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The Requirements
For the modern battlefield, an effective range out to 600 meters, and preferably 900 meters, is ideal. The ability for the round to penetrate common barriers, such as glass or concrete, is of particular importance, and for example contributed to U.S. difficulty in Vietnam, where the dense jungles often stopped the bullets. The new round idealistically needs to be easily compatible with existing 5.56mm weapon platforms, to make the conversion as easy as possible. Finally, the weapon needs sufficient stopping power at close range as well, where less accuracy (and therefore skills) often puts soldiers at risk if their enemies are not immediately incapacitated from their rifle fire. It has been known that under particularly intense combat soldiers can continue fighting for hours after being shot (despite a wound being lethal), especially after taking various drugs which give them a higher resistance to pain (which is a known strategy of the terrorists, such as with their use of Synthetic Heroin), and physical stops of opponents through sheer brute force over psychological elements such as pain or fear has become increasingly more important. Furthermore, given the assortment of weapons used by modern irregular militia groups, such as terrorists, it is possible for U.S. troops to be facing off against many different types of weapons that are far from uniform, ranging from bolt action rifles to submachine guns. Often using old WWII era soviet weapons, such as the mosin nagant or Ak-47, it is not actually unreasonable to conclude that the U.S. may be facing off in many situations against enemies using weapons that are actually better than the 5.56mm at their given ranges, even though most modern militaries have abandoned such "antiquated" weapons. The 7.62mm x 54mmR for instance, while no longer used as a primary rifle cartridge by most countries, is quite commonly used in PKM's, dragnuv's and mosin nagants and commonly used by terrorists and irregular military's, which has a range in excess of 1000 meters. This has become so much of a problem that the military is now developing a .338 NM machine gun in order to reach out to the same ranges as the 7.62mm x 54mmR, which has been proven to be troublesome even for the .308. At the same time, various pistol cartridges of the soviets are still commonly in use in submachine guns, which gives the terrorists an advantage over larger, less maneuverable weapons in close quarters combat. The wide variety of weapons on the battlefield and the environment itself places extreme demands on the average soldier and their weapon, given that the enemy does not use a uniform series of weapons and it can become necessary to have a weapon that can both be used in close quarters combat, or at long ranges in order to be prepared against every major threat. Given the difficulty of outfitting a soldier with every possible weapon they could need at any range or predicting the requirements of every battle, a one-size-fits all approach is essentially necessary to be prepared for any engagement. While the type of weapon for this job is important, there is also no short selling the importance of the cartridge itself.

So, what cartridge possibly possesses the means to fullfill all of these roles while also being light weight and low recoil enough to serve in a small enough firearm, to be usable as both a carbine and long range rifle round? There are a few options, with the 7.62mm x 51mm NATO capable of fulfilling all of these goals, albeit being substantially heavier and with far greater recoil than is generally found to be acceptable. The 6.8mm remington and 6.5mm Grendel are likely the best candidates, designed and used in tandem with the U.S. marksmen units and special forces, and that have shown considerable promise in the field. The rounds are only 30% heavier than the 5.56mm with comparable recoil from the same weapons, while still maintaining the effective barrier penetration, stopping power and range needed. To some extent, if less rounds were needed to incapacitate an enemy individual, this slight loss in the number of rounds capable of being carried could actually translate in to a net gain. Less rounds can be carried due to a slight increase in weight, but less rounds are needed; furthermore the slight increase in weight would be negligible enough to warrant the shift (with the average soldier's combat pack only increasing by 3 pounds for the same amount of ammunition). Heavier rounds are less dependent on fragmenting and yawing for stopping power, less dependent on velocity, and lose less velocity at longer ranges due to the higher inertia and better aerodynamics. The rounds are both roughly twice as heavy as the 5.56mm, 7.5 to 8 grams (compared to 3.6 to 4.1 grams for the 5.56mm), and about as heavy as the 7.62mm x 39mm round while being more accurate, aerodynamic and having a higher velocity. However the the full cartridge with the brass and gun powder are around 16.3 grams each, compared to 12.5 grams for the 5.56mm, or around 30% heavier (and roughly equivalent in weight to the 7.62mm x 39mm).

The muzzle energy is 2,600 joules for the 6.5mm grendel and 2,700 from the 6.8mm remington, from a full length barrel, while approximately 1,800 joules for the 5.56mm and 2,200 for the Ak-47. This is 45% more energy than the 5.56mm and 30% more than the 7.62mm x 39mm, which gives it substantially more power even at close ranges. The recoil is comparable to these weapons and easily manageable by most shooters, making it ideal for carbine length weapons, while the 7.62mm x 51mm NATO has approximately 3-4 times the recoil of a standard 5.56mm. In addition, modern day recoil buffers and muzzle breaks that also dampen the noise and flash of the weapon (and prevent kickback normally associated with muzzlebreaks) can reduce this down to the same recoil of the 5.56mm, albeit it the 5.56mm can also have it's recoil reduced further by these means. The rounds are heavier and more powerful than the 5.56mm, but still have a velocity around 800 to 850 m/s, which is roughly comparable to most sniper rounds between 750 and 880 m/s, the 7.62mm x 51mm NATO, and the Mk. 262 5.56mm sniper round, giving it more than adequate range and bullet drop to remain viable. For most practical purposes, there is no real advantage to extremely high velocity cartridges, which burn out most of their energy in the first 200 yards due to their reduced aerodynamics, tend to strike targets with too much velocity to reliably penetrate (the high velocity actually reduces penetration at above certain speeds), and tend to be easily deflected, and also increases the erosion of the barrel, as well as increase pressures and friction in the gun in general. The somewhat paradoxical nature of velocity is that a faster bullet should have a longer range, but faster bullets experience greater aerodynamic resistance, thus reducing their range. This phenomnina is why most sniper rounds are actually heavier than their standard counterparts, with the M118 sniper 7.62mm x 51mm NATO being approximately 11.3 grams vs. 9.7 grams for the standard cartridge, and 5.56mm Mk. 262 being 5 grams vs. 4.1 grams for te standard cartridge. The 6.8mm Remington and 6.5mm Grendel have roughly the right balance of velocity for range and flat trajectory, but sufficient penetration and good aerodynamics to remain competitive out to long range. This is partially achieved due to the high Ballistic coefficient or, BC of the cartridges, with the standard 5.56mm having approximately .2 BC or .35 BC with the sniper cartridge, the 7.62mm having approximately .35 and .5 BC for the standard and sniper cartridge, and the 6.8mm Remington and 6.5mm Grendel having .35 BC and .5 BC comparatively, being roughly on par with the standard sniper rounds of the 5.56mm and 7.62mm, respectively.

At close ranges, the rounds are extremely powerful with more than enough power to reliably incapacitate targets in 1-2 hits, and penetrate through common urban barriers (possessing greater energy and penetration than the 7.62mm x 39mm). They have sufficient velocity to reach long ranges but not such a highly velocity they fail to penetrate certain barriers (like the 5.56mm), are heavy and big enough not to be deflected by certain targets (such as glass or ceramics) but do not tend to overpenetrate soft targets (such as people). The rounds are above average in performance in nearly every category, but excel tremendously at long range. Due to the high accuracy (both rounds get over 1 MOA in accuracy with standard factory ammunition, which is roughly sniper grade) and good aerodynamics, both the rounds have an incredibly long range in comparison to the 5.56mm, and to some extent even the 7.62mm x 51mm NATO. At 600 yards, the 5.56mm has approximately 420 joules, which is less than a 9mm. The 6.8mm Remington has 690 joules, while the 6.5mm Grendel has 1080. This gives the grendel roughly the same power as a full power .357 magnum at 600 yards, and exceeds the power and penetration of the 7.62mm x 51mm NATO cartridge at approximately 650 yards. The U.S. military found the effective range of the 6.8mm Remington to be approximately 500 meters, although this was from an M4 carbine length barrel, which is substantially shorter than that commonly used by the Grendel (14.5 inches compared to 20 inches, respectively). The 6.5mm grendel is at least as effective as the 7.62mm x 51mm NATO cartridge at 600 yards and beyond, giving it roughly comparable ranges to the cartridge or about a 1000 yard range, despite significantly less recoil and muzzle energy. This gives it a fairly realistic engagement distance of 900 meters, well within the parameters desired for an ideal carbine cartridge, and gives the 6.8mm Remington sufficient performance out to 300-600 meters, or well within the parameters the study found the 5.56mm marksmen weapons had trouble with. The rounds are as heavy and even more powerful than an Ak-47 round, with superior barrier penetration and stopping power, yet have superior accuracy, range and even armor penetration than the 5.56mm. While being slightly heavier and with slightly more recoil, none of these drawbacks are particularly difficult to overcome for the average shooter, and their benefits seem to outweigh any potential drawbacks, being a far superior compromise in the intermediate cartridge category. In addition to this, most 5.56mm weapons can easily be converted to using the 6.5mm Grendel and 6.8mm Remington, using the same magazines, lower receiver and generally sharing parts commonality with most 5.56mm rifles. This in particular makes the round attractive for this role.


6.8mm Remington vs. 6.5mm Grendel
The main dilemma comes down to the 6.5mm Grendel and the 6.8mm Remington, in the assault rifle or carbine platform. Both are very accurate, with 1 MOA, have roughly the same energy and recoil, are designed for similar purposes with similar barrier penetration, and can easily be fit in to any 5.56mm weapon's receiver with a few minor changes (namely the bolt, barrel and spring). They can be used in any carbine length weapon with little issues of recoil. The main difference comes down to one main factor; preferred combat role. The 6.8mm Remington is inferior to the 6.5mm grendel in regards to aerodynamics and general ballistics, having substantially less energy at longer ranges and dropping off quickly at long ranges (with a G2 BC of approximately .35, vs. .5 BC for the 6.5mm Grendel). However, while the 6.5mm Grendel would make an ideal sniper cartridge, being inherently accurate and having the same range as much larger rounds, the 6.8mm Remington is superior from shorter barrels. Designed specifically with faster burning powders designed to work well with shorter barrels, like those used by the m4 carbine, the weapon achieves approximately 2,000 to 2,300 joules from a 14.5 inch barrel, and still maintains it's high accuracy and firepower. This is more than the 7.62mm x 39mm, and the round still has an effective range out to 500 meters, slightly under the desired range, from a carbine sized weapon, and far outdoes the 5.56mm at 1,450 joules. The 6.5mm grendel on the other hand usually neccessitates longer barrels, and while delivering superior terminal ballistic performance, is not as effective from shorter barrels.

This essentially gives two options; choose a carbine length weapon, such as the M4 which is 30 inches long, with an effective range just under 600 meters, or a full-length rifle like the M16, 40 inches long, and have an effective range of out to 1000 meters. Given the U.S. military's current tend towards maneuverable carbine rifles, the 6.8mm Remington would seem to be the more preferential cartridge if used in the M4 carbine. However, there is an option that would satsify both requirements. Using a bullpup weapon, such as the tavor, a 30 inch long carbine-length weapon that has a 20 inch long barrel is theoretically possible. If a bullpup, like the Steyr Aug or Tavor is used, this would effectively eliminate the need for a carbine length weapon like the M4, allowing for both a long barrel and a carbine length weapon, lending favor to the 6.5mm Grendel. This in turn would make the 6.5mm Grendel the superior cartridge choice, if a bullpup weapon was used. As the Tavor has proven exceptionally popular worldwide and is known for both reliability, ergonomics and accuracy, it likely would be the best choice in the 6.5mm Grendel, but virtually any bullpup is viable in this role.

In the end, a lot of options need to be weighed. However, the 6.5mm Grendel seems to be the best caliber choice if a bullpup weapon is chosen, which generally is superior to traditionally laid out firearms. With incredible range, stopping power and accuracy at only 30% increased weight and slightly greater but still tolerable recoil, the caliber would be the ideal choice over the 6.8mm remington. Despite this, the 6.8mm Remington is a more proven round with the 6.5mm Grendel only recently entering the trials and being designed outside of the military, and the M4 carbine has proven to be a more popular weapon in the military. Either option would be an extremely beneficial replacement over the 5.56mm. Superior stopping power, accuracy, range, and barrier penetration would be worth the mild trade off of a 30% increase in the weight of the ammunition and a slight increase in recoil.


Possible 5.56mm Variants
While the best option would ideally be the 6.8mm Remington or 6.5mm Grendel to replace the 5.56mm, realistically this may prove to be a far more difficult task. While theoretically easy to convert a standard 5.56mm assault rifle over to either the 6.5mm Grendel or 6.8mm Remington caliber, few military's or companies have done so and the relative complexity, while minor, could be enough of a logistical hassle to deter widescale adoption. It is simply easier to switch out the 5.56mm ammunition as long as it's compatible with existing rifles, and use that than to develop an entirely new upper receiver, bolt, barrel and recoil spring for a new rifle. While not ideal there are some 5.56mm variants that, at least in the meantime, could vastly improve the performance of most assault rifles. While they would not necessary increase the range or firepower dramatically, it would be a very useful boost especially in regards to lethality or stopping power. Furthermore older weapons that have not yet been converted to the newer cartridges could still benefit from the improved 5.56mm rounds.

Three options that are available seem to provide dramatically better performance over the standard M855 rounds, the M855A1 enhanced performance round, the MK. 252 77 grain blackhills ammunition, and the Mk. 318. With the M855A1 and Mk. 318, both rounds provide more consistent performance, tumbling and fragmenting reliably in targets even at long ranges (beyond 300 yards from an m4 carbine) from as short as a 10.5 inch barrel, while also providing armor piercing capabilities. Both rounds are more powerful and reportedly achieve greater power from a shorter barrel using more modern gunpowder, with the Mk. 318 achieving approximately 2925 fps (891 m/s) and the M855A1 2,984 fps (910 m/s), compared to 2788 fps (850 m/s), or produce 1,623 and 1,700 joules in the M4 carbine compared to 1,481 joules for the standard M855. This was achieved by using faster burning propellant which delivers more of the energy from a shorter barrel. The M855A1 of course is the most powerful, generating velocities of 910 m/s from the M4 carbine, or 1700 joules with a 4.1 gram round, which is quite a bit above the standard M855 and the Mk. 318, and similar in power to a full sized M16 with a 20 inch long barrel (1800 joules).

One primary advantage given by M885A1 round is its consistent performance against soft targets. While the older SS109/M855 was yaw-dependant, which means its effectiveness depends on its yaw angle when it hits a target, the M855A1 delivers the same effectiveness in a soft target no matter its yaw angle. The new SMP-842 propellant in the round burns quicker in the shorter M4 carbine barrel, ensuring less muzzle flash and greater muzzle velocity. ). The round is also reported to have superior barrier penetration than the standard 5.56mm round, penetrating 3/8th inch of steel at 300 meters, which the standard 5.56mm can only do at 160 meters, and generally passing through plate glass or masonry with little to no issue. While the M855A1 possessed superior armor penetration, the Mk. 318 reportedly possessed superior terminal performance and barrier penetration. The front of the Mk. 318 round is an open tip backed up by a lead core, while the rear half is solid brass. When the bullet hits a hard barrier, the front half of the bullet smooshes against the barrier, breaking it so the penetrating half of the bullet can go through and hit the target. With the lead section penetrating the target and the brass section following, it was referred to as a "barrier blind" bullet. While the Mk. 318 and M855A1 are not identical, they are very similar in performance and have reliable performance in soft tissue, allowing them to readily incapacitate their targets while also maintaining good armor penetration. The Mk. 318 more reliably fragments in soft tissue, while the M855A1 is better at piercing armor.

The black hills 77 grain, 5 gram ammunition is heavier and more accurate than a standard 5.56mm round, while also possessing a ballistic coefficient of .35, compared to only roughly .2 for the standard 5.56mm. The round maintains higher levels of energy at longer ranges and has a more consistent and flatter bullet drop, making it ideal out to longer ranges, and the larger open-tip cartridge tends to provide more stopping power. The round is said to increase the range of the weapon out to 700 meters, and is said to have significantly more stopping power than the standard 5.56mm, being heavier and having more momentum and force (along with a hollow point, open-tip). While the round achieves sub 1-MOA and was reportedly effective on long ranges, being used by the Navy Seals in the Mk. 12 special purpose rifle, it's performance was reportedly disappointing and the navy continued a search for a superior cartridge that was somewhat more powerful. Currently the Navy is in the process of replacing most of these rifles with the SCAR Mk 17, although the round achieved markedly improved performance over the standard 5.56mm round especially in the designated marksmen role.

While all great options, all of them are still somewhat underpowered and still suffer from poor barrier penetration and stopping power (albeit are significantly better than the M855). An option of course is to combine the merits of each cartridge together to take advantage of all the potential benefits. Use the 5.56mm Mk. 262 bullet and combining it with the M855A1 gunpowder (or something roughly equivalent) to allow for a bullet that has more power from shorter barrels, capturing the benefit of both rounds, is a possibility for example. For that matter, the Mk. 262 bullet could be designed in a similar manner as the Mk. 318 or M855A1, more consistently fragmenting and with even better barrier penetration that matter, combing elements of all three rounds. With that thought process in mind, it would also be possible to use the .25-45 sharp gun powder and case with the smaller 5.56mm round to achieve more power. Achieving 2,350 joules with a 6.5mm round, it would be possible to use a similar, thicker brass case with the stronger propellant simply necked down to fire the 5.56mm, like used in the .25-45 sharp. As the .25-45 sharp uses the 5.56mm case, it would simply be a matter of adding the same gunpowder to the same brass case to produce the same results (ideally with thicker brass walls,however). This could allow for substantially more power and give the round a much longer range. As the velocity would be unusually high, this would be best combined with the 5 gram, 77 grain round or an even heavier bullet, to help compensate for it. Theoretical ballistics would be 970 m/s with a 5 gram cartridge or 915 m/s with a 5.6 gram round, retaining the same armor penetration but also the superior power of the heavier 5.56mm bullet. This would come at the expense of slightly higher chamber pressures, but would be comparable to the Mk. 318 or M885A1 and well within the margins of a traditional 5.56mm rifle.

Ultimately these options would not be as good as he 6.5mm Grendel or 6.8mm Remington, but it would be a dramatic improvement over the standard M855 round, especially considering the military's transition to the M4 carbine ,especially the Army. In any of the choices, it's clear that dramatic improvements to the 5.56mm in regards to barrier penetration and stopping power, as well as accuracy and performance from a short barrel, would benefit U.S. and NATO soldiers against clear threats they are currently facing. While ideally the 6.8mm Remington or 6.5mm Grendel would replace the 5.56mm, or an even newer caseless round, any kind of improvement would greatly benefit our soldiers, who have a clear and pressing need for a superior cartridge.

Another option might be certain variants of the the 5.56mm, but with newer gunpowder that's more powerful and a different bullet than the M855. A newer Mk. 318 variant seems to have more consistent fragmentation and be more powerful from a shorter barrel with faster burning gun powder, while the mk. 252 is a hollow point sniper round with sub 1 MOA and a range potentially of 700 yards. While the navy seals were apparently unsatisfied with the round's performance in comparison to the 6.8mm or 6.5mm Grendel which are now in trial phase, it is a viable option, and if with newer gun powder could not only be more powerful but also better from shorter barrels. The gun powder could come from the Mk. 318, or from the .25-45 sharp which from the same case a was able to produce 2,350 joules, or more than an Ak at 2,200. Granted it would be a smaller bullet, but a larger 5 gram round that seems to have better stopping power could certainly benefit, from that. It would give it a velocity of 970 m/s, slightly more than the standard 5.56mm at 940 m/s, and still be fairly powerful and have a fairly long range. The only real fear would be overpenetration but as it seems to provide more consistent performance and be a hollow point round, that may not be as big of an issue.

Even the other gunpowder would mean better performance from a shorter barrel so it would do well. On the flip side, all you have to replace is the upper receiver with these newer rounds which means you can preserve them for the most part. Guns themselves are relatively inexpensive by comparison to other military weapons (for all 2.4 million soldiers, 2.4 million new guns would only be about 2.4 to 4.8 billion dollars, which out of a 600 billion dollar annual budget is not that much for literally every soldier to have a new gun), so it's not that impossible to just buy new stuff or give the old stuff to the recruits and the new stuff to the frontline soldiers. xp