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...versus "the other" 6.X caliber.
From the "Wildcatting the 6.8 SPC" thread
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6.8 SPC VS 6.5 GRENDEL
At the 2004 Shot Show in Las Vegas, two new cartridges - Remington's 6.8mm Special Purpose Cartridge and Alexander Arms' 6.5mm Grendel - were introduced for use in the renowned AR-15 rifle and its many variants. Ever since, individuals have engaged in often heated debate on Internet sites like ar15.com, tacticalforums.com, and 65grendel.com, arguing which would be the better choice to succeed 5.56 NATO as the U.S. armed forces' next small arms caliber.
Both cartridges were initially offered with "open tip" bullets, a type that is of limited usefulness in land warfare. This study will attempt to provide an objective evaluation as to whether the 6.8 SPC or 6.5 Grendel is truly the superior alternative when using full metal jacket (FMJ) projectiles suitable for general military use. To accomplish this task will require not only a comparison of the known characteristics of each round, but also a look at what measures are needed to utilize these cartridges in variants of the M16 rifle, M4 carbine, and M249 light machine gun.
AMMUNITION CHARACTERISTICS
Ballistic Performance
The 6.8mm 115-grain projectile has a listed ballistic coefficient (BC) of 0.325, which can only be honestly described as mediocre, especially in contrast to the unusually high 0.636 BC of the 6.5mm 144-grain bullet. At normal engagement distances (under 300 meters) this disparity may not have a significant effect, but at very long ranges (up to 1000 meters) the more streamlined 6.5mm projectile produces a much flatter trajectory and smaller wind drift.
Advantage: The 6.5 Grendel is clearly superior in this category, due to the ability to use bullets of high ballistic efficiency.
Terminal Performance
The 6.8mm FMJ bullet is nearly identical in configuration to that of the "open tip" combat projectile originally loaded for testing by special operations, so could be expected to also have early onset of yaw (often referred to as "tumbling") after entering soft tissue. Gelatin test of the 6.5mm FMJ appear to show a slightly slower initiation of yaw, but the effect on the gel block was still impressive.
Advantage: The 6.8 SPC may have a slight edge at shorter engagement distances, but since the 6.5 Grendel retains velocity better, the smaller caliber will likely have more stopping power at long range.
Penetration Capability
In urban combat, such as has occurred in Iraq over the last two years, the M249 machine gun will often be used to engage enemy personnel in vehicles and buildings. The intense firefights in Somalia during 1993 first showed all too dramatically that the 5.56 NATO round was deficient at punching through such obstacles. The capability of a bullet to penetrate "hard" targets is determined by several factors, including impact velocity, sectional density, jacket toughness, and core hardness.
Advantage: All other factors being equal, a smaller diameter bullet has greater sectional density and higher retained velocity, which combine to give the 6.5 Grendel superior penetration potential compared to the 6.8 SPC.
Tracer Performance
Tracer ammunition is used primarily in machine guns so that the gunner can observe the bullets' trajectory and adjust his aim. In rifles and carbines, tracers are also employed by small unit leaders to show supporting weapons where to direct their fires. Another use for this type of round is to load a couple of them into the magazine before filling the magazine with ball ammo. Then, when the rifleman has shot so many rounds that a tracer goes downrange, he knows that he has almost run dry and it's time to reload.
Advantage: Tracer projectiles can be manufactured in nearly all calibers, but the greater length/diameter ratio of the bullets loaded in the 6.5 Grendel offer the promise of a longer burn time than may be possible with the 6.8 SPC.
WEAPON MODIFICATIONS
Rifle Magazines
Since neither the 6.8 SPC nor the 6.5 Grendel stack properly in government issue magazines, it has been necessary to develop new, cartridge-specific magazines for each caliber. Precision Reflex markets 15-rd, 20-rd, and 25-rd high-capacity, steel magazines for the 6.8 SPC. At present, Alexander Arms offers steel, 17-rd mags for the 6.5 Grendel, although aluminum, 24-rd magazines are reportedly being developed.
Advantage: For military use, the greater the number of rounds that can be stuffed into a magazine, the better. This makes the 6.8 SPC the obvious leader in this category.
Stripper Clips
Most civilian shooters fill their detachable box magazines with loose rounds taken from a carton of 20 cartridges, which are loaded one round at a time. Military personnel, however, have a faster method. Stripper clips (also known as chargers) enable a soldier to more quickly load ammunition into the magazines. There are currently no stripper clips made specifically for either round, but Remington Arms marketed a stripper in the early 1900s for the Model 8 rifle, which was chambered in .30 Remington, the parent cartridge of the 6.8 SPC. In the 1950s, magazines for the Czech vz52 rifle were loaded via 5-rd strippers that, conveniently, are just the right size to accept 6.5 Grendel rounds.
In order to hold stripper clips in position above magazine feed lips requires the use of clip guides. Military 5.56 mags are made with an impressed reinforcing rib near the rear, into which a stripper clip guide can be locked. Because 6.8 SPC magazines also feature an impressed rib at the same location, a U.S. G.I.-type clip guide could be used. Mags for the 6.5 Grendel, on the other hand, are manufactured without the rear rib in order to provide sufficient internal space for proper cartridge stacking, which makes it necessary to use a bulkier clip guide that clamps over the entire top of the magazine.
Advantage: The 6.8 SPC has a slight advantage, solely because it can use clip guides that are lighter, more compact, and possibly less expensive.
Belt Feed
Ammunition intended for use in rifles and carbines should also be adaptable to firing in belt-fed, squad automatic weapons like the M249 light machine gun (LMG). Achieving this goal with the minimum of alterations to the M249 feed mechanism will require that the cartridges fit into metallic links of the same length as that of the standard M27 links used to feed 5.56 NATO ammo. The M27 link can easily be redesigned so that the loops will be of the appropriate diameter to fit either the 6.5 Grendel or 6.8 SPC. However, for the round to correctly interface with the forward belt feed pawl and the feed tray's cartridge stop, it is also vital that the case shoulder protrude in front of the link loop by roughly a quarter of an inch.
Advantage: The 6.5 Grendel case has insufficient protrusion in front of an M27-type link, but the 6.8 SPC fits perfectly, so the latter seems the better choice for belt feed.
IN CONCLUSION
The heavy, highly-streamlined projectiles fired by the 6.5 Grendel deliver terrific long-range performance, which would make that cartridge a superb choice for standard-length weapons like the Marines' M16A4 rifle. However, the 6.8 SPC has less velocity loss when fired in the short-barreled M4A1 carbine that is used in relatively large numbers by Army units.
The degree of difficulty in converting the current family of 5.56mm military small arms to either 6.5 Grendel or 6.8 SPC is nearly the same for each, with the apparent exception of the M249 light machine gun. Because of the noted complications with belt feed of 6.5 Grendel, and the fact that a high-capacity magazine for that round is not yet available, the unavoidable conclusion is that, at the present time, the 6.8 SPC would be the better alternative to replace the 5.56 NATO cartridge.
From the "Wildcatting the 6.8 SPC" thread
Paul, the pending article is an update of one that was published in 2005. So as to not go further off-topic in the wildcatting thread, I'm posting the 2005 article here. Please note the conclusion.Can you give us a little hint? I'm pretty sure which side it will be favoring.stanc said:Anyway, that might be changed to infamous when my "6.5 Grendel vs 6.8 SPC" article gets printed in an upcoming (June? August?) issue of SPECIAL WEAPONS.
==========================================
6.8 SPC VS 6.5 GRENDEL
At the 2004 Shot Show in Las Vegas, two new cartridges - Remington's 6.8mm Special Purpose Cartridge and Alexander Arms' 6.5mm Grendel - were introduced for use in the renowned AR-15 rifle and its many variants. Ever since, individuals have engaged in often heated debate on Internet sites like ar15.com, tacticalforums.com, and 65grendel.com, arguing which would be the better choice to succeed 5.56 NATO as the U.S. armed forces' next small arms caliber.
Both cartridges were initially offered with "open tip" bullets, a type that is of limited usefulness in land warfare. This study will attempt to provide an objective evaluation as to whether the 6.8 SPC or 6.5 Grendel is truly the superior alternative when using full metal jacket (FMJ) projectiles suitable for general military use. To accomplish this task will require not only a comparison of the known characteristics of each round, but also a look at what measures are needed to utilize these cartridges in variants of the M16 rifle, M4 carbine, and M249 light machine gun.
AMMUNITION CHARACTERISTICS
Ballistic Performance
The 6.8mm 115-grain projectile has a listed ballistic coefficient (BC) of 0.325, which can only be honestly described as mediocre, especially in contrast to the unusually high 0.636 BC of the 6.5mm 144-grain bullet. At normal engagement distances (under 300 meters) this disparity may not have a significant effect, but at very long ranges (up to 1000 meters) the more streamlined 6.5mm projectile produces a much flatter trajectory and smaller wind drift.
Advantage: The 6.5 Grendel is clearly superior in this category, due to the ability to use bullets of high ballistic efficiency.
Terminal Performance
The 6.8mm FMJ bullet is nearly identical in configuration to that of the "open tip" combat projectile originally loaded for testing by special operations, so could be expected to also have early onset of yaw (often referred to as "tumbling") after entering soft tissue. Gelatin test of the 6.5mm FMJ appear to show a slightly slower initiation of yaw, but the effect on the gel block was still impressive.
Advantage: The 6.8 SPC may have a slight edge at shorter engagement distances, but since the 6.5 Grendel retains velocity better, the smaller caliber will likely have more stopping power at long range.
Penetration Capability
In urban combat, such as has occurred in Iraq over the last two years, the M249 machine gun will often be used to engage enemy personnel in vehicles and buildings. The intense firefights in Somalia during 1993 first showed all too dramatically that the 5.56 NATO round was deficient at punching through such obstacles. The capability of a bullet to penetrate "hard" targets is determined by several factors, including impact velocity, sectional density, jacket toughness, and core hardness.
Advantage: All other factors being equal, a smaller diameter bullet has greater sectional density and higher retained velocity, which combine to give the 6.5 Grendel superior penetration potential compared to the 6.8 SPC.
Tracer Performance
Tracer ammunition is used primarily in machine guns so that the gunner can observe the bullets' trajectory and adjust his aim. In rifles and carbines, tracers are also employed by small unit leaders to show supporting weapons where to direct their fires. Another use for this type of round is to load a couple of them into the magazine before filling the magazine with ball ammo. Then, when the rifleman has shot so many rounds that a tracer goes downrange, he knows that he has almost run dry and it's time to reload.
Advantage: Tracer projectiles can be manufactured in nearly all calibers, but the greater length/diameter ratio of the bullets loaded in the 6.5 Grendel offer the promise of a longer burn time than may be possible with the 6.8 SPC.
WEAPON MODIFICATIONS
Rifle Magazines
Since neither the 6.8 SPC nor the 6.5 Grendel stack properly in government issue magazines, it has been necessary to develop new, cartridge-specific magazines for each caliber. Precision Reflex markets 15-rd, 20-rd, and 25-rd high-capacity, steel magazines for the 6.8 SPC. At present, Alexander Arms offers steel, 17-rd mags for the 6.5 Grendel, although aluminum, 24-rd magazines are reportedly being developed.
Advantage: For military use, the greater the number of rounds that can be stuffed into a magazine, the better. This makes the 6.8 SPC the obvious leader in this category.
Stripper Clips
Most civilian shooters fill their detachable box magazines with loose rounds taken from a carton of 20 cartridges, which are loaded one round at a time. Military personnel, however, have a faster method. Stripper clips (also known as chargers) enable a soldier to more quickly load ammunition into the magazines. There are currently no stripper clips made specifically for either round, but Remington Arms marketed a stripper in the early 1900s for the Model 8 rifle, which was chambered in .30 Remington, the parent cartridge of the 6.8 SPC. In the 1950s, magazines for the Czech vz52 rifle were loaded via 5-rd strippers that, conveniently, are just the right size to accept 6.5 Grendel rounds.
In order to hold stripper clips in position above magazine feed lips requires the use of clip guides. Military 5.56 mags are made with an impressed reinforcing rib near the rear, into which a stripper clip guide can be locked. Because 6.8 SPC magazines also feature an impressed rib at the same location, a U.S. G.I.-type clip guide could be used. Mags for the 6.5 Grendel, on the other hand, are manufactured without the rear rib in order to provide sufficient internal space for proper cartridge stacking, which makes it necessary to use a bulkier clip guide that clamps over the entire top of the magazine.
Advantage: The 6.8 SPC has a slight advantage, solely because it can use clip guides that are lighter, more compact, and possibly less expensive.
Belt Feed
Ammunition intended for use in rifles and carbines should also be adaptable to firing in belt-fed, squad automatic weapons like the M249 light machine gun (LMG). Achieving this goal with the minimum of alterations to the M249 feed mechanism will require that the cartridges fit into metallic links of the same length as that of the standard M27 links used to feed 5.56 NATO ammo. The M27 link can easily be redesigned so that the loops will be of the appropriate diameter to fit either the 6.5 Grendel or 6.8 SPC. However, for the round to correctly interface with the forward belt feed pawl and the feed tray's cartridge stop, it is also vital that the case shoulder protrude in front of the link loop by roughly a quarter of an inch.
Advantage: The 6.5 Grendel case has insufficient protrusion in front of an M27-type link, but the 6.8 SPC fits perfectly, so the latter seems the better choice for belt feed.
IN CONCLUSION
The heavy, highly-streamlined projectiles fired by the 6.5 Grendel deliver terrific long-range performance, which would make that cartridge a superb choice for standard-length weapons like the Marines' M16A4 rifle. However, the 6.8 SPC has less velocity loss when fired in the short-barreled M4A1 carbine that is used in relatively large numbers by Army units.
The degree of difficulty in converting the current family of 5.56mm military small arms to either 6.5 Grendel or 6.8 SPC is nearly the same for each, with the apparent exception of the M249 light machine gun. Because of the noted complications with belt feed of 6.5 Grendel, and the fact that a high-capacity magazine for that round is not yet available, the unavoidable conclusion is that, at the present time, the 6.8 SPC would be the better alternative to replace the 5.56 NATO cartridge.