Exploration of General Purpose Cartridge Concept, Take Two

Tony Williams has published a provocative and compelling article, The Case for a General-Purpose Rifle and Machine Gun Cartridge (GPC).  In it, he outlined the need and advantages for an intermediate-caliber general-purpose cartridge which could better fill the roles currently filled by 5.56x45mm and 7.62x51mm in infantry rifles and machine guns.
A month ago I presented some preliminary thoughts inspired by Tony's article, in Comparing 5.56x45mm, 6mm PPC, and Undercharged 6.5x47mm Lapua, Assuming 14.5" Barrel.  Since then, a few people have asked me to consider some additional characteristics and cartridge types.  My software really wasn't up to it (and I wasn't very happy with the graphs it produced in the previous writeup, either), so I've spent the last month upgrading my software.  It's still a bit rough around the edges, but it at least generates combined graphs, and incorporates functions for calculating momentum and penetration.  I also have some estimates of munition weight so that the burden of carrying a given number of rounds can be assessed.
As before, I have no conclusions, but this article summarizes the state of my investigations so far.  The 6mm PPC and 6.5x47mm Lapua continue to impress.
Please read my previous article if you haven't already (linked above), where I describe the 6mm PPC, 6.5x47mm Lapua, and Hornady's A-Max line of projectiles.
As before, to make the comparisons equitable, reported cartridge performances were adjusted to fit those expected when fired from a 14.5" long barrel (as seen on the United States Army's M4 infantry rifle).  However, in a separate analysis, per request a variety of full-power cartridges were also compared, assuming use of a 24" barrel (as seen on United States Army's M240 machine gun).
Displayed below are graphs of projectiles' velocity, momentum, penetration, and kinetic energy vs distance.
Some notes about a couple of these graphs:
    Momentum vs Distance - Momentum is purported to be a good metric for predicting the lethality of a projectile striking a living target.  I believe other factors are at least as important (early onset of bullet yaw, fragmentation, shot placement, etc), but all other factors being equal, and lacking a good model for predicting lethality, momentum is probably a fair criteria for comparison.  See Fackler's excellent paper on wounding patterns for more information on terminal effects.  Momentum is presented in Kboles (thousands of boles, where a bole is equal to 1 g * cm / second).
    Penetration vs Distance - Take this one with a huge grain of salt.  I plotted the predicted penetration of the bullet into RHA with the assumption that it had a mild steel core (which is typical of many military rounds, including some of America's own "ball" rounds).  This flies in the face of the actual composition of A-Max projectiles, which have a lead core.  It's not too much of a stretch to think bullets of the same general quality, dimensions, and weight could be fashioned with a steel core, but it's not a given, either.  Furthermore, it is known that the actual penetration power of a projectile is much lower early in its trajectory, because it exhibits significant yaw coming out of the rifle barrel and takes some time and distance (100 to 200 yards) to stabilize.  Its actual penetration distance will be at its maximum at these latter ranges.  Efforts to reflect this in my ballistic software is a work in progress.
    Also, note: The 14.5"-barrel graphs show projections to 1200 yards, whereas the 24"-barrel graphs show projections to 2000 yards.
Legacy and Intermediate Cartridges, From 14.5" Barrels

The cartridges and projectiles plotted above are:
  • 5.56x45mm NATO, 70-grain Semi-Spitzer, G1 ballistic coefficient of 0.214, muzzle velocity 2735 feet/second
  • 6mm PPC, 70-grain A-Max, G1 ballistic coefficient of 0.307, muzzle velocity 3060 feet/second
  • 6.5x47mm Lapua, 123-grain A-Max, G1 ballistic coefficient of 0.510, with 90% charge (to reduce recoil, per Williams' criteria), muzzle velocity 2605 feet/second
  • 7.62x51mm NATO, 149-grain M80 (BT-FMJ), G1 ballistic coefficient of 0.393, muzzle velocity 2637 feet/second
    Legacy and Full Power Cartridges, From 24" Barrels

    The cartridges and projectiles plotted above are:
  • 6.5x47mm Lapua, 123-grain A-Max, G1 ballistic coefficient of 0.510, full charge, muzzle velocity 2916 feet/second
  • 7.62x51mm NATO, 149-grain M80 (BT-FMJ), G1 ballistic coefficient of 0.393, muzzle velocity 2820 feet/second
  • .338 Norma Magnum, 180-grain A-Max, G1 ballistic coefficient of 0.432, muzzle velocity 3291 feet/second
  • 7.62x54mm Russian, 180-grain Spitzer, G1 ballistic coefficient of 0.484, muzzle velocity 2566 feet/second
    Highlights of comparisons:
      Intermediate cartridges:
      For a recap of velocity and KE comparisons of the intermediate-power cartridges (from the 14.5" barrels), see the previous paper on this topic.
      Looking at the momentum developed by the intermediate-power cartridges, the picture is quite clear.  The 6mm PPC develops and maintains marginally more momentum than the 5.56mm NATO at all ranges.  It can be surmised that, all other factors being equal, the PPC might be as lethal at 250 yards as 5.56mm NATO is at 100 yards.  At longer ranges, the PPC retains the same momentum at 800 yards as the 5.56mm does at 480 yards.  Thus the PPC's advantage over the 5.56mm is incremental, but significant.
      The 6.5x47mm Lapua and 7.62x51mm NATO are obviously in a totally separate class, with both retaining more momentum at 600 yards than the PPC or 5.56mm develop at their muzzles.  At shorter ranges the 7.62x51mm vastly outperforms the Lapua, but the significance of this is unclear since both deliver ample damage at short range.  The 7.62x51mm, launched from a 24 inch barrel, delivers 384 Kboles at 800 yards, and has well-documented lethality at this range. The Lapua, launched from the 14.5 inch barrel, delivers 384 Kboles at 675 yards, and 7.62x51mm with the 14.5 inch barrel delivers 384 Kboles at 725 yards.  If it is assumed that both are amply effective at these ranges, then these cartridges can be assumed roughly equivalent at all ranges, since their momentum at range closely converge at about 650 yards and do not significantly diverge through 1200 yards.
      In summary, the 6mm PPC provides a modest but significant advantage over 5.56mm NATO at all ranges, with no obvious drawbacks aside from a 33% increase in recoil energy.  The 6.5x47mm Lapua provides performance arguably equivalent to 7.62x51mm NATO but considerably lower weight and recoil, and represents a more aggressive alternative for a general-purpose cartridge.
      Full power cartridges:
      At popular request, full-power cartridges for machine guns with full-length (24 inch) barrels were added to the analysis.  The legacy 7.62x51mm NATO and 7.62x54mm Russian cartridges are compared to the 6.5x47mm Lapua and .338 Norma Magnum cartridges.  7.5x55mm Swiss (GP11) was originally included as well (per request), but was subsequently removed as its performance proved almost identical to 7.62x54mm Russian across the board.
      The machine gun serves two functions: It suppresses combatants in its area of fire so friendly troops are free to maneuver, and it defeats targets which rifle fire cannot (fortifications and armored vehicles).
      In terms of putting rounds on target, the Lapua and Norma maintain a higher velocity at all ranges, providing a flatter trajectory and shorter time of flight relative to the legacy cartridges.  The heavier rounds are known to produce a superior psychological effect, which is important when used in the suppressive role.  Suppressed targets are simply more likely to duck and stay ducked when the larger, heavier projectiles zip overhead or land nearby.  The Lapua's intermediate-weight 123gr bullet is untried in this role, being 75% heavier than 5.56x45mm NATO but 17% lighter than the 7.62x51mm, and 30% lighter than the Norma and Russian.
      For penetration of vehicles and heavy cover, the penetration chart paints a complex and surprising picture.  The Lapua and legacy cartridges offer very close performance at most ranges, with the 7.62x51mm NATO showing a dip relative to the other contenders in the intermediate ranges (roughly 600 to 1200 yards).  Of these three, none show sufficient capacity to penetrate the front or side armor of APC's, much less IFV's or heavier vehicles.  The Lapua should chew through a somewhat (30%) greater thickness of brick or concrete cover than the legacy NATO round at range, but it never quite approaches the performance of the legacy 7.62x54mm Russian.  All remain capable of defeating thin-skinned vehicles out to at least 1500 yards.
      The .338 Norma Magnum provides a curious counterpoint in the penetration role, defeating up to a 50% greater thickness of armor at short range, compared to the other three cartridges.  But how meaningful is this, really?  Of all the armored vehicles of the last forty years, only the venerable BTR-70's side armor fails to stop this threat, and only at very short range.  Beyond about 250 yards, the .338 is in the same boat as the others -- incapable of posing a threat to vehicles with even a mediocum of armor protection.
      On the other hand, RHA hasn't figured significantly in the lives of western infantry in the 21st century.  The ability of the .338 to defeat heavy cover is arguably more relevant.  The performance gap between the .338 and 7.62R closes rapidly, and the two perform identically beyond about 1200 yards, but at shorter ranges the .338 punches deeper holes in brick and mortar -- 33% deeper at most ranges.  To put this into perspective, at 200 yards where the 7.62x51mm NATO can defeat eight inches of cinderblock or two inches of brick, the .338 should be able to defeat up to twelve inches of cinderblock or three inches of brick.  In most of the world, firebrick is standardized on 4.25 inch width, so neither cartridge would be capable of shooting through a brick wall.  Cinderblocks are standardized on 8.5 inch width, rendering legacy cartridges marginal at penetrating cinderblock walls at a perpendicular angle, while the .338 should be able to defeat a single-layer cinderblock wall at 45 degrees of obliquity.  As an aside, while penetration does increase with higher projectile mass, projections predict the .338 Norma Magnum remains incapable of penetrating 4.25 inches of firebrick at 200 yards with projectile masses below 350 grains.
    Weight burden comparisons:
    One of the typical arguments against adoption of a heavier cartridge is the added burden imposed on the infantryman who needs to carry it all.  This is not a small issue!  Soldiers are chronically overburdened, and will shave the handles of their toothbrushes just to lighten their load an ounce.  The added burden imposed by heavier munitions is worth inspection.
    Changes in cartridge size and power impacts the weights of rifles, magazines, and munitions.  Rifle weights fluctuate considerably between models and between improvements within a family line, so they will not be considered here.  Magazine weight is calculated relative to the Heckler & Koch 30-round STANAG box magazine, expanded in all axes in proportion with cartridge dimensions relative to 5.56x45mm NATO.  It is assumed here that an infantryman carries six full 30-round magazines.
    MunitionBulletCartridgePowderEmpty magazineFull magazineSix full magazines
    5.56x45mm 70gr Semi-spitzer70 grain
    (4.5 gram)
    95 grain
    (6.2 gram)
    26 grain
    (1.7 gram)
    8.8 ounce
    (250 gram)
    21.9 ounce
    (621 gram)
    8.2 pound
    (3.7 kg)
    6mm PPC 70gr A-Max70 grain
    (4.5 gram)
    90 grain
    (5.8 gram)
    29 grain
    (1.9 gram)
    9.6 ounce
    (271 gram)
    22.5 ounce
    (638 gram)
    8.4 pound
    (3.8 kg)
    6.5x47mm Lapua 123gr A-Max123 grain
    (8.0 gram)
    166 grain
    (10.8 gram)
    40 grain
    (2.6 gram)
    11.5 ounce
    (327 gram)
    34.1 ounce
    (967 gram)
    12.8 pound
    (5.8 kg)
    7.62x51mm NATO 149gr M80149 grain
    (9.7 gram)
    182 grain
    (11.8 gram)
    47 grain
    (3.1 gram)
    12.2 ounce
    (346 gram)
    38.1 ounce
    (1081 gram)
    14.3 pound
    (6.5 kg)

    Two facts leap out of this table:  First, the short, wide case of the 6mm PPC is a big win for increasing powder capacity at a net weight reduction (relative to 5.56mm).  Second, the excellent performance of 6.5x47mm comes at a high price in added weight.
    Increasing the soldier's burden by four and a half pounds (the difference between six magazines of 5.56mm and six magazines of 6.5x47mm) is a non-starter, and takes 6.5x47mm out of consideration (at least for the infantry GPC -- it still looks promising as a machine gun cartridge, at considerable weight savings compared to 7.62x51mm and 7.62x54mmR).
    On the other hand, the 3.6 ounce weight difference between six full 30-round magazines of 5.56mm and 6mm PPC is almost negligible, offering a significant boost in infantry rifle performance with no drawbacks.
    This suggests future considerations of potential GPC candidates should focus on short, wide cartridges, as these provide higher performance with minimal weight impact.
    Discusson on the GPC topic is ongoing in the TankNet forums.