounds in this category, some of which have already been listed. Like the Hornady TAP series which has several varieties and is in frequent use by LEO. The Federal Trophy Bonded Bear Claw (TBBC) is effectively a unicorn as I have never seen it available, but IIRC it is an FBI approved load and those who have it, swear by it. The 50gr TSX by Black Hills (Black Hills is a smaller manufacturer, all their stuff is GREAT but expensive) is an awesome all copper barrier blind expanding round reviewed here.
Over-penetration
First, it is a very important fact, that any projectile capable of inflicting lethal damage on a human-sized target will 100% go through at least one interior wall (two sheets of drywall, likely no insulation, and not hitting a stud), and very likely go through several walls before it hits an exterior wall or has shed enough momentum to where if it hits something it would be non-lethal.
This is a physics problem. The heavier a projectile is, the better it retains its momentum and remains lethal longer. The lighter the projectile is, the less it retains its momentum and does not stay lethal for as long.
This is why people who are really concerned about over-penetration might choose birdshot from a shotgun, since those extremely tiny and lightweight pellets barely make it through one wall. However, birdshot is at best debateable whether it would stop someone, and you now have 300+ projectiles instead of 9 from buckshot, or 1 from a handgun/rifle that you have to account for. So its not a trade-off without consequences
This applies to AR15 defensive ammunition, because the very lightweight 55-77gr projectiles will go through LESS walls than a 9mm 115-147gr projectile, a .45ACP 200-230gr projectile even though those handgun projectiles are travelling at lower velocities.
If you really want to have ammunition that is extremely capable, and will offer the least amount of penetration then a fragmenting 5.56mm/.223 round like the TMKs listed above would be among your best choices.
I have previously posted on this topic and the link below has more info and links to several tests around the internet. To again, be 100% clear… I am NOT saying 5.56mm/.223 ammunition doesn’t go through walls. What I AM saying is that 5.56mm/.223 ammunition goes through LESS walls than almost anything else except for birdshot from a shotgun.
I dont mean to make light, but the best solution to over-penetration concerns is hitting your target. Second best solution is knowing your home layout and where to place yourself so that any errant shots go in a safe(ish) direction or into a solid object that is capable of containing the projectile.
From my own home defense plans, my 1st choice is to make a stand is the top of the stairs and shoot downwards where the only thing I might hit with a miss is the hardwood floor. My fallback from there is a bedroom at the end of a hallway and shooting down that hallway if any projectiles happen to leave the house (unlikely after passing through 1 wall/door and a brick exterior wall) it would have to go through about 50ft of forest before maybe hitting a neighbors house (also brick) if i was really unlucky.
PDN happened to have this video not too long ago, I bookmarked it as they show some of what I mean about knowing your layout. Knowing where your furniture and people may be (and maybe willfully placing some in strategic locations) can do a lot to mitigate over-penetration concerns.
I hope that helps, I’m happy to answer any questions, especially on the over-penetration issue as it is often misunderstood and there is lots of empirical data out there.