One thing that seems apparent from the ARDEC presentation on the CTSAS program is the excessive capability and weight of the 6.5mm CT ammunition, as well as its use of lead-cored projectiles as opposed to more modern (and less dense) EPR-type projectiles. Further, the 6.5mm configuration explicitly uses the same case as the 7.62mm CT round, to allow for shorter development times. These facts together indicate that the 6.5mm CT round demonstrated in the ARDEC presentation is not well optimized for minimum weight. For this post, we’ll create an estimate of a new, lighter round that still should be powerful enough to replace both 5.56mm and 7.62mm as a universal caliber. As mentioned in the previous post, we’ll call it the “LW 6.5mm CT” to differentiate it from ARDEC’s 6.5mm CT round.

Fortunately for me, I have already modeled a 6.5mm* low-drag EPR-type projectile, which has made cameos in the title images of a few of my previous articles:

*Quick fun fact for those reading this who believe I am a 5.56mm fanboy: .264″/6.5mm is actually the default caliber I use for projectile design in SolidWorks, and the products then get scaled up and down to other calibers if I need them.

This projectile weighs 7 grams (108 grains) and has a calculated i7 Form Factor of 0.86 giving it a G7 BC of 0.257, although for the purposes of this article we will use the more modest 0.90 i7 FF and 0.246 G7 BC to account for difference between theoretical and actual values. It has an overall length of just over 34mm even (1.339 in), and is based on the shape of the Berger 130gr AR Hybrid 6.5mm bullet, but with construction patterned after the M855A1 and M80A1 EPRs developed by the Army.

To determine the weight of the ammunition that will use this bullet, we will need to determine what capability it will need. The 6.5mm CT ammunition in the ARDEC presentation matches the energy produced by 7.62mm M80 Ball at a kilometer out to 1,200m, which seems excessive. At the very least, matching 7.62 NATO at a kilometer, instead, would seem to meet the needs of a single-caliber system, but due to the smaller diameter of the 6.5mm projectile, less energy is likely needed to achieve the same penetration. While one could do an exhaustive study to determine the ideal muzzle velocity and energy, I will instead just eyeball it. 6mm SAW was a caliber designed in the 1970s as the “ideal” squad automatic weapon caliber, and it launched a 105 grain bullet with a similar ballistic coefficient at a muzzle velocity of about 2,550 ft/s. We’ll use this muzzle velocity, too, which gives 2,115 Joules of muzzle energy, and – using JBM’s excellent ballistics calculator – 344 J of energy at 1,000 meters, and more specific energy (energy divided by bullet frontal area) than 7.62mm M80 Ball – and coincidentally a pleasantly flat trajectory, as well.

To estimate the weight of this round, we can create a modifier to determine the mass of a plastic telescoped case based on the figures given to us in the ARDEC presentation. We know, for example, that the 5.56mm CT round weighs 127 grains, or 8.23 grams, whereas a standard round of M855 weighs 12 grams. Subtracting the components that remain the same between the two, we get:

12 grams – 4 gram bullet – 0.25 gram primer – 1.72 gram propellant = 6.03 gram case weight

8.23 grams – 4 gram bullet – 0.25 gram primer* – 1.72 gram propellant* = 2.26 gram case weight

*We actually do not know the weight of primer and propellant for 5.56mm CT, but the error shouldn’t be significant for the purposes of this article.

That means case weight was reduced by 62.5%, so we can multiply the case weight of an existing round by 0.375 to get the approximate weight of the PCT case. Let’s check this process using 7.62 NATO and 7.62 CT:

23.1 grams – 8.43 gram bullet – 0.35 gram primer – 2.98 gram propellant = 11.34 gram case weight

11.34 grams * 0.375 = 4.25 gram CT case weight

4.25 grams + 8.43 grams + 0.35 grams + 2.98 grams = 16.01 grams CT cartridge weight

Actual weight of 7.62mm CT is 15.6 grams, so this process does produce a fairly accurate result. Now, we need to select a case to start with, ideally one with similar muzzle energy and the same caliber. A suitable candidate exists in the form of 6.5 Grendel:

17.8 grams – 8 gram bullet – 0.25 gram primer – 1.90 gram propellant = 7.65 gram case weight

7.65 grams * 0.375 = 2.87 gram CT case weight

Now, we’ll be using a different weight projectile for the example (7 grams instead of 8), but that’s a simple matter of plug-and-play:

2.87 grams + 7 gram bullet + 0.25 grams + 1.90 grams = 12.02 grams CT cartridge weight

I’ll go ahead and round that to an even 12 grams. We can also estimate the weight of the LW 6.5mm CT belt link by averaging the weight of the 7.62mm and 5.56mm CT belt links:

0.84 gram 5.56mm CT belt link + 1.46 7.62mm CT belt link = 2.30 grams

2.30 grams / 2 = 1.15 grams

Now, we just need to find the weight of magazines. To do that, we need some idea of the size of the LW 6.5mm CT case, but that’s straightforward enough. Let’s take the approximate case weight for 7.62mm CT and divide the weight of the LW 6.5mm CT case by it:

2.87 grams / (15.6 grams – 8.43 grams – 0.35 grams – 2.98 grams) = 2.87 / 3.84 = 0.747

This indicates how much smaller a 20 round LW 6.5mm CT magazine would be than a 20 round 7.62mm or 6.5mm CT magazine. We got a value of 150 grams before for the 20 round 6.5mm CT magazine, and while I don’t like to multiply an estimate by an estimate, it’ll have to do:

0.747 * 150 = 112 grams

Now, I would like to use 30 round magazines for this, and we know that a 30 round PMag weighs about 47% more than a 20 round PMag, so we can estimate a 30 round magazine by:

1.47 * 112 grams = 165 grams

So then our values are:

Weight of 1 round of LW 6.5mm CT: 185 grains / 12.0 grams
Weight of 1 belt link for LW 6.5mm CT: 17.7 grains / 1.15 grams
Weight of 1 30-round LW 6.5mm CT magazine: 2,546 grains / 165 grams

Now we can simply plug them in:

 

Infantry Squad:

Squad Leader: 210 rounds LW 6.5mm CT in 7 magazines totaling 3.675 kg
Team Leader: 210 rounds LW 6.5mm CT in 7 magazines, 200 rounds linked LW 6.5mm CT totaling 6.305 kg (x2)
Automatic Rifleman: 800 rounds linked LW 6.5mm CT totaling 10.520 kg (x2)
Grenadier: 210 rounds LW 6.5mm CT in 7 magazines, 200 rounds linked LW 6.5mm CT totaling 6.305 kg (x2)
Rifleman: 210 rounds LW 6.5mm CT in 7 magazines, 200 rounds linked LW 6.5mm CT totaling 6.305 kg (x2)

Weight of ammunition of infantry squad: 62.545 kg (x3)

 

Weapons Squad:

Squad Leader: 210 rounds LW 6.5mm CT in 7 magazines totaling 3.675 kg
Machine Gunner: 300 rounds linked LW 6.5mm CT, totaling 3.945 kg (x2)
Assistant Gunners: 210 rounds LW 6.5mm CT in 7 magazines, 400 rounds linked LW 6.5mm CT totaling 8.935 kg (x2)
Ammunition Bearer: 210 rounds LW 6.5mm CT in 7 magazines, 300 rounds linked LW 6.5mm CT totaling 7.620 kg (x2)

Weight of ammunition of weapons squad: 44.675 kg

 

Platoon HQ:

Platoon Leader: 210 rounds LW 6.5mm CT in 7 magazines totaling 3.675 kg
Platoon Sergeant: 210 rounds LW 6.5mm CT in 7 magazines totaling 3.675 kg
Radio Operator: 210 rounds LW 6.5mm CT in 7 magazines totaling 3.675 kg
Combat Medic: 210 rounds LW 6.5mm CT in 7 magazines totaling 3.675 kg

Weight of ammunition of platoon headquarters: 14.700 kg

 

So with our LW 6.5mm CT example cartridge, the infantry platoon’s load is reduced by 12% to 247.010 kilograms, and the weight of the weapons squad is absolutely slashed down to 44.675 kilograms from 80.911 kilos, a savings of 45%! Versus ARDEC’s 6.5mm CT, moving to a less ambitious universal caliber reduces weight in the infantry platoon by 23%, given the same number of rounds (I’m including those 15 spare magazines from last time), a considerable improvement.

Lighter ammunition paradigms are yet possible, though. Let’s see what happens if we retain the dual-caliber system, and use 5.56mm CT alongside the CTSAS 6.5mm CT, replacing one-for-one the two current conventional brass-cased calibers. Remember the values for these rounds are:

Weight of 1 round of 6.5mm CT: 237 grains / 15.4 grams
Weight of 1 belt link for 7.62mm/6.5mm CT: 22.5 grains / 1.46 grams
Weight of 1 20-round 6.5mm CT magazine: 2,320 grains / 150 grams

Weight of 1 round of 5.56mm CT: 127 grains / 8.2 grams
Weight of 1 belt link for 5.56mm CT: 13 grains / 0.84 grams*

We’re missing the weight of a 30 round magazine for 5.56mm CT, but a value of 110 grams seems reasonable based on the weight of 30 round polymer magazines for brass-cased 5.56mm. I also simplified the ammunition bearer’s loadout by having him only carry 6.5mm ammunition, 140 rounds of it in magazines, presumably for a lightweight 6.5mm Designated Marksman’s Rifle.

 

Infantry Squad:

Squad Leader: 210 rounds 5.56mm CT in 7 magazines totaling 2.492 kg
Team Leader: 210 rounds 5.56mm CT in 7 magazines, 200 rounds linked 5.56mm CT totaling 4.300 kg (x2)
Automatic Rifleman: 800 rounds linked 5.56mm CT totaling 7.232 kg (x2)
Grenadier: 210 rounds 5.56mm CT in 7 magazines, 200 rounds linked 5.56mm CT totaling 4.300 kg (x2)
Rifleman: 210 rounds 5.56mm CT in 7 magazines, 200 rounds linked 5.56mm CT totaling 4.300 kg (x2)

Weight of ammunition of infantry squad: 42.756 kg (x3)

 

Weapons Squad:

Squad Leader: 210 rounds 5.56mm CT in 7 magazines totaling 2.492 kg
Machine Gunner: 300 rounds linked 6.5mm CT, totaling 5.058 kg (x2)
Assistant Gunners: 210 rounds 5.56mm CT in 7 magazines, 400 rounds linked 6.5mm CT totaling 9.236 kg (x2)
Ammunition Bearer: 140 rounds 6.5mm CT in 7 magazines, 300 rounds linked 6.5mm CT totaling 8.264 kg (x2)

Weight of ammunition of weapons squad: 47.608 kg

 

Platoon HQ:

Platoon Leader: 210 rounds 5.56mm CT in 7 magazines totaling 2.492 kg
Platoon Sergeant: 210 rounds 5.56mm CT in 7 magazines totaling 2.492 kg
Radio Operator: 210 rounds 5.56mm CT in 7 magazines totaling 2.492 kg
Combat Medic: 210 rounds 5.56mm CT in 7 magazines totaling 2.492 kg

Weight of ammunition of platoon headquarters: 9.968 kg

 

This arrangement is the lightest yet, with a total weight of 185.844 kilograms for the platoon, a savings of almost 100 kg, or 34%, versus the current brass-cased dual caliber system, and a savings of 42% – almost 135 kilograms! – versus a single-caliber system using the 6.5mm CT proposed by ARDEC. That’s an average weight reduction of over 3.5 kilograms (almost 8 pounds) per person, a very substantial difference.

It doesn’t, however, address the concerns some have about the infantry squad’s ability to project fire to longer ranges. One possible (and very light) solution to that would be to issue a unified belt-fed caliber for a new weapon that would replace both the 7.62mm M240 machine gun and 5.56mm M249 automatic rifle, while issuing carbines in a lighter, smaller caliber. That might look like this:

 

Infantry Squad:

Squad Leader: 210 rounds 5.56mm CT in 7 magazines totaling 2.492 kg
Team Leader: 210 rounds 5.56mm CT in 7 magazines, 200 rounds linked LW 6.5mm CT totaling 5.122 kg (x2)
Automatic Rifleman: 800 rounds linked LW 6.5mm CT totaling 10.520 kg (x2)
Grenadier: 210 rounds 5.56mm CT in 7 magazines, 200 rounds linked LW 6.5mm CT totaling 5.122 kg (x2)
Rifleman: 210 rounds 5.56mm CT in 7 magazines, 200 rounds linked LW 6.5mm CT totaling 5.122 kg (x2)

Weight of ammunition of infantry squad: 54.264 kg (x3)

 

Weapons Squad:

Squad Leader: 210 rounds 5.56mm CT in 7 magazines totaling 2.492 kg
Machine Gunner: 300 rounds linked LW 6.5mm CT, totaling 3.945 kg (x2)
Assistant Gunners: 210 rounds 5.56mm CT in 7 magazines, 400 rounds linked LW 6.5mm CT totaling 7.752 kg (x2)
Ammunition Bearer: 210 rounds LW 6.5mm CT in 7 magazines, 300 rounds linked LW 6.5mm CT totaling 7.620 kg (x2)

Weight of ammunition of weapons squad: 41.126 kg

 

Platoon HQ:

Platoon Leader: 210 rounds 5.56mm CT in 7 magazines totaling 2.492 kg
Platoon Sergeant: 210 rounds 5.56mm CT in 7 magazines totaling 2.492 kg
Radio Operator: 210 rounds 5.56mm CT in 7 magazines totaling 2.492 kg
Combat Medic: 210 rounds 5.56mm CT in 7 magazines totaling 2.492 kg

Weight of ammunition of platoon headquarters: 9.968 kg

 

That gives a weight of ammunition carried by the platoon of 213.886 kilograms, which is less of a reduction in weight than the previous example, though it still eliminates over 67 kilograms (24%) versus the current system. Complementing that, it would not only reduce the number of weapons in service (as the GPMG and SAW would be combined into one), but would also give the automatic riflemen organic to the infantry squads capability and range very close to current GPMGs but with a far greater ammunition load, while still retaining the good handling and recoil characteristics of current 5.56mm carbines and reducing weight beyond what is possible with a one caliber system.

To conclude, that’s what the weight breakdowns of different future ammunition paradigms look like for the infantry platoon. It should be noted that I am backing no one concept, though I have insisted for some time now that the benefits of the two-caliber system should not be ignored moving forward. These examples help make clear why, although they make very clear the benefits of one-caliber systems, as well.

Mostly, though, they illustrate what a promising technology polymer cased telescoped ammunition really is. In all cases, the weight carried by the most burdened unit – the weapons squad – was significantly reduced, and in particular all examples dramatically reduced the weight carried by the ammunition bearer in the squad, who currently carries the most weight in ammunition of any member of the platoon.