Let's start by saying that throughout history there has likely never been a more modified or easily customizable platform as the AR-15 series of rifles. You can make a short barrel rifle for home defense or CQB work, a long range rifle geared towards collecting prairie poodles or competition and virtually anything in between. There are likely more accessories for the AR-15 than the Chevy 350 and Honda Civic combined. Everything from quad rails, triggers, barrels, lights and even chainsaws and flamethrowers (Yes Virginia, even flamethrowers!). With all of this modularity and customization that can be done by the end user one might ask:
~"Wes, where in the world do you get that the AR-15 has a problem that can't be readily solved by the current aftermarket!?"~
The answer to that question is caliber selection.
~"Wes you must be crazy, we can choose from all types of calibers and chamberings, surely there is one that fits the bill for the end user!"~
It would seem that way but we are working with an inherited handicap in the platform and that is the Cartridge Overall Length (COAL). The AR-15, as designed by Eugene Stoner almost 70 years ago, utilized a magazine length capable of supporting cartridges only as long as 2.250" The rifle's design, along with adoption into military service, helped to establish the 223 cartridge and solidify its place in the mainstream shooting world. This level of widespread acceptance was fueled by the availability of inexpensive brass and surplus ammunition.
If we compare the cartridge capability of the AR-15 to the short action Remington 700 it becomes readily apparent that we cannot utilize many of the available cartridges on the market due to either the COAL or the case head diameter.
~"But Wes, the AR-10 or AR-308 exists to 'fix' this perceived issue"~
Yes the AR-10 platform corrects for the caliber issue but comes at a great cost over a comparable Remington 700 and that is overall weight. One of the great things about the AR-15 is that it can fit into many roles where a sub 8lb rifle shines. Roles such as a hunting rifle or truck gun, a home defense rifle or, arguably more importantly, as a civilian rifle capable of allowing an armed populace to resist an authoritarian government while allowing for freedom of movement.
In short, unless the rifle is mounted to a tripod on a truck, weight matters.
Now it is said that "necessity is the mother of invention" and nowhere else can that be as readily seen as when it comes to finding new cartridges to stretch the capability of the AR-15. Cartridges such as the 6.5 Grendel, 6mm ARC, 22 Nosler, 22 Valkyrie, 300 Blackout, 6.8 SPC, etc. All of these cartridges fit into the COAL limitation of the AR-15's magwell and allow the platform to outperform the 5.56/223 in various niche roles. The handicap of the COAL becomes clear when we ask the question of "Which of these cartridges would not be better served with even more case capacity by going from 2.250" to 2.500"?" The short answer is that ALL of these cartridges, even the 300 Blackout, would benefit from having a longer case capable of supporting more case capacity. Let's take a look at just how a few popular cartridges could benefit from a simple change to the platform:
300 Blackout - The ability to push the lighter bullets faster while surpassing the supersonic ammo in 7.62x39, the ability to load the heaviest 30 caliber bullets to magazine length meaning that subsonic 300 Blackout would pack more punch than a 45 ACP.
6.5 Grendel - More powder capacity to push the standard 123gr bullet faster for more energy and bucking the wind better, the ability to run the ballistically superior 140+gr 6.5mm bullets without losing precious case capacity.
6mm ARC - The ability to push the heaviest 115gr 6mm bullets inside of normal magazine lengths, more powder capacity and therefore speed on normal hunting weight bullets.
22 Valkyrie/22 Nosler - Heavier bullets don't eat into case capacity as much allowing higher speeds, less wind drift and better performance on target.
Just about every cartridge can benefit from increased case capacity so this is an easy exercise. The real fun comes when we start looking at what could be stuffed into this new design. Forgotten cartridges like the 6, 6.5, or 7mm TCU cartridges, which take the 223 case to it's peak performance in bullet selection and energy, would now fit into the platform and offer a much needed performance boost. The 7 TCU, for example, can push a 120gr bullet over 2650 fps while the 5.56 could push a 77gr bullet to 2800 fps. The performance of the TCU, carrying over 1-1/2 times the bullet weight, on target is greatly improved and this is with the slowest of the TCU style calibers (The 6 TCU could very likely push a 95gr Matchking to velocities similar to the 77gr Matchking out of a 5.56).
But what if we keep in the spirit of "what could have been" if the original design encompassed a 2.500" magazine length. Would we see brass cases that are longer be available to usher in a new wave of wildcat cartridges?
Imagine if the 6mm ARC or 6.5 Grendel cases were available with an untrimmed length of 2.000" that were capable of running a 9mm bullet? This would make the 350 Legend look like a pip-squeak and allow for a new wave of short barreled home defense rifles/pistols in the platform. This could even completely supplant the entire Pistol Caliber Carbine (PCC) idea of AR-15's.
How about that same case necked down to allow the use of midrange 7mm bullets such as the 162gr Hornady ELD-X? This would allow for one of the highest BC bullets to be run in the AR-15 platform and would likely make for a hunting cartridge that would stomp all over the 6.8 SPC or a long range bullet that could deliver more energy and buck the wind better than either the 6mm ARC or 6.5 Grendel.
What about a 22 Nosler case lengthened and blown out to 6mm or 25 caliber for a flat shooting medium game round or for a national defense rifle that would stretch further than the 7.62x39 or 5.45x39 cartridges. This would provide for our nations defense by allowing the average rifleman to engage a potential enemy beyond any distance where they could effectively return fire. Or perhaps a 22 Nosler necked down to 20 caliber and making essentially a 204 Ruger Super Magnum that could run 40gr bullets another 300-400 fps faster or allow for manufacturers to make high BC long for caliber 20 caliber bullets (light kicking 20 caliber PRS rifle anyone?).
~"Well Wes, if this is such a good idea then why doesn't any manufacturer "stretch" the AR-15 platform and benefit from this superiority?"~
The answer is pretty simple; institutional inertia. The platform has been around since the 1950's and the public, manufacturers, and military are already tooled up and heavily invested in the current design. Imagine a company like Magpul deciding to support a move to a new platform with something as simple as a magazine change. This effort would require a large amount of capital in the form of design hours, injection mold tooling, testing R&D, marketing, etc. Very few manufacturers in the firearms world would be willing to invest in a change such as this without a guarantee that the public sector would flock to the new design which brings us to our next reason.
With the average gun owner already supplied with magazines, ammo, spare parts, etc. they would need to see adequate real-world performance, instead of marketing hype (which tends to be the norm), before leaving the tried and true design of the current AR-15. The military has all of the same investment issues as the public sector but on a much grander scale. With millions of rounds of 5.56, hundreds of thousands of rifles and magazines and the integration of NATO in both parts of the current system, the military would be talking about a wholesale shift from not only the AR-15 but also the M-249 SAW.
In a nutshell, the price constraints is what kills this type of advancement and we have only looked at the firearms manufacturing side of things. The ammunition side of the market would have many of the same growing pains but with a potentially higher upside to the scenario in ammunition sales for groundbreaking newer cartridges and possible military contract work should the design take off.
The most logical path forward for a design change such as this is a new platform altogether. This breaks the mental ties of comparison to the AR-15 and is simply billed as a replacement platform with enhanced capabilities over an aging design that has been upgraded beyond its useful lifecycle. This would allow for other items to be upgraded and brought into the 21st century such as a buffer system that would allow for a folding stock or a magazine catch design that more easily allows for ambidextrous use. I envision that manufacturers would pattern the magazine to work in AICS patterned rifles to allow cartridges such as the 6 Dasher or 6mm PPC to be utilized in the platform but also allowing the magazines to be used for competition focused precision bolt rifles as well to offset any marketing risk. This would have a side benefit of allowing a paddle style magazine release such as the HK or CZ Scorpion's use.
Just imagine, for a moment, what could be if the industry was pushing the platform outside of the 2.250" box and being truly innovative; The next 20 years would be a revival of wildcat designs and accessories that could change the future landscape of the small arms world!