New Army "Caliber Configuration Study"

[Tony Williams]

You are making an academic comparison of the cartridges under an artificial restriction (equal barrel lengths). This methodology is used in order to show one's pet cartridge concept in the best possible light, but it seldom provides a realistic comparison of the cartridges in the barrel lengths actually used on the battlefield.

In John's hypothetical ambush scenario, he discussed 65G ARs and LMGs. The current US Army individual weapon is the M4 carbine and the LMG is the M249, both with 14.5" barrels. The USMC's M27 IAR has a 16" barrel, as does SOCOM's Mk46 LMG. Therefore, in the best case scenario, I see 16" as the maximum likely barrel length for 65G weapons.

Which means the question that really needs to be answered is how would 6.5 Grendel (7g/108gr bullet) performance from a 16" barrel compare to 7.62x54R in the 25.4" barrel PKM?

I see your point, but I completely disagree with it. The performance of cartridges needs to be compared on a like-for-like basis - i.e. using the same barrel lengths - because that demonstrates what they are capable of. Barrel lengths are not fixed in the way that cartridge dimensions are: it is a simple matter to fit a longer barrel to an MG (e.g. the British Army is planning to acquire longer barrels for its 5.56mm Para Minimis in order to improve their effective range). But once you've chosen a cartridge, you're stuck with it for a long time. The ability to use barrels of different lengths (depending on the balance between effectiveness and portability required) provides tactical flexibility for an army, but they do need to choose a cartridge which will deliver the performance they need from a barrel of acceptable length.

We have been discussing long-range performance here, in which 5.56mm weapons are frankly irrelevant - the comparison is between the Grendel and 7.62mm. And the barrel length of the M240 MG (i.e. probably 95+% of the 7.62mm MGs in US service) is - guess what? 24.5 inches! So I will concede that I maybe shouldn't have picked 20" as the comparator - it's too short, and shows the Grendel at a disadvantage.

[LRRPF52]

PKM averages about 2700fps with 147.8gr 7.62x54R light ball steel core USSR 1986. G1 BC is .325

The SVD with Russian 1997 7.62x54r 152 gr. "7N1" Sniper load, .498 G1 BC is a different animal. 2723fps from SVD, and 2657fps from SVDS.



http://7.62x54r.net/MosinID/MosinAmmo030.htm

This is why I like the 130gr cup and core bullets with BC's in the mid 5's, but I could almost care less about US Army Caliber Configurations at this point.

[stanc]

PKM averages about 2700fps with 147.8gr 7.62x54R light ball steel core USSR 1986. G1 BC is .325

I presume you got that info from http://7.62x54r.net/MosinID/MosinAmmo001.htm

The problem is, http://7.62x54r.net/MosinID/MosinAmmo.htm shows the BC of steel-core, light ball varying from an incredibly low 0.242 to an unbelievably high 0.538, depending on where and when produced.

And to add to the confusion, I found this info on THR:

148-gr steel-core ball, 57-N-323-S: 0.381 G1 --- 0.192 G7 --- 820-835 m/s http://www.thehighroad.org/archive/index...19407.html

[LRRPF52]

Sounds like the BC variations are consistent with production in that part of the world...Normalnyui

[stanc]

I see your point, but I completely disagree with it. The performance of cartridges needs to be compared on a like-for-like basis - i.e. using the same barrel lengths - because that demonstrates what they are capable of.

Unfortunately, that does not necessarily show what they are capable of with the barrel length(s) actually used on the battlefield.

For instance, to compare performance of 7.62x51 and 7.62x54R using the same barrel lengths would give a false comparison of ballistics as actually used in the L129A1 (16" bbl) and SVD (24" bbl) rifles in Afghanistan.

We have been discussing long-range performance here, in which 5.56mm weapons are frankly irrelevant - the comparison is between the Grendel and 7.62mm. And the barrel length of the M240 MG (i.e. probably 95+% of the 7.62mm MGs in US service) is - guess what? 24.5 inches!

I agree the discussion is about 6.5G vs 7.62mm. However, 6.5G was designed to fit 5.56mm weapons. As such, it would be used in something like the Mk46, not the M240, so barrel length of the latter is completely irrelevant.

If you object to citing the (16") barrel lengths of recent 5.56mm weapons like the Mk46 LMG and M27 IAR, look at those in 7.62mm: Mk17 Std, 16"; HK417 Std, 16"; L129A1, 16".

Plus, the German MG5A2 Infantry machine gun has an 18" barrel, the M60E6 adopted by Denmark has a 17" barrel, and the Polish UKM-2013 has a 17" barrel option.

Face it, the trend is to barrels significantly shorter than 20" length, so it would seem prudent to plan accordingly.

[Tony Williams]

The problem is, http://7.62x54r.net/MosinID/MosinAmmo.htm shows the BC of steel-core, light ball varying from an incredibly low 0.242 to an unbelievably high 0.538, depending on where and when produced.

And to add to the confusion, I found this info on THR:

148-gr steel-core ball, 57-N-323-S: 0.381 G1 --- 0.192 G7 --- 820-835 m/s http://www.thehighroad.org/archive/index...19407.html

I presume that there is some confusion between G1 and G7 BCs. G1 figures are approximately double the G7 results.

[Tony Williams]

I agree the discussion is about 6.5G vs 7.62mm. However, 6.5G was designed to fit 5.56mm weapons. As such, it would be used in something like the Mk46, not the M240, so barrel length of the latter is completely irrelevant.

I do agree that the Army should be presented with data on how the Grendel (and any other rival proposals) performs from barrels of different lengths, with the ballistics of each compared with the PKM. They could then decide what they wanted. I suspect that they would be unwilling to make a choice which would leave their troops outranged by the PKM, and would have to face up to the fact that choosing a short barrel for any desired ballistics would mean having to select a more powerful cartridge, resulting in more recoil and muzzle blast and a greater ammunition weight - they wouldn't want that either. They would be pushed into making a reasoned decision about the relationship between cartridge design and barrel length, in full awareness of the negative implications of short barrels.

I suspect that they might end up choosing one design of belt-fed MG in two versions: light (with a light barrel of 16-18 inches and a bipod) and heavy (with a heavy barrel of 20-24 inches and normally on a tripod). Since the gun would of course be modular, they could easily attach a bipod to a heavy-barrel version if they needed to combine long range with portability. They would thereby get the benefits of a common cartridge and ammo belt and a common MG (except for the choice of barrels and mountings).

Edit to add: the same choice would of course apply to rifles. The logical solution would be a bullpup, otherwise they would have a choice between a long barrel for the best ballistics (DMR) or a short barrel with reduced ballistics for compactness.

[JASmith]

+ 1

[LRRPF52]

If you look at the Mk.48, it has a 19.75" barrel:



If you bullpup or semi-bullpup a smaller LMG, using the best component systems of LMG's to-date, chamber it in something with 6.5mm 130gr at 2500-2600fps, then you really have something. The Russians are ahead of the game with that PKM bullpup, although it is an afterthought, rather than purpose-built from scratch, and the ammunition container is in the wrong place.

[stanc]

If you bullpup or semi-bullpup a smaller LMG, using the best component systems of LMG's to-date, chamber it in something with 6.5mm 130gr at 2500-2600fps, then you really have something.

Indeed it would. I don't know if a belt-fed bullpup (with acceptable ergonomics) is feasible, but the M60 series clearly shows that a semi-bullpup LMG can be done.

I wish I could say the same about your proposed 130gr load. I just don't see how a lead-free 6.5mm bullet heavier than ~110gr can be viable. A 130gr LF Ball projectile would be extremely long, and a matching LF Tracer even longer. Perhaps too long to be practical?

[JASmith]

...I just don't see how a lead-free 6.5mm bullet heavier than ~110gr can be viable. A 130gr LF Ball projectile would be extremely long, and a matching LF Tracer even longer. Perhaps too long to be practical?

This may be the conclusion we all come to. But, a lot depends on how the lead-free and the tracer option are implemented and whether it can be done within acceptable cost constraints.

After all, the 130 gr performance would be awesome!

[stanc]

I do agree that the Army should be presented with data on how the Grendel performs from barrels of different lengths, with the ballistics of each compared with the PKM. They could then decide what they wanted. I suspect that they would be unwilling to make a choice which would leave their troops outranged by the PKM...

The question is, would 6.5 Grendel w/108gr bullet be outranged by 7.62x54R w/148gr bullet, when 65G is fired from a 16" barrel?

I see no need to have a trajectory match. The 16" barrel L129A1 and 20" barrel Mk48 cannot have as flat of trajectory as the SVD and PKM, yet presumably are enabling UK and US troops to not be outranged?

IMO, a new Squad Common Cartridge/General Purpose Cartridge does not need to equal or exceed M80 ballistics, it just needs to provide the same engagement range capability. It seems to me that what's important is hit probability, not any specific trajectory, as indicated by use of the L129A1 and Mk48 weapons.

...choosing a short barrel for any desired ballistics would mean having to select a more powerful cartridge, resulting in more recoil and muzzle blast and a greater ammunition weight - they wouldn't want that either.

They might not want it, but that doesn't mean they wouldn't accept it. Remember, the relationship between barrel length versus recoil and muzzle blast has been known for a very long time, yet barrels keep getting shorter and shorter.

I suspect that they might end up choosing one design of belt-fed MG in two versions: light (with a light barrel of 16-18 inches and a bipod) and heavy (with a heavy barrel of 20-24 inches and normally on a tripod). Since the gun would of course be modular, they could easily attach a bipod to a heavy-barrel version if they needed to combine long range with portability. They would thereby get the benefits of a common cartridge and ammo belt and a common MG (except for the choice of barrels and mountings).

One major flaw with that idea: The CLAWS and LDAM programs indicate they want a LMG for the rifle squad, and a MMG for the platoon, with each in a different caliber. That means two different cartridges, two different belts, and no common MG.

Edit to add: the same choice would of course apply to rifles. The logical solution would be a bullpup, otherwise they would have a choice between a long barrel for the best ballistics (DMR) or a short barrel with reduced ballistics for compactness.

I agree that a bullpup rifle -- providing someone comes out with a really good design -- would be the logical choice. And I think you've hit on an excellent approach to presenting a case for a bullpup that might actually have a chance of convincing the decision makers, where previous arguments have failed.

I'm talking about your idea to show the relationship between cartridge size and power versus barrel length and weights of weapon, ammo, and magazines. My understanding is the 6.5 GPC prototype (second from left, in the photo below) delivers approximately the same muzzle velocity from a 16" barrel as 6.5 Grendel (center, below) from a 24" barrel. These two cartridges would seem to be perfectly suited to making your case.

[cory]

Indeed it would. I don't know if a belt-fed bullpup (with acceptable ergonomics) is feasible, but the M60 series clearly shows that a semi-bullpup LMG can be done.

I wish I could say the same about your proposed 130gr load. I just don't see how a lead-free 6.5mm bullet heavier than ~110gr can be viable. A 130gr LF Ball projectile would be extremely long, and a matching LF Tracer even longer. Perhaps too long to be practical?

The tracer does not need to be a matching 130gr projectile. I 115-120gr tracer can be loaded to ~match the ballistic trajectory.

[cory]

The question is, would 6.5 Grendel w/108gr bullet be outranged by 7.62x54R w/148gr bullet, when 65G is fired from a 16" barrel?
Legitimate question. However I'm thinking you're narrowing the scope down to much, as a projectile hasn't been designed yet. I'd think we'd keep it open to the 105-115gr range, at a minimum.

...

IMO, a new Squad Common Cartridge/General Purpose Cartridge does not need to equal or exceed M80 ballistics, it just needs to provide the same engagement range capability. It seems to me that what's important is hit probability, not any specific trajectory, as indicated by use of the L129A1 and Mk48 weapons.

Completely agree.

...

...

[stanc]

The tracer does not need to be a matching 130gr projectile. I 115-120gr tracer can be loaded to ~match the ballistic trajectory.

Yes, tracer bullets are often lighter than ball projectiles. For example, Swedish 6.5x55 Ball is 140gr, but 6.5x55 Tracer is only 120gr.

However, I did not mean equal weight. Perhaps "companion Tracer" would've been a better choice of words.

In any case, the point is that a lead-free Tracer bullet would almost certainly be longer than a lead-free 130gr Ball round, which itself would be quite long. What rifling twist would be needed to stabilize such a long projectile? Would it be stable in arctic temperatures? Would it be suited to the M855A1/M80A1-type construction?

[stanc]

The question is, would 6.5 Grendel w/108gr bullet be outranged by 7.62x54R w/148gr bullet, when 65G is fired from a 16" barrel?

Legitimate question. However I'm thinking you're narrowing the scope down to much, as a projectile hasn't been designed yet. I'd think we'd keep it open to the 105-115gr range, at a minimum.

I am open to a different weight. In my example, I was using Tony's hypothetical, scaled-up 7N6 bullet, which he says would weigh 7 grams (108 grains) in 6.5mm.

Consensus of the subject matter experts on his MG&A forum is that 7.2 grams (111 grains) is about the maximum practical weight for a lead-free 6.5mm bullet. Since I have no information to the contrary, 108 grains seemed like a reasonable weight.

I would note that the steel-core FMJ bullet in the mythical Wolf steel-cased ammo is even lighter, at 100 grains.

[JASmith]

Yes, tracer bullets are often lighter than ball projectiles. For example, Swedish 6.5x55 Ball is 140gr, but 6.5x55 Tracer is only 120gr...

Do you have any pictures of those two bullets?

[stanc]

Do you have any pictures of those two bullets?

Good pics at: http://www.amkat.se/index.php?Env=Text-2...u_Value_B=

-------------------------- BALL ----------------------------------------------------------- TRACER -------------------------

[JASmith]

Stan,

Thanks!

It seems there is more to the story. The other bullet pictures, including the slightly lighter AP round, all seem to be about this same length as the two you pulled up for us.

If I read the labels correctly, the white-tipped bullet is the 7.78 gram tracer bullet. The copper-colored boattail bullet is a heavier ball round.

[stanc]

The other bullet pictures, including the slightly lighter AP round, all seem to be about this same length as the two you pulled up for us.

Yeah, the AP and Ball projectiles appear to be the same length, with the Tracer bullet maybe just a hair longer.

If I read the labels correctly, the whit-tipped bullet is the 7.78 gram tracer bullet. The copper-colored boattail bullet is a heavier ball round.

Correct. I've so labeled the pics in my previous post.

A pity the AP and Tracer bullets don't seem to be available in shooting quantity. They'd be interesting to load in 6.5 Grendel cases for some possibly informative testing.