Deer hunting with an AR-15

A friend of mine recently went deer hunting for the first time with me. This gentleman is a well trained shooter, but primarily experienced with modern sporting rifles. In our area the whitetail deer go up to 180 lbs or so, with most being in the 100 to 120 range, and many consider the most prevalent cartridge used in modern sporting rifles, the .223 / 5.56, to be borderline for whitetail hunting. So my friend chose to use a traditional stocked .308 bolt action for his hunting rifle. 308 is right in the sweet spot for deer, being powerful enough to be very effective without being overkill.

After the hunt, my friend pondered using a AR-15 for hunting, since that is the platform he has trained on the most and it's operation is natural to him. We started discussing common caliber offerings for that platform and I decided to lay out those details here. For purposes of this discussion, I will be considering only cartridges chambered for AR-15's, not AR-10's. As well, I will stick to the more common offerings- chamberings in which both parts and ammunition are offered by multiple major companies. Sure, if you look hard enough you can find an AR-15 chambered in 50 Action Express and then go about modifying magazines to work with it, but that's not very practical, so I will be avoiding boutique offerings like that. If you think I have missed something that fits this (I'm guessing the 458 SOCOM fans won't like not being included here), please respond and make your case. There may well be offerings for which I am not fully informed.

In this post I will be comparing:
5.56 NATO / .223 Remington
6.5 Grendel
6.8 SPC
300 Blackout (subsonic and supersonic)
7.62x39
450 Bushmaster

What qualities make an ideal hunting cartridge is a topic that has been hotly debated for decades. Does impact energy matter? Do you want something that expends all of it's energy inside the animal or want something that shoots through? A light fast bullet that creates a very large temporary wound cavity or a larger diameter heavy bullet that punches a larger permanent wound channel? Is hydrostatic shock even a thing? Do you need a "brush busting" caliber that isn't directed off target by twigs or grass?

Based on a few decades of hunting experience and reading on the topic, I'm going to suggest an ideal hunting cartridge has the following attributes:

• A bullet that holds together. Fragmenting bullets make a mess of the meat and you spatter the animal with lead bits, many of which make it into the final product. And for ethical reasons, using non-lead bullets just makes sense. This rules out bullets or cartridges that deliver their terminal energy via fragmentation.
• A cartridge that is suitable for the range. In the woods of the midwest a long shot is 75 yards, any further and the object is quite likely to be completely obscured by trees. If you are hunting a field with longer shots that doesn't apply, but I know very few hunters that have any success hunting fields. Most deer are taken in the woods. This means that just about anything will work, since no rifle cartridges have significant drop at 75 yards.
• A cartridge that has enough power to pass through the animal. If the shot doesn't drop the quarry immediately, having two holes makes it easier to track.
• A cartridge that offers good terminal ballistics. This can be through making a large permanent cavity, a large temporary wound cavity, or both. A properly placed shot should do enough damage to the heart and / or lungs to drop the animal in a speedy and humane way.

In general, the class of cartridges commonly available in AR-15's are intermediate power rifle rounds or pistol caliber rounds. They aren't going to be as powerful as high power rifle rounds that are traditionally used for deer hunting today. That said, more deer have probably been taken in the US with the 30-30, which really straddles the class of high power and intermediate power rifle rounds. The most common loading for 30-30 is a 150 gr round nose bullet moving at 2390 fps. That results in 1900 ft-lbs of energy at the muzzle and 1275 at 100 yards. I think we can all agree that this is adequate power for whitetail hunting so I will use this as a benchmark to compare other cartridges to. Here is a chart with 30-30 at the top and the difference in 100 yard energy as the last column.

Cartridge	Weight	Type	Velocity	MuzzleEnergy	100ydEnergy	MuzzleDifference	100ydDifference
30-30 (standard)	150	Soft Point	2390	1900	1275	Standard	Standard
.223 Remington	55	Soft Point	3240	1282	974	-33%	-24%
6.5 Grendel	123	Hornady SST	2580	1818	1586	-4%	+24%
6.8 SPC	110	Nosler Soft Point	2700	1780	1464	-6%	+14%
300 Blackout Sub	208	Hornady A-Max	1020	480	467	-75%	-63%
300 Blackout Super	110	Hornady GMX	2350	1349	1060	-29%	-17%
7.62 x 39	123	Hornady SST	2350	1508	1136	-21%	-11%
450 Bushmaster	250	Hornady SST	2200	2686	1879	+15%	+47%

One of the disadvantages the 30-30 has is due to how it feeds, it has to use round nose bullets, which shed velocity faster than pointed bullets. So the 30-30 is more powerful than most of the options available in the AR-15 at zero yards, but it loses that advantage quickly as the yards stack up. By 100 all the options except .223, 300 Blackout and 7.62 x 39 have more power than the 30-30. But energy is just one factor to consider. We have to consider wound channels and evaluate whether each of these has the power to pass through a deer. This is where it gets a little harder to compare since there are varying opinions on what is best.

First, let's consider wound channel. Assuming we use the same bullet type in each that holds together and gets good expansion, we should get similar levels of expansion in each. For purposes of this evaluation, we will assume each bullet will expand 70%. Some manufacturers advertise 100% expansion, but I'm going to be more conservative here. I will say I have gotten excellent results with both Barnes TSX and Hornady GMX all copper bullets. So wound channel compared to our 30-30 standard is:

Cartridge	Bullet Diameter	Wound Channel	Difference
30-30 (standard)	.308	.524	Standard
.223 Remington	.224	.381	72%
6.5 Grendel	.264	.449	85%
6.8 SPC	.277	.471	90%
7.62 x 39	.312	.530	101%
450 Bushmaster	.452	.768	147%

From this chart we can see most of the cartridges lag behind the 30-30, if we assume the bullet expansion is a constant factor. That leaves us with two questions- First, will each of these rounds be able to expand and penetrate far enough? Second, how does the temporary wound cavity / hydrostatic shock play a role and how is it different for each of these cartridges? That is a much harder question to answer. We know that speed plays a factor but I'm not aware of any publicly available mathematical formula to project this.

I do know that over the weekend I shot a deer broadside, through both lungs, with an all copper bullet that did not fragment, and the lungs were a mess. Was it from rib fragments that shot through them? Damage from being stretched by the temporary wound channel as the bullet passed through? Hydrostatic shock that some people swear exists and others swear it doesn't? I can't say. But there is another factor or three at play. And observation over the weekend tells me that a 139 gr bullet of .277" diameter impacting a target somewhere around 2550 fps will turn a set of whitetail lungs into various size chunks of tissue and cause a deer to expire quite rapidly.

To answer this question we have to look at performance in ballistic gelatin. This will give us an idea about both penetration and temporary wound channels. Since none of the cartridges being compared here are quite at the power of my 7mm-08, expecting them to create the same results is a guess. Is there a gradual curve at play? A lower velocity threshold that has to be met? For this part we have to use our judgment where we can and just take a best guess for the rest. For consistency, I will be using Brass Fetcher videos when possible.

30-30 Winchester, 160gr Hornady FTX. Much greater than 18" penetration. The bullet left the block with lots of velocity.

6.8 SPC, 110 gr Barnes TSX. Penetration was greater than the 18" block.

6.5 Grendel, 120 gr Barnes TSX. Penetration was just greater than 18". It didn't come out of the block with much energy.

300 Blackout Subsonic. 220 gr Open Tip Match. 24" of penetration with very little disruption compared to other cartridges.

300 Blackout Supersonic. 110 gr Barnes Tac-TX (TSX). Penetration greater than 18".

7.62x39, Federal 123 gr Powershock. Penetration was greater than 18".

450 Bushmaster, two loads, one supersonic and one subsonic.

A test with 223, 6.5, 6.8 and 50 Beowulf.

All of the cartridges can penetrate the medium to an adequate distance. Will that translate into full penetration of a deer? It depends. Deer have bones that could slow down a bullet greatly. Lighter bullets slow down or get knocked off course easier than heavy bullets. Do all of these cartridges have the penetration power to punch through a shoulder and still offer enough damage to the internal organs to bring about a speedy death? What about a rib? Will a rib deflect or slow down the bullet enough that it doesn't do it's job properly? That's up to you to decide. Any of these will do the job in an ideal circumstance, but an ethical hunter needs to plan around shots that aren't quite perfect, animals that are moving or not broadside.

I think we can see if we look at the tests from 450 Bushmaster and 300 Blackout subsonic loads, that velocity certainly disrupts the medium. The heavy bullets moving at 1100 fps have very good penetration, but do not create temporary wound cavities or disrupt the medium (whether you call it hydrostatic shock or not). It seems to follow that more velocity = more disruption. This should be balanced against the permanent wound channel in the chart above when you make a choice. Do you want a larger permanent wound channel? More tissue disruption? Both? Both seems to suggest the 450 Bushmaster supersonic. But any of these choices seem to be an improvement over the .223, so pick one and practice marksmanship.

Lastly, I want to talk to trends. A dozen years ago 6.5 Grendel had little adoption and it looked like 6.8 SPC was going to take off. Now that trend is quite the opposite. 300 Blackout is here to stay. 450 Bushmaster is up and coming. 7.62x39 is here to stay and lately AR-15's have been chambered in that quite reliably. We live in a golden age of firearms and there are many good choices.