New Army "Caliber Configuration Study"

[nincomp]

Huh? Steel core bullets in the same form factor as lead core bullets will have a LOWER BC than the heavier lead core bullet. If you maximize the optimum shape for two bullets of the same weight, the denser bullet will always win on BC.

You are correct, assuming that the bullets are of the optimum shape. The problem here is that BC gets better with a better form factor which ends up with longer and longer bullets. When we are looking at magazine-fed weapons, the overall length is limited. This limits the form factor. The perfect example of this is seen when the 6.8 SPC is compared to the 6.5 Grendel. The 6.8 SPC is limited to shorter, lower form factor bullets, so no matter how much the shape is optimized, the BC will be lower than that achievable with the Grendel.

Another good example is the 5.56x45 and the 5.45x39. When they use bullets of roughly the same weight and sectional density (55gr M193 / 53gr 7N6), the 5.45 permits a longer ogive nose, giving a better form factor and BC.

As has been noted by John and Stan, if the performance of a lighter projectile is to match the M80 at long range, the bullet will be too long to fit within the Grendel specs. It is not out of the question that the new cartridge would be the same length as the current 7.62x51 to allow enough powder space and a long-ogive bullet.

In the end, several different parameters need to be balanced, among them are the shape of the bullet and ogive length, bullet weight, case shape and capacity, recoil impulse, etc. Then come the issues of twist rate and bullet stability at low temperatures. When it comes down to it, the goal of Caliber Configuration Study is to figure out what the parameters will be.

[nincomp]

There has been a lot of discussion about Russian bullet construction. Here are some useful links:

5.45x39: http://gunsru.ru/rg_patron_5_45x39_eng.html
Evolution of the bullet: http://translate.google.com/translat...-text=&act=url

7.62x39 : http://gunsru.ru/rg_patron_7_62x39_eng.html

7.62x54: http://gunsru.ru/rg_patron_7_62x54_eng.html

I found it interesting that one of the newest bullet types has a lead core: but it is a special issue for reduced richochet. Most of the regular issue ammo has a steel penetrator of some sort, but is was found that they tended to richochet back at soldiers who were performing "special operations in settlements."

While looking for these links, I stumbled across the ATF ruling on the 5.45 7N6 ammo -ouch:

http://www.atf.gov/press/releases/2014/04/040714-special-advisory-test-examination-and-classification-7n6-545x39-ammunition.html

[Tony Williams]

A comment on bullet design: it does not necessarily follow that a lengthened bullet has a better Form Factor, if the extra length just goes into the shank, leaving the nose and tail profiles the same.

This can be demonstrated by comparing the 7.62mm M80 with the M118 sniper loading (both lead-cored). The M118's FF is almost exactly the same; it has a better BC simply because it's heavier.

Extra length can permit a better FF by allowing a longer, more tapered ogive - but only if the COAL permits this. Which it doesn't in the 7.62x51 (or the 5.56x45, or the .300WM, or the .338LM).

The US has a long tradition of designing cartridges with COALs too short to permit the use of high-BC bullets with long ogives. The 6.8mm Rem is perhaps the most extreme recent example of this - the 6.5mm Grendel is an exception.

One of the key design points of any new military cartridge should be that the specification must allow the use of long-ogive bullets, since they can provide a significant improvement in long-range performance without any extra power, weight or recoil. Apart from slightly longer gun actions and magazines, it provides extra performance at no cost.

Choosing the .338NM rather than the .338LM for the LDAM MMG would be a big step in the right direction.

[BluntForceTrauma]

I want to run some numbers, making a comparison between this GS Custom 6.5mm 112gr monolithic with a PKM 7.62x54R round.

Any educated guesses as to a realistic velocity from a 16" barrel for a 112gr with at least a 0.523 BC from the 6.5 Grendel case? I'm thinking if we have a Grendelized M855A1 that it would have a similar form factor to the GS Custom 6.5 112gr, perhaps a bit lighter if the steel core is less dense than copper.

John

P.S. I'm going to compare it to a 7.62x54R round from a PKM 25.4" barrel, with a 148gr bullet at 2,717 fps with a 0.350 BC. I've guesstimated the BC, since internet sources vary so widely. Sierra bullets lists a BC of 0.397 for its .308 150gr FMJBT. If I'm being unfair to the Russian, speak now.

[stanc]

A comment on bullet design: it does not necessarily follow that a lengthened bullet has a better Form Factor, if the extra length just goes into the shank, leaving the nose and tail profiles the same.

This can be demonstrated by comparing the 7.62mm M80 with the M118 sniper loading (both lead-cored). The M118's FF is almost exactly the same; it has a better BC simply because it's heavier.

Thanks for that info, Tony. I would note that the M118 Match bullet has a tangent ogive, whereas that of M80 Ball is secant. A better comparison would be to steel-core M59 Ball (left, below), which also has a secant ogive, and weighs approximately the same as lead-core M80 Ball (right, below). Do you happen to know the FF of the M59 bullet?

Extra length can permit a better FF by allowing a longer, more tapered ogive - but only if the COAL permits this.

The US has a long tradition of designing cartridges with COALs too short to permit the use of high-BC bullets with long ogives. The 6.8mm Rem is perhaps the most extreme recent example of this...

One of the key design points of any new military cartridge should be that the specification must allow the use of long-ogive bullets, since they can provide a significant improvement in long-range performance without any extra power, weight or recoil.

I wholeheartedly agree. We can only hope the military will, too.

Apart from slightly longer gun actions and magazines, it provides extra performance at no cost.

Yes. It causes me to wonder how much performance would improve by simply increasing cartridge OAL of 5.56 and 7.62 NATO.

On your MG&A forum, Badcow454 stated that the 7.62x51 case loaded with a scaled-up 7N6 bullet would have a 2.94" COAL. Since the FN MAG was originally adopted by Sweden in 6.5x55, which has a 3.15" COAL, it would seem a relatively easy task to adapt MAG variants to use 7.62x51 with long-ogive bullets.

Of course, this concept could also be applied to 6.8 SPC, as well as variations like 6.5 SPC.

Question: What do you get if you scale up the 5.45x39 round by a factor of 1.1?
Answer: A long-loaded 6mm SPC.

[stanc]

I'm going to compare it to a 7.62x54R round from a PKM 25.4" barrel, with a 148gr bullet at 2,717 fps with a 0.350 BC. I've guesstimated the BC, since internet sources vary so widely. Sierra bullets lists a BC of 0.397 for its .308 150gr FMJBT. If I'm being unfair to the Russian, speak now.

Speaking now.

Yes, I think it's unfair. One of the SME's on Tony's MG&A forum says he uses 0.406 BC and 2750 fps (for MV from the PKM).

IMO, 0.406 BC for 7.62x54R 148gr steel-core Ball seems reasonable. It's consistent with the middle of the BC range in test data on 7.62x54R.net, as well as published BC's for 7.62x51 147gr M80 Ball.

[stanc]

I want to run some numbers, making a comparison between this GS Custom 6.5mm 112gr monolithic with a PKM 7.62x54R round.

Any educated guesses as to a realistic velocity from a 16" barrel for a 112gr with at least a 0.523 BC from the 6.5 Grendel case?

I'll hazard a guess of 2550-2600 fps, based on 16" velocities @ http://www.alexanderarms.com/images/...ballistics.pdf

[BluntForceTrauma]

Obviously, I want to put the 65G in the best light, and am jealous of any competing data, so let's just get that on the table now. Having said that, I intend to have very strong rationales, openly and objectively argued for my data.

What is the rationale for your SME's data about 7.62x54R? I'm skeptical that his BC is better than Sierra's, given that Sierra is actually tested, has a denser lead core that sheds velocity slower, and has a secant ogive. Got a link to his data?

I'm gonna go with 2550 fps for the 112gr 65G @ 0.523 BC (this is a conservative BC because that's the 2400 fps BC). Comparable 110-115gr 6.8 loads get about 2550 fps from a 16".

John

[stanc]

Obviously, I want to put the 65G in the best light, and am jealous of any competing data, so let's just get that on the table now. Having said that, I intend to have very strong rationales, openly and objectively argued for my data.

What is the rationale for your SME's data about 7.62x54R? I'm skeptical that his BC is better than Sierra's, given that Sierra is actually tested, has a denser lead core that sheds velocity slower, and has a secant ogive. Got a link to his data?

I'm gonna go with 2550 fps for the 112gr 65G @ 0.523 BC (this is a conservative BC because that's the 2400 fps BC).

ARRRRRGH! I just spent two hours composing a reply, only to lose it all because of the forum's %&@#ing auto-log out!

Okay, abbreviated version.

SME apparently does a lot of official government testing of military small arms ammo, and is held in high regard on Tony's MG&A forum.

Links to SME's 7.62x54R LPS data:

* BC http://forums.delphiforums.com/autog...es/?msg=5922.7
* MV http://forums.delphiforums.com/autog...es/?msg=5922.9

Did a lot of searching, but was unable to find any manufacturer listing of .311" bullet of required weight and shape. Closest fit is Nosler .308" 150gr E-Tip, a solid copper bullet (much closer in material density to steel-core LPS) with boattail and tangent ogive. BC = 0.469, so I still think 0.406 is reasonable for the LPS.

Am skeptical that BC of a 110-112gr, 6.5mm military ball projectile will get above 0.480 (if even that high), but I nevertheless look forward to seeing your ballistic comparison.

[BluntForceTrauma]

Here are my charts for a 6.5 Grendel carbine with commercial off-the-shelf lead-free projectile versus a Russian 7.62x54R PKM machine gun, based on the following assumptions:

6.5 Grendel: 112gr GS Custom copper monolithic (0.523 BC) @ 2550 fps from 16" barrel.

PKM: 148gr 57-N-323S mild steel core (0.381 BC) @ 2725 fps from 25.4" barrel.

As with models for the global warming debate, one's models depend on one's assumptions and initial inputs, but I think mine are defensible. Data was generated using the online JBM Ballistics Calculator and charts were made after inputting the data into an Excel spreadsheet.

The news here is how well a 6.5 Grendel CARBINE stacks up against the threat medium machine gun.

While starting out slower, the 6.5 Grendel velocity starts beating the PKM after 300 yards. 6.5 Grendel hits harder at range, aiding penetration.

The 6.5 Grendel's trajectory drop is neck-and-neck with the PKM until it pulls ahead after 650 yards.

The 6.5 Grendel drifts less at all ranges in crosswinds; this is key for getting on target faster.

As is to be expected, 6.5 Grendel doesn't match or exceed the energy of the bigger cartridge until after 800 yards.

John

[nincomp]

P.S. I'm going to compare it to a 7.62x54R round from a PKM 25.4" barrel, with a 148gr bullet at 2,717 fps with a 0.350 BC. I've guesstimated the BC, since internet sources vary so widely. Sierra bullets lists a BC of 0.397 for its .308 150gr FMJBT. If I'm being unfair to the Russian, speak now.

I tend to believe the fellow on the the MG&A forum. He has access to a range with doppler radar measuring equipment, and had been a missile designer. Ballistics is his job.

I eventually got the russian photo of the 7.62x54 LPS and the Sierra 150 FMJBT pretty much to scale (what a pain). As you would expect, the steel cored bullet is longer. You can see that the ogive on the LPS is longer and it has a longer boattail. This makes the form factor pretty much the same as the Sierra. This is what I meant in my earlier post.

I have heard that GS Custom's BC's tend to be optimistic. I find it odd that I never saw those bullets being used by long range competitors. Based upon the published BC's they would have been idiots not to.

good combined 762.jpg

[Tony Williams]

I'm thinking if we have a Grendelized M855A1 that it would have a similar form factor to the GS Custom 6.5 112gr, perhaps a bit lighter if the steel core is less dense than copper.

Forgive me for being picky, John, but the Form Factor is unaffected by bullet weight or the density of the materials - it is only concerned with the shape. The weight of the bullet is taken into account in calculating the Sectional Density which is then multiplied by the FF to get the Ballistic Coefficient.

[BluntForceTrauma]

Tony, perhaps I wasn't clear, but what I meant is that I'd like to postulate a 6.5 M855A1, with its two-part copper and steel construction, that exactly matches the shape or form factor of the 6.5mm 112gr GS Custom monolithic in order to preserve that bullet's BC, as nearly as possible.

Photo concept attached.

Nincomp, thanks, valid points. The 7.62x54R BC of 0.381 I quoted is, supposedly, translated from Russian military manuals. I might add that it could simply be the GS Custom bullets are too pricey for competitors?

Anyway, as always, my main point is that these hypotheses look promising and I wish I had money to burn playing them out!

John

[stanc]

Nice work, John! The graphs illustrate the ballistics quite nicely.

But, since I think the GSC bullet gives an overly optimistic basis for comparison, I ran my own tables, using the 110gr Barnes brass solid (second from right, below) as a military ball surrogate.



6.5 Grendel LMG: 110gr Barnes banded solid (0.452 BC) @ 2550 fps from 16" barrel.

7.62x54R PKM: 148gr 57-N-323S mild steel core (0.406 BC) @ 2750 fps from 25.4" barrel.

Data was generated using the online JBM Ballistics Calculator.

_______ 65G _ 762 ____ 65G _____ 762 ____ 65G _____ 762 ___ 65G __ 762
Range _ Drop _ Drop _ Windage _ Windage _ Velocity _ Velocity _ Time _ Time
(yd) ___ (in) __ (in) ___ (in) _____ (in) ____ (ft/s) ___ (ft/s) ___ (s) ___ (s)

000 ____ 0.0 ___ 0.0 ___ 0.0 ____ 0.0 ____ 2550.0 __ 2750.0 __ 0.000 _ 0.000
100 ___ 48.4 __ 44.7 ___ 0.8 ____ 0.8 ____ 2356.8 __ 2525.8 __ 0.122 _ 0.114
200 ___ 90.5 __ 84.0 ___ 3.5 ____ 3.5 ____ 2172.0 __ 2312.4 __ 0.255 _ 0.238
300 __ 125.2 _ 116.7 ___ 8.1 ____ 8.2 ____ 1995.5 __ 2109.3 __ 0.399 _ 0.374
400 __ 151.1 _ 141.6 __ 15.1 ___ 15.3 ____ 1828.1 __ 1916.9 __ 0.556 _ 0.523
500 __ 166.6 _ 157.0 __ 24.6 ___ 25.0 ____ 1670.6 __ 1736.1 __ 0.728 _ 0.688
600 __ 169.7 _ 160.8 __ 37.0 ___ 37.8 ____ 1524.5 __ 1568.6 __ 0.916 _ 0.869
700 __ 157.7 _ 150.5 __ 52.5 ___ 54.0 ____ 1391.8 __ 1416.9 __ 1.122 _ 1.071
800 __ 127.8 _ 122.8 __ 71.5 ___ 74.0 ____ 1274.8 __ 1284.0 __ 1.347 _ 1.293
900 ___ 76.4 __ 74.0 __ 93.9 ___ 97.9 ____ 1176.0 __ 1173.5 __ 1.593 _ 1.538
1000 ___ 0.0 ___ 0.0 _ 119.8 __ 125.5 ____ 1097.3 __ 1087.7 __ 1.857 _ 1.804

Energy @ 1000 yards
65G: 294 ft-lbs
762: 389 ft-lbs

[BluntForceTrauma]

Thanks, Stan, for taking the time to generate the data points. So, worst case, our carbine is still almost going neck-and-neck with their MMG?

Something went awry at the 700-yard mark on your drop data?

John