The Pistol Caliber Carbine (Part 1)

The Pistol Caliber Carbine (PCC) is debated in terms of its practical use.  Some view it as a range toy, others include this firearm as part of their survival plan.  In exploring this topic we need to first look at hard data before we can really explore things like philosophy of use.  In Part One of the Pistol Caliber Carbine I will explore the physics involved with this class of firearm versus the conventional SHTF rifle, the AR15.
In Part 2, we will use this data to discuss philosophy of use.
In Part 3, we will talk about common PCCs in Canada.

There are two empirical methods of assessing the terminal performance of a round
1) Kinetic energy
2) Momentum
You can skew the results in favor of your favorite firearm just by selecting a method of measurement that supports the strength of your bullet.  The kinetic energy method holds that a rounds potential is based on the physics formula for kinetic energy where energy is equal to half the mass times velocity squared.  With this measurement method, bullets that go fast are considered superior to those with mass.
On the other end, momentum is based on s simple velocity times mass equation.  People who gravitate towards heavier calibers prefer this method of measurement.
Speaking anecdotally, real world users of light but fast bullets such as HK’s 4.7mm in the MP7, tell about having to expend half a magazine to quickly put someone down.  Similar to this, Delta force operators in Mogadishu found that their heavy hitting 1911s in 45ACP were more effective then their M4 carbines in putting the drug addled Somalis down.
Secondly, if a bullet’s energy expands into an empty body cavity or it exits the body having not transmitted all its energy, then energy kinetic is not an appropriate measurement.
This is why I prefer momentum and will use this as my measurement standard for the following analysis.

In Canada, AR15 users (in an effort to look more cool) seemingly have gravitated towards the 10” barrel on their AR15s.  I don’t know anyone who goes out and shoots anything other than 55grain bullets (except for me) so I will use that as my bullet weight.
Using momentum, the numbers read as follows… 2616fps (797m/s) on a 55 grain (3.56g) projectile.
Final result is 2.837 Newton Seconds.

Out of a handgun, the average 115gr 9mm projectile reads as follows… 1100fps (335m/s) on a 115 grain (7.45g) projectile.
Final result is 2.496 Newton-Seconds.

Out of a pistol, the 9mm is a lot worse than the 10” AR15.

Now if we up the barrel length to 18” on our 9mm PCC to make a non-restricted firearm in Canada our velocity jumps to 1297fps (395m/s)
Final result is 2.942 Neuton-Seconds.

Oh ho!  That is better than the 10” barrel AR15!

How about your standard 230gr (14.90g) 45ACP projectile out of a 18” barrel?
Final result is 4.472 Newton-Seconds.

Wow…. The 45ACP has ballistics like a rainbow with its low speeds but at close ranges, it has an unbeatable punch to it.


Specter Arms sells 14.75” barrel AR15s.  Longer than what is cool but let’s run the numbers on those…
Final result is 3.122 Newton-Seconds.  At this point, the AR15 beats the 9mm PCC but still falls short against the 45ACP PCC.

So the truth is, from a physics perspective, the PCC is just as effective as a AR15 at close ranges.

Now long ranges (100m+) is where the PCC falls down.  This is where we need to look at philosophy of use…  but that is for the next article.

223/556 and the AR15 Barrel

In the world of the AR15, shorter is cooler.  This runs contrary to everything else in life.  Guys want giant trucks, giant houses, giant dogs, big big big!  Even in firearm calibers there are those that would rather have giant calibers such as 458 SOCOM or 50 Beowulf.  Yet when it comes to the AR15 it seems that the opposite holds true or should I say, the opposite holds false  - The AR15 is something where size actually does matter either in a bigger caliber or a longer barrel.

The original M16/AR15 had a 20 inch barrel.  With the 55grain rounds of the time they would achieve a muzzle velocity of 3259 feet per second.  It was determined at the rifle’s conception that a velocity no less than 3000 feet per second would be needed to achieve fragmentation and thus maximum lethality in the 55grain M193 round.

During the Vietnam conflict Special Operation Groups or SOG Teams would conduct clandestine operations in Vietnam ranging from black propaganda to direct action.  A shorter, handier rifle was developed to lighten the load and compensate for SOG’s long term reconnaissance patrols and for rear echelon troops not needing a full rifle.  The new M4 Carbine had a barrel length of roughly 14.5” giving a muzzle velocity just a touch over the needed 3000 ft/s.  This meant that the M4 could rarely have the needed velocity past 100 meter mark to achieve the lethality that the round was developed for.  With this limitation a new round was developed.  The slower moving but longer 62 grain round designated the SS109 was a fair way to fix some of the fragmentation problems.  It may seem counter intuitive, but the longer SS109 (although slower) would fragment easier as it was a longer bullet.  Think of it as trying to break a short stick and trying to break a longer stick – the longer acts as a better lever and snaps easier.

So the M4 was…. Well….. better now that it had a new round but after the battle of Mogadishu, Delta reported that their M4 carbines had little effect on the Somalis.  They often switched to their heavy hitting 1911s to bring the insurgents down.  It was forgotten that simply having a longer barrel on the M4 would of given them back the lethality that their rifles needed.

USSOCOM found that the higher unlock pressures generated by the shorter M4 gas system, caused premature wear on the rifle causing parts needing to be replaced or repaired at a rate 4 times faster then that of the M16.  For a government agency it is not a big issue… for an individual on a budget, this seems like a big deal.

But for reasons I simply can’t understand the mentality of “smaller is cooler” concept took the AR15 platform in a ridiculous direction; 10” and 7.5” AR15 barrel lengths.   Essentially if you were to stick the barrel of such an AR against someone’s flesh, you MIGHT get lethal bullet velocity but I would argue the blast is more effective at that point in dealing damage.  10” AR15s simply turn bullets into noise.  There is no advantage at that point other than LCF – Look Cool Factor.  If you really need something that short, switch to a SMG or a pistol caliber carbine.

The main people I deal with are not military and don’t have milspec M193 or SS109 and are using the similar (but not identical) .223 Remmington round.  I can only conclude that the minimum length for the lower pressure and lower velocity 55gr .223 Remington, a barrel length should be no less than 18 inches to get the needed effect out of the round.  Should a user opt for a heavier 62gr bullet, you could go as short as 16”.  But always remember – bigger is better.