Grendel LMG

[longdayjake]

Is it safe to say that nobody is going to try this with a full auto?

[Tony Williams]

Now here's an interesting idea: http://www.thefirearmblog.com/blog/2...onversion-kit/

The US Machine Gun Armory have developed a set of kits to convert the MK46 / M249 SAW into a range of 7.62mm calibers including the 7.62x39mm, 7.62x40mm Wilson Tactical, .300 AAC BLK and 7.62x51mm. All that is required is swapping out the top-cover, barrel and a few other parts.

What I'd like to see is them producing a 6.5 mm Grendel version...

[stanc]

Very interesting find, Tony. Do you suppose they use RPD belts for the 7.62x39 conversion?

Contact info, if anybody wants to "lobby" for a 6.5 Grendel kit:

U.S. Machine Gun Armory, LLC.
Suite 415
6300 Sagewood Drive
Park City, UT 84098 USA

sales@machinegunarmory.com

ETA: I'm not sure it matters, though. After thinking about it some more, it doesn't seem likely that a 6.5 Grendel LMG can provide sufficient performance to replace the 7.62 MMG. Even if you compare 20-inch barrels, you're still looking at 150gr @ 2700 fps vs 110gr @ 2600 fps, which makes for a substantial difference in terminal energy.

Plus, the 6.5 bullet will probably have a lower BC, so trajectory wouldn't be as flat, nor effective range as long. And if that isn't bad enough, a 6.5 LMG likely would have a barrel of no more than 16 inches, which would cut velocity down to about 2500 fps.

It seems like it'll be necessary to go with something like the 6.86x46 UIAC in order to have a LMG with 7.62 MMG performance.

[LRRPF52]

I would run at least a 130gr for a machinegun load, and with a half-bullpup design like the M60 or full bullpup, barrel length would not be limited. Even with a conventional design, the 19-20" barrels used on the Mk48 are still maneuverable, while a 20" 7.62 NATO MAG58 is not.

You will need higher pressures to get any 6.86 projectiles to keep up with 7.62 NATO. 6.5mm is the caliber node in short actions when it comes to best trajectory, best wind drift, and lower pressures, with substantial bullet weights.

[stanc]

I would run at least a 130gr for a machinegun load...

Wouldn't that reduce MV and make trajectory even worse?

You will need higher pressures to get any 6.86 projectiles to keep up with 7.62 NATO. 6.5mm is the caliber node in short actions when it comes to best trajectory, best wind drift, and lower pressures, with substantial bullet weights.

My understanding is that, all other things being equal, it takes higher pressure for a smaller caliber bullet to achieve the same velocity as a bigger caliber, not less.

[JASmith]

If I recall correctly, much of the Grendel military story is based on the 139 grain Scenar bullet and long range performance.

The current interest in bullets of 123 grains and less is largely a result of interest by the hunting community where most of the shots are taken at well under 300 yards.

So, going to 130 grains and slightly really represents a return to the thinking behind the "Grendel as a Universal Military Cartridge."

[stanc]

Hmm. My recollection is that the 123gr Scenar was always the primary basis for comparison, while the 139gr Scenar and the 144gr FMJBT were regarded as too heavy for general military use.

Either way, a 130gr Ball projectile @ 2400 fps just ain't gonna match 7.62 M80 trajectory and energy.

[bwaites]

Hmm. My recollection is that the 123gr Scenar was always the primary basis for comparison, while the 139gr Scenar and the 144gr FMJBT were regarded as too heavy for general military use.

Either way, a 130gr Ball projectile @ 2400 fps just ain't gonna match 7.62 M80 trajectory and energy.

True, but what if the military does what it did with 5.56 ammunition and ups the ante? I keed, I keed.....LOL!!

[JASmith]

...My understanding is that, all other things being equal, it takes higher pressure for a smaller caliber bullet to achieve the same velocity as a bigger caliber, not less.

We have to be careful about what we mean by "other things being equal." In the present instance, the sectional density is the operative item to be kept equal simply because that quantity largely determines velocity retention and penetration potential. The .277 bore has right at ten percent more area than does the .264 meaning that a 110 gr .277 bullet will have the same sectional density as a 100 grain .264 bullet.

The smaller bore and same cartridge volume means that a small diameter bullet will see higher pressures longer than would a larger diameter bullet of the same sectional density. This is because the total cartridge and barrel volume is smaller for the smaller diameter bullet.

Restating for clarity: The same total mass of gas in a smaller volume means higher pressures.

[LRRPF52]

An LMG will have a much larger bolt, and won't be limited by AR mag-length COAL. There also won't be a need to save the brass or case for reloading. Basically, you're looking at bolt gun velocity potential, and my friend is getting 2650-2700 fps with 139gr Lapua Scenars out of his Cz527 Grendel, so a 130gr at 2650 should be quite doable.

[stanc]

An LMG will have a much larger bolt, and won't be limited by AR mag-length COAL. Basically, you're looking at bolt gun velocity potential, and my friend is getting 2650-2700 fps with 139gr Lapua Scenars out of his Cz527 Grendel, so a 130gr at 2650 should be quite doable.

Loaded to longer than AR mag length, and to substantially higher than AR pressure? Perhaps.

Although since it's standard practice to have rifles and LMGs in the same caliber, that would rule out conversion of 5.56x45 and 7.62x39 rifles, necessitating development and acquisition of new rifles.

Another point to consider in advocating a 130gr projectile: for the US and NATO, it will need "green" construction (steel tip and copper slug, most likely), while the V4 and Russia would doubtless continue to use steel core. For both, a 130gr bullet would be very long for caliber, longer than the Hornady 120gr GMX (1.4").

Would that present stability problems?

Would it permit a matching Tracer bullet? (Tracers are typically longer than Ball rounds.)

[LRRPF52]

I wouldn't want downrange LMG performance in close-range carbines anyway...makes for disastrous results in the house/CQB, and increases recoil over the current systems unacceptably. Trying to make a cartridge do what 5.56 does within 100m and what 6.5mm Swede does at 700m ain't gonna happen in the same gun with the same load.

Those GMX's are very long indeed. An alloy could address the solids and keep them within length. Tracers only need length for maximum burn distance, and there is plenty of that in 6.5's. I'd be more concerned about throat/barrel burn than anything else really, but the Grendel is very mild on throats in its current state.

[stanc]

I wouldn't want downrange LMG performance in close-range carbines anyway...makes for disastrous results in the house/CQB, and increases recoil over the current systems unacceptably.

IIRC, we've been over this point before. Advocating two different loads -- heavy ball for machine guns and light ball for carbines -- is a nice theory, but the reality is that nearly all the world's armies (China being a notable exception with the 5.8x42) use only one loading for carbines and LMGs.

IMO, any proposal for the use of two loads is a non-starter.

[Tony Williams]

IIRC, we've been over this point before. Advocating two different loads -- heavy ball for machine guns and light ball for carbines -- is a nice theory, but the reality is that nearly all the world's armies (China being a notable exception with the 5.8x42) use only one loading for carbines and LMGs.

IMO, any proposal for the use of two loads is a non-starter.

I agree that a single universal load is very desirable.

Having said that - how often, in the real world of military operations, do you get sections breaking up MG belts to feed rifles, or hastily belting MG ammo from rifle mags? Does it really matter if the belted ammo is different from the rifle ammo? That might be a compromise worth making to obtain the other benefits of one calibre for all rifles and magazine-fed guns.

However, we must also bear in mind that to be worth changing from 7.62mm in belted MGs, the new ammo needs to offer a substantial reduction in weight.

[stanc]

I agree that a single universal load is very desirable.

The main advantage I see is for simplicity in manufacturing.

Having said that - how often, in the real world of military operations, do you get sections breaking up MG belts to feed rifles, or hastily belting MG ammo from rifle mags?

I'd think that it happens very seldom. IIRC, my best friend told me there was one time in Vietnam when he de-linked M60 rounds to feed his XM21 sniper rifle.

Does it really matter if the belted ammo is different from the rifle ammo?

IMO, there's no technical reason to rule out using different weight ball rounds (or for that matter, even having different calibers) for rifle and LMG. After all, rifle ammo comes pre-packed in stripper clips, and LMG ammo in links. But, good luck convincing military leadership to abandon the SOP of having a single load.