In an earlier post I mentioned the advantages of using a pistol caliber carbine over a handgun for home defense. The next question I often get asked is how pistol caliber carbines stack up against more typical carbines chambered in intermediate power rifle cartridges. The thinking is, if you are going to use a rifle, why not use one that uses a cartridge that has more power? I'll attempt to explain some advantages and disadvantages of an intermediate cartridge carbine contrasted against a pistol caliber carbine. To keep some order, I'll break this down into categories.
One of the most popular rounds in the USA today is the 300 Blackout (aka 300 BLK). While based on a .223 case, it's design parameters were quite different, as it was designed to work well with subsonic loads. The BLK uses very heavy bullets for the powder load, half or less powder than you find in a .223. The powder to bullet weight ratio is more like a pistol caliber than a rifle caliber. In a 16" barrel you aren't going to have any unburnt powder left when the bullet exits the barrel. For these reasons, subsonic 300 Blackout is much quieter than .223, 5.56 NATO or 7.62x39 rounds. 300 BLK also works well with lighter bullets at higher velocities, but increased velocity gets you additional noise. So in this post, I want to compare a pistol caliber carbine against a 300 BLK carbine for both subsonic and supersonic loads. I will break each out into several categories. Because each person and their requirements are different, each category may be more or less important to you than other people.
Ease of Handling - Winner 9mm
How easy a firearm is to handle is determined by several factors. First we can look at barrel length. Both firearms can be reliable in firearms with shorter barrels, 9mm runs perfectly fine in 4" barrels or 16" barrels. For most loads, 9mm gets peak velocity out of a barrel that is around 14". 300 Blackout also works well out of shorter barrels, 8" is common, but about as short as one can go and ensure reliability. Velocities drop off steeply for 300 BLK if you go shorter than 12.5" for many loads.
If both firearms are AR-15's, then the size and weight will be very similar. However, 300 Blackout is only available in AR's and bolt action rifles, while 9mm carbines are available in a number of different rifles. The Hi Point carbine is very popular, as is the Kel-Tec Sub-2000. The CZ Scorpion is a newer offering and the Beretta Storm Carbine was quite popular for a time. There are carbine conversion kits from Roni and Mako that are basically an exoskeleton for a 9mm pistol. There are more options in 9mm, which increases the likelyhood that a given design will best suit the needs of a user.
Sig MPX 9mm Carbine pictured
The AR platform is very ergonomic, but some of the 9mm carbine options come in at a lighter weight or have other packaging advantages. Even in a 9mm AR, if you get a dedicated receiver design it shortens up the length. So for this first category of ease of handling, the nod goes to the 9mm platform.
Cost - Winner 9mm
Again, there are many affordable 9mm carbine options, many at lower price points as you can get a 300 Blackout rifle. As well, 9mm ammunition is around half the price of 300 BLK, which means ongoing operating costs will be lower. You can put together a budget 300 BLK AR-15 for less than a budget 9mm AR-15, but that's the only case where 9mm costs more. Overall cost goes to 9mm.
Muzzle Blast - Tie
Muzzle blast is encompassed of the noise and flash of a firearm firing. The noise is a combination of the powder burn, unburnt powder flashing outside the muzzle, a supersonic crack (if there is one) and the noise of the firearm action. Muzzle blast is a category where we must consider what ammunition is being used- for both 9mm and 300 BLK we have both supersonic and subsonic offerings. Subsonic is quieter for 2 reasons- you don't get the crack of the bullet breaking the sound barrier and subsonic loads tend to use faster burning powder that is all burned in a short length of barrel. This means less blast from powder that burns outside the barrel.
9mm and 300 BLK are both pretty quiet relative to other carbine cartridges when subsonic loads are chosen. Based on my experience and reviewing real world data, 300 Blackout subsonic does have an advantage. Both are louder with supersonic loads. The BLK is much louder with supersonic ammunition, the 9mm has less of a delta between sub and supersonic loads. Of the 4 options, only 300 Blackout supersonic has any real muzzle flash. You can move to a faster 9mm load with less of a penalty than 300 BLK. Keep in mind muzzle flash and noise can be disorienting at night, especially indoors. A muzzle flash that ruins night vision is not optimal if you or your family are being attacked. Since both of these cartridges are among the most pleasant to shoot when it comes to muzzle blast, I'm rating this category a tie.
Terminal Ballistics - Undecided
On this category I've gone back and forth several times. I'm not ready to declare a tie, but neither do I think there is a clear cut winner for all uses. First let me be clear- 300 Blackout has more energy in almost all barrel lengths than 9mm. In fact, 300 BLK can have twice the energy in many cases. That's significant. However, extra energy can come with disadvantages in muzzle blast and even in practical considerations of terminal ballistics. One must consider efficiency- using enough tool to get the job done without overkill can often be advantageous.
For this section, I'll point you at some videos where you can view videos of each of these rounds fired into ballistic gelatin. Is this a perfect comparison tool? Certainly not. But it's one of the better data points we can consider. First up, we will look at our problem child, 300 Blackout subsonic. Why do I call this the problem child? Overpenetration and lack of expansion. The currently available 200 - 220 grain 30 caliber bullets just don't expand at subsonic velocities. So you get a .3" hole, with maybe a twist a foot in, with a lot of energy still in the bullet as it leaves the target. This increases the change you could shoot through an attacker and the bullet could harm a person or damage property. That's a serious attribute to consider in populated areas.
Contrast that against 9mm subsonic with a permanent would channel of .7" or more. 9mm bullets are designed to expand at lower velocities and do so reliably. Notice also how it sheds velocity quickly inside the gel block. So while your 9mm bullet has half the energy when it hits the target, it dumps most of it's energy into the target rather than zipping straight through and carrying that energy into some other target. It has a larger permanent and temporary cavity than the 300 BLK subsonic. In this case the 9mm is both more terminally effective and less likely to harm something past the target.
9mm supersonic offers similar results to subsonic.
300 Blackout with lighter bullets pushed to supersonic velocities are the top performer. They hit the gel block much harder and create a noticeably larger wound channel. It's easily the top performer in this category.
Final Thoughts
I almost included a chart with velocities over barrel lengths, but I decided to summarize instead since this is getting long already! 9mm cartridges his peak velocity around 14" of barrel length, and get 90% of their velocity in 10" of barrel they have at 16" of barrel. They use less powder than 300 Blackout and it's often a faster burning powder, so it accelerates the bullet in a shorter time period. 300 BLK takes a longer barrel to get top velocity, especially with supersonic loads. Supersonic loads suffer greatly in barrels that are 8" or 10" long. 16" barrels are better suited. For subsonic loads, 12" or so seems to be a nice balance between handiness and reliability.
9mm kicks more. Is it substantial? It's not. But due to the faster powder and many 9mm carbines being straight blow back in action, 9mm carbines offer more felt recoil. Both of these options are pretty low when it comes to recoil though, so it's not a significant factor, perhaps.
Is 9mm enough? Does it's quality of being less likely to overpenetrate give it an edge for use in urban or suburban areas? Is the increased terminal effectiveness of supersonic 300 Blackout worth the extra muzzle blast? Only an individual can answer that for themselves. I'd suggest subsonic 9mm has the advantage if one was running less than 12" of barrel length and 300 Blackout supersonic loads have the advantage at 16", where muzzle blast is minimized and velocity is maximized. But either one in any barrel length is a fine choice, perhaps among the best possible choices, when selecting a home defense firearm.
Thanks for your time!