Optimum cartridge pondering

Every choice is a trade-off. “You want armor to be light,
effective, and cheap. Pick any two.” So, sometimes you have to figure out what
are the most critical limiting factors, and go from there.

An ideal gun is light weight, accurate, shoots flat, hits
hard, has little recoil and comfortable ergonomics, has long barrel life, is reliable,
is low maintenance, has inexpensive and light weight ammo, and is easy to
operate… Yaahhh…. Riiiight…..

Back to reality.

The bullet does the work – everything else is just delivery
system. So, to stop a person or other living target (or set off a boomer), the bullet needs enough energy when it hits to do the job. Launching
the bullet imparts the energy into the bullet, and that causes recoil, requires
a gun, etc. Generally speaking, the greater the muzzle energy, the more the
recoil, the more wear on the gun, the greater the cartridge weight required, the
higher the chamber pressure, the more difficulty there is in noise suppression,
etc. So, an ideal cartridge would have some maximum tolerable muzzle energy,
and a minimum retained energy out to some desirable range.

What should those three numbers be? It depends on the
application. For the moment, I’ll consider military rifle cartridges (and perhaps Boomershoot guns). Maybe a
future essay will consider other applications.

If you generate much more than about 2000 ft·lb
of ME, a lot of smaller or less experiences shooters may have a problem flinching
or bruising from significant use, unless well trained and given sufficient
practice. Also, at closer range most bullets with more than 2000 ft·lb
will just waste an increasing percentage of their energy beyond the target,
after full penetration, on the backstop. (For comparison, 2000 ft·lb
is a typical muzzle energy for a .243 Winchester). Much less than about 400 ft·lb
is getting into a very marginal area for stopping power, cover or body armor penetration,
etc. (around 400 ft·lb is a typical 9 mm or 45 ACP round ME). For most
shooters, anything beyond a thousand yards is problematic for all sorts of
reasons, but out to that range an argument can be made, especially in places
like Afghanistan or Iraq, or in farm country with large fields, where distances
are long.

Challenge Summary: Muzzle energy less than 2000 ft·lb, greatest possible retained energy at
1000 yards, preferably at least 400 ft·lb.

It’s easy to find cartridges with less than 2000 ft·lb
muzzle energy. The problem is that most of them in larger calibers (30 cal and
up) are relatively fat, light, low BC bullets, or slow heavy ones that have a
trajectory like a rainbow and a time-of-flight measured in cups of coffee. The
smaller calibers (like .223), bullets are too light to carry much energy for
the distance, and start having severe wind problems at significant ranges. (For
comparison, a 5.56 NATO 77 gr bullet has a bit less than 1400 ft·lb
ME, and a 7.62 NATO 175 gr bullet has about 2600 ft·lb ME.)

It’s also easy to find cartridges that retain at least 400
ft·lb
at 1000 yd: just GO BIG. Heavy bullet, big brass, lots of powder, good to go.
But that generates more recoil, higher pressures, needs heavier guns, has
heavier ammo, more recoil, shorter barrel life, and so-forth.

Retaining energy argues that only high ballistic coefficient
bullets will likely manage to meet this challenge. A 6.5mm mid-weight bullet
with a high BC, like a Lapua 123 gr Scenar (BC of .547) launched at moderate
velocities, can be loaded to have both a ME less than 2000 ft·lb,
and have more than 400 ft·lb
at 1000 yards. One of the few current cartridges that meet this challenge is
the 6.5mm Grendel. It still has 372 ft·lb at 1000 meters in a factory loading, shot from a mid-length barrel. For
comparison, at 1000 yards, a 5.56 NATO 77 gr bullet has less than 200 ft·lb
of energy (similar to a .32 Auto), and a 7.62 NATO 175 gr has retained a bit
under 600 ft·lb
energy (similar to a typical 40 S&W shot from a 5” barrel). Also note that for reliable boomer detonation, a velocity of at least 1500 fps is generally required, and a typical 6.5 Grendel round is still moving faster than that at 700 yards (unless you are using a fairly short barrel).

The 6.5 mm cartridges have an excellent reputation with
hunters, as well as target shooters, and smokeless powder 6.5mm cartridges have
been around for well over a century, so there are a wide range of bullets
available for loading your own for any particular application you might have.

Ponder, think, consider, contemplate….

10 thoughts on “Optimum cartridge pondering

  1. This is an issue I’ve spent a lot of time on. Without putting numbers on it, I’ve been thinking in terms of a practically-man-carried rifle of decent MV (2600 or better) and high BC. Different targets call for different MEs. If you’re thinking targets like man, common vehicle, deer, and maybe elk, then the fast 6.5s call for your attention. The 6.5-284 speaks to me. Boomers– no problem. Materiel targets like trucks or APCs demand more muzzle energy, or specifically target energy (TE?) so we’re talking heavier rifles, but even there the term “Practical man carried” is somewhat flexible, depending on the distances carried, the other load requirements (food, ammo round count, etc.) and the person doing the carrying.

    Then comes the question; is it for a fighting type rifle (available in semi auto)? No doubt if it isn’t available right now, the AR-10 could be chambered for the 260 Remington.

    Then there is that traumatic shock wound threshhold, around 2000 fps impact velocity, that medical doctors speak of.

  2. I’ve thought a lot about the higher velocity 6.5mm rounds, too. But with that increased velocity comes rapidly diminishing returns, more so than in 30 cal and larger. Increased throat erosion, decreased barrel life, much higher pressures, much louder report, much increased recoil relative to the increase in energy, a need for a longer barrel to utilize all the extra powder, etc. Don’t get me wrong, I think the 6.5×284 is a GREAT goat-gun (as in mountain goats), target rifle, and boomer plinker, and I’d love to have one. But the downsides of using it as a common-issue general purpose rifle are problematic. Suppressing 62k PSI is harder than 50k PSI, ammo is a lot heavier and fatter (meaning fewer in the basic load and magazine), you get about 1/3 the accurate barrel life, etc. As I said, ANY choice is a compromise of competing priorities, and I was just putting forth one possible take on them. If the US military were to ever replace the 5.56 NATO, I’d vote for the 6.5 Grendel.

  3. Rolf; you are right as usual. As always it comes down to the role or task of the rifle. For regular infantry it is one thing. For the lone operator, or the two person sniper team it is quite another. Each has to decide. If I were in harms way with a 6.5-284, I’d want someone with an AR-15 as backup. If winning the Big Match is the goal, rebarreling once in a while, or carrying bigger cartridges, is not a huge deal. That’s why we have so many rifles and calibers (well that and marketing). And as we as individuals get older that role changes too, doesn’t it? Not long ago I would have said (did say) that I could ruck it 20 miles a day in the mountains. Been there, done it. Now; not quite so much.

  4. Funny, isn’t it, how well the olde 6.5×55 Swede holds up in comparison? The only real drawback is that it’s OAL makes it a long-action proposition.

    To that end, the Grendel is a damn good choice, (in the standard AR-16/M-16 sized action), but I’ll choose to go up to the AR-10’s mid-length winner, the .260 Remington.

    Further, I’ll chose the .260 Remington, as it’s also a fine bolt-action cartridge, rifles so chambered are catalog items. Bolties offered in 6.5 Grendel are damn few, at least for the time being.

    Ultimately, my personal best would be a true 6.5-308 Ackley Improved type design. While basically just a goosed .260 Remington, it would likely offer the ability to max out the benefits of the mid-length action, balanced with the most efficient projectile choices in a bore complimentary to the case.

    Next steps up from there get into the 6.5×284 zones, and much bigger rifles withal.

    All that said, my custom 6.5 Swede on a pristine, virgin, Commerical Zastava Mauser 98 receiver is in the works.

    Anyone know where I can score a Parker Hale other Interarms Mk.X, steel floorplated ’98 to strip as a parts donor for the bolt assembly and bottom metal? Thanks!

    Jim
    Sunk New Dawn
    Galveston, TX

  5. With the powders that are available today, I don’t see why something couldn’t be done with the old 6.5(X50)Italian round. The original load used a 140gr pill that looked a lot like the pill used in the 6.5 Swedish round – damn near an inch long. Also the case is shorter then even the 7.62X51 and smaller diameter.
    I know I fired an original round at a water soaked 24 inch oak log once and picked up the projectile on the groud ehind the log. I could have reloaded it and used it again(well except for the rifling grooves on it), heh, heh, heh

  6. Having fired something on the order of 10,000 rounds of Grendel in the last 5 years, I’d be hard put to name another cartridge that meets your criteria. The 6.8 works really well out to 400 yards, but the ballistics of that bullet start acting more like a parachute out there. There are 6mm versions that would also work, but no commercial loadings.

    The Grendel does need a good bolt action rifle complement, and Mossberg could sell a ton of MVP’s if it would slide it into that rifle.

    The Grendel really does mimic the traditional 6.5×55 pretty well, though the newer powders and bullets can make the 6.5×55 into another whole ballgame.

    However, I don’t think we shall see a government changeover until we have true caseless ammunition.

    Rolf, do you have a Grendel? If not, come shoot mine at Boomershoot next go around! Or sooner if you like.

  7. Bill – nope, no Grendel yet. I like my 6.5x55s (Ruger M77, and an old Swedish mil-surp). I’ve used it both with hand-loads and mil-surp ball. Very fun to shoot. Mostly I just thought that since Joe was gracious enough to let me post occasionally here on his blog (not having one of my own), I’d try to stick mostly fairly well-contained essays on various topics that somehow relate to the blog masthead. This is just one of those various things that rolls around in my head from time to time, and I just try to see things from a slightly different azimuth.

    The specific trigger for this particular blurb was a screenplay I’m working on, and when something comes up like “what sort of guns /cartridge do the troops use?” it seemed like a natural thing to write up, as much to formalize my own thought about what goes into the story as to make for something someone else might like to read.

    Thanks for the boomershoot offer. I’ll be in position 42 again next year, if all goes according to plan.

    EMDFL – are you referring to the 6.5×52 Carcano? Yes, looks like shooting a pencil with a 160 gr bullet in it, just the 6.5×55. With most of them, though, the rifling is rather fast for bullets under 130 gr, IIRC. And great penetration is a benefit of great sectional density.

    Jim- yes. For a battle rifle, the 260 Rem would be great, but for a lighter assault rifle, it’s still a bit hot and heavy, IMHO.

  8. Rolf,

    3 of the 4 of us that run the Precision Rifle Clinic shoot Grendels. Gene has shot the barrels out of several, as have I, Paul has a 16″ that he really likes. I usually have a couple with me at and given Boomershoot. In fact, it was the rifle that got me interested in long range. My first Boomershoot we shot a 20″ out to 700 and had success.

    Americans have been infatuated with .30 cal since the turn of the 20th century and the advent of the 30-30 and then the 30-06 and have basically ignored the superiority of smaller caliber but better sectional density and BC bullets and calibers.

    The 6.5×55, .260, 6.5×47 and 6.5 Creedmoor all fill that role very nicely in bolt action or AR10 frames.

    BUT…the Grendel gives you those great bullets in an AR15 frame, with all the attendant advantages. Sure, that smaller case can’t give you great ballistics out past 1000 yards, but its still very effective at 800-1000 in comparison to the 5.56, and actually has less wind drift with decent bullets than the .308.

    If you want a chance to shoot one before Boomershoot, drop me an email and we can try to figure out a place/time to get together. Heck, there is an F Class match in TriCities the second weekend in September, come on down and shoot my 28″ in a match! I’ll even supply ammo!

  9. Bill – I plan on being at the Friday Boomer clinic. Hopefully by September I’ll be employed, and that’s a busy time of year for a teacher, otherwise I’d be happy to take you up on the F Class match. In the mean time, if there is somewhere closer to the Seattle area, rolf at boomershoot dot org will get to me.

  10. Sounds good, I’ve never shot over there, but there may be a match or two this fall I’ll try!

    I thought I had seen a registration for the clinic with your name on it!

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