New Army "Caliber Configuration Study"

[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=Te...&Menu_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.

[Tony Williams]

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.

Yes, as I've mentioned before (in this thread IIRC, but possibly in another - there are so many of them I find it hard to keep track), I regard hit probability as being the key comparator within ballistics, but that does involve trajectory, flight time and wind-bucking qualities. It is not necessary to match the 7.62mm's retained energy, particularly since a smaller-diameter bullet needs less energy to achieve the same penetration.

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.

Can you refresh my memory as to where that one came from, please?

[LRRPF52]

What if you could increase hit probability, reduce wind deflection & trajectory, with a lighter projectile that has way less recoil, and exceed M80 or 7.62x54R terminal energy beyond 300yds all in the same ball of wax?

Oh yeah, we've kicked that horse before.

130gr 6.5mm .570 G1 BC, 2600fps

       Range    Velocity    Energy     Trajectory         TOF          Drift
      (yards)     (fps)     (ft-lb)       (MILS)         (sec)         (MILS)
          0      2600        1951           0.00         0.0000         0.00
        [B]100      2444[/B]        1724           0.00         0.1190        -0.18
        200      2294        1519          -0.60         0.2457        -0.37
        [B]300 [/B]     2149        [B]1333[/B]          -1.41         0.3808        -0.57
        400      2009         1165          -2.34         0.5252        -0.78
        500      1875        1015          -3.38         0.6797        -1.01
        600      1747         881          -4.53         0.8454        -1.25
        700      1626         763          -5.80         1.0234        -1.51
        800      1512         660          -7.22         1.2148        -1.78
        900      1407         572          -8.78         1.4205        -2.08
       1000      1312         497         -10.53         1.6414        -2.39
       1100      1227         435         -12.46         1.8781        -2.71
       1200      1155         385         -14.61         2.1304        -3.04

USSR 1986 7.62x54r Steel Core Light Ball .325 G1 BC, 2706fps

       Range    Velocity    Energy     Trajectory         TOF          Drift
      (yards)     (fps)     (ft-lb)       (MILS)         (sec)         (MILS)
          0      2706        2403           0.00         0.0000         0.00
        [B]100      2430[/B]        1937          -0.00         0.1170        -0.30
        200      2170        1546          -0.61         0.2476        -0.64
        [B]300[/B]      1928        [B]1220[/B]          -1.50         0.3943        -1.01
        400      1703         952          -2.60         0.5599        -1.43
        500      1500         738          -3.93         0.7477        -1.89
        600      1324         575          -5.54         0.9608        -2.41
        700      1181         457          -7.47         1.2013        -2.97
        800      1075         379          -9.79         1.4683        -3.56
        900      1000         328         -12.52         1.7583        -4.13
       1000       941         291         -15.67         2.0682        -4.69
       1100       893         262         -19.24         2.3960        -5.23
       1200       852         238         -23.23         2.7406        -5.75

You could even drop the pressures and go with a 2500fps mv and still smoke it.

[stanc]

What if you could increase hit probability, reduce wind deflection & trajectory, with a lighter projectile that has way less recoil, and exceed M80 or 7.62x54R terminal energy beyond 300yds all in the same ball of wax?

Oh yeah, we've kicked that horse before.

130gr Lead Core 6.5mm .570 G1 BC, 2600fps

USSR 1986 7.62x54r Steel Core Light Ball .325 G1 BC, 2706fps

You could even drop the pressures and go with a 2500fps mv and still smoke it.

Uh huh. Question is, with the switch to lead-free bullets, can you get even 120gr 6.5mm, let alone 130gr?

Plus, you're picking one of the lowest BCs for LPS Ball shown on 7.62x54r.net. Try it with 110gr 6.5mm .450 G1 BC vs Polish 1986 7.62x54r Steel Core Light Ball .538 G1 BC, 2800fps

[JASmith]

Uh huh. Question is, with the switch to lead-free bullets, can you get even 120gr 6.5mm, let alone 130gr?

Plus, you're picking one of the lowest BCs for LPS Ball shown on 7.62x54r.net. Try it with 110gr 6.5mm .450 G1 BC vs Polish 1986 7.62x54r Steel Core Light Ball .538 G1 BC, 2800fps

Enough cherry picking to "prove" a point!

[stanc]

Can you refresh my memory as to where that one came from, please?

http://www.thinkdefence.co.uk/2013/11/usa-force-rest-nato-change-calibres/

[LRRPF52]

Uh huh. Question is, with the switch to lead-free bullets, can you get even 120gr 6.5mm, let alone 130gr?

Plus, you're picking one of the lowest BCs for LPS Ball shown on 7.62x54r.net. Try it with 110gr 6.5mm .450 G1 BC vs Polish 1986 7.62x54r Steel Core Light Ball .538 G1 BC, 2800fps

That would be some nice ammunition if the data isn't a typo. It's got a long boat tail, nice ogive profile.

[stanc]

Enough cherry picking to "prove" a point!

That is the point I was trying to make, Joe. I wouldn't actually use the cited BC and MV of the Polish LPS round. I'd want the nominal values. In an attempt to get such data, yesterday I went to the MG&A forum and asked. One of the SME's reported LPS G7 BC as 0.208 (which translates to 0.406 G1 BC), and MV from the PKM machine gun as 840 mps (~2750 fps).

http://forums.delphiforums.com/autogun/messages/?msg=5922.7

http://forums.delphiforums.com/autogun/messages/?msg=5922.9

These numbers are in line with the mid-range LPS rounds tested by 7.62x54r.net, and seem much more reasonable to use if one wishes to have a realistic comparison.

And, while I really like Paul's 6.5 LMG concept, I just don't see how a 130gr lead-free bullet is practical. The Swedish 6.5mm AP bullet is nearly 5 calibers long, but weighs only 113gr. Make it completely lead-free, and it'd weigh less than 110gr.