Modern Cartridge Design is a collection of tenets that information how firearms and ammunition engineers have created a lot of the centerfire rifle rounds unveiled in recent times. This philosophy is without doubt one of the main drivers behind the accuracy positive aspects in as we speak’s rifles, whether or not we’re speaking about customized rigs with eye-popping worth tags or price range weapons you possibly can decide up at a big-box retailer.
Cartridges that Embody Modern Cartridge Design
- Are optimized for aero-ballistically environment friendly, heavy-for-caliber, excessive BC bullets
- Use quicker twist charges to stabilize these projectiles
- Propel these bullets at comparatively average velocities
- Have circumstances with satisfactory neck size to help these projectiles
- Can seat these bullets to specified cartridge size with out impinging on the powder column
- Employ minimal physique taper
- Headspace off sharp shoulders
- Have cosy throats with a standardized leade geometry
While not each cartridge unveiled within the final decade or two adheres to all these factors, many do. And any spherical meant for precision taking pictures and long-range looking checks all—or practically all—these bins.
Good examples of cartridges following the Modern Cartridge Design (MCD) blueprint embody: 6.5×47 Lapua, the 6mm and 6.5mm Creedmoor, the PRC household (6.5 PRC, 7 PRC, .300 PRC), 6.8 Western, 6mm GT, and 6mm Dasher.
Why Modern Cartridge Design Works
Maybe the easiest way to consider MCD is that it employs quite a few finest practices—lots of them derived from benchrest competitors–to tip the chances within the cartridge’s favor. Individually, every of those tenets may not transfer the needle very a lot when it comes to efficiency, however mixed collectively they end in cartridges which have set new benchmarks when it comes to accuracy and sensible efficiency in long-range competitors and within the area.
Cartridge Alignment
One key to accuracy is consistency. The straighter you possibly can align a cartridge in a barrel’s chamber, the higher. This is the explanation MCD cartridges decrease physique taper, and headspace off sharp shoulders. Both assist the cartridge to take a seat centered within the chamber and parallel to the bore.
In the previous, many rifle cartridges have been designed with vital taper and shallow shoulder angles to assist them feed and extract easily.
This method is especially evident with rounds just like the .30/30, however you additionally see it with the .30/06 and .308 Winchester, which integrated physique taper and shallower shoulder angles to work higher within the semi-auto and totally automated weapons of the day. And, in fact, each the .308 and ought-six spawned quite a few cartridges that exhibit these traits too.
With a number of taper and shallow shoulders on a cartridge you’re extra liable to getting tolerance slop and a case that doesn’t lie completely within the chamber.
The steep shoulders present in MCD (30 levels or extra) additionally assist set up exact, constant headspace, which is the gap from the bolt face and the purpose of contact between the case and the chamber. (For circumstances that headspace off the shoulder, the gap is often measured from the midpoint of the shoulder.) Cartridges that headspace off a shallow shoulder just like the .30/06 (17.5 levels) and .308 Win. (20 levels) usually are not as straightforward to get to headspace precisely. The identical is true with belted cartridges just like the 7mm Rem. Mag. and .300 Win. Mag. that headspace off a raised rim on the base the case. (Though, in recognition of this shortcoming, some fashionable chambers for these magnums are lower so the spherical headspaces off the shoulder.)
Sharper shoulders additionally profit handloaders, because the circumstances received’t stretch as rapidly lowering the necessity for case trimming.
Interior Ballistics
The chain of occasions that happens when taking pictures a rifle is surprisingly advanced. Rather than element the entire course of, we’ll decide issues up because the firing pin strikes the primer and discuss the way it pertains to Modern Cartridge Design.
When the primer touches off it sends a jet of sizzling slag into the propellant mattress that ignites the powder. The gasses launched by the powder are what push the bullet out of the case and into the barrel.
For peak accuracy we wish this occasion to be as constant as attainable. If the bottom of our bullet extends previous the cartridge neck and into the physique of the case a pair unfavorable issues can happen. One, the bottom of the bullet may be pushed sideways because the gasses increase inflicting it to enter the rifling at a tilt. And as soon as it begins within the rifling at a tilt, it stays tilted because it spirals down the bore. There’s even proof suggesting a bullet may be bent if sufficient of it protrudes beneath the juncture the place the case neck and physique meet. Needless to say, neither situation is conducive to accuracy.
This turns into a difficulty when attempting to load lengthy, excessive BC bullets in circumstances designed to drive lighter bullets as quick as attainable by maximizing powder capability. In order for the cartridge to slot in the journal and within the chamber, these excessive BC bullets have to be seated far again with a portion of the bullet within the powder chamber.
Take the 28 Nosler, as an example. This 7mm behemoth launched in 2016 does a very good job with 150- to 160-grain bullets, spitting them out round 3300 fps. But with bullets weighing 175 grains or extra you run into the difficulty described above. Shooters both must throat out their chambers to accommodate the longer bullets (and probably find yourself needing to single-load the rounds) or seat these bullets means again within the case. This design shortcoming is one purpose you don’t see the 28 Nosler utilized in long-range competitors in any significant numbers.
The different downside with seating these lengthy smooth bullets far again within the case is that the neck may not grip the bullet in the very best place. The bearing floor on these bullets may not be totally supported inside the case neck, which additionally isn’t ideally suited.
This is why the 7mm PRC, as an example, has a notably shorter physique than the 28 Nosler—in order that it might use bullets just like the 175-grain ELD-X and ABLR, the 180-grain ELD-M, the 190-grain A-Tip, and the 195-grain Berger EOL, and seat them appropriately so they’re supported by the case neck and don’t have the bottom of the bullet extending into the case physique.
Short, Fat Efficiency
We’ve additionally discovered that brief, fats circumstances—which actually gained traction with the rollout of the brief mags within the early 2000s by Winchester and Remington—are very environment friendly. If you are taking a long-skinny case and a short-fat case with the identical quantity, the short-fat case will ship quicker muzzle velocities with the identical powder cost.
This is one more reason MCD cartridges are thicker across the center in proportion to their size than classic case designs.
Modern Cartridge Design Throat Geometry
One small however vital tweak within the evolution of cartridge design is throat geometry. (The throat is the unrifled part of the bore that surrounds the bullet within the chamber.) MCD cartridges have throats which might be simply barely bigger than the bullet diameter. This promotes good alignment between the bullet and bore because the projectile enters the rifling—serving to make sure that the bullet is straight because it begins its journey down the barrel.
Many older cartridges have throats considerably bigger than the bullet diameter, and that slop could cause accuracy points.
Putting some numbers to this, MCD cartridges spec their throat diameters at lower than .001 inch over bullet diameter, typically .0005 to .0006 inches—so half a thousandth of an inch or so. This contains the 6mm Creedmoor (.0005 in.), 6.5 Creedmoor (.0005 in.), 6.5 PRC (.0005 in.), 6.5-284 Norma (.0006 in.), 6mm GT (.0006 in.), 7 PRC (.0006 in.), .300 Norma Mag. (.0007 in.), and .300 PRC (.0008 in).
Contrast that with the SAMMI spec throats for these cartridges: .243 Win. (.0033 in.), .264 Win. Mag. (.004 in.), 6.5×55 Swede (.0053 in.), .308 Win. (.002 in.), .30/06 (.0026 in.), and .300 Win Mag. (.007 in.) Comparatively, placing a bullet down these throats is like throwing a hotdog down a hallway. Of course, you may get customized reamers for these outdated cartridges to make barrels with tighter throats—and might lengthen the throats to seat longer bullets and use quicker twist charges on the rifling to make them extra MCD-like. But that does contain a variety of effort and might result in different points, reminiscent of issue getting the rounds to slot in the magazines they have been initially designed for.
Standardized Leade Angle
The leade is the the place the throat transitions into the rifling and is machined at an angle. The normal with MCD cartridges is 1.5 levels with a pair exceptions. This can also be expressed as 1° 30′ (one diploma, thirty minutes, since there are 60 minutes in a level).
Many older cartridges have the identical 1.5-degree leade angle, such because the .243 Win., whereas others are steeper (7mm Rem. Mag., 3 levels) and others are shallower (.30/06, 1° 22′).
How a lot of a distinction leade angle makes when it comes to measurable precision is debatable. But it’s value noting, because the 1.5-degree leade has change into practically common with MCD rounds.
High BC Bullets
Whether speaking about big-game looking or long-range goal taking pictures, we’ve seen a marked shift towards bullets which might be longer, sleeker, and usually heavier than these used up to now.
These bullets are outlined by their excessive ballistic coefficients (BCs) which make them extremely environment friendly downrange. Though they won’t begin on the identical blistering velocities because the lighter bullets utilized in conventional magnum rounds, they typically catch as much as these projectiles downrange as a result of they do a greater job retaining their velocity.
This additionally makes these bullets extra wind resistant than the lighter, quicker bullets, which is a large benefit when taking pictures below sensible area situations.
Take, as an example, my Allterra Arms 6.5 PRC that shoots Hornady 143-grain ELD-X bullets at 2950 fps. Even although this lighter bullet begins on the identical velocity as a 180-grain Nosler Accubond from a .300 Win. Mag.—and due to this fact has a a lot decrease muzzle power—by the point you get to 600 yards the 143 ELD-X has dropped 65 inches versus 76 inches for the 180-grain bullet. More spectacular, the retained velocity and power on the 143 ELD-X at 600 yards (2202 fps / 1539 ft-lb) is best than the 180-grainer (1937 fps / 1500 ft-lb). Go to 800 yards, and the 6.5 PRC has left the .300 Win. Mag. within the mud: 143-grain drop, velocity, and power (137 in. / 1967 fps / 1229 ft-lb) versus the 180-grainer (163 inches / 1656 fps / 1096 ft-lb).
Best of all, you get this downrange efficiency at a fraction of the recoil, which is why each bullet measurement (that means caliber diameter) and muzzle velocity aren’t the easiest way to gauge cartridge efficiency.
Faster Twist Rates
To stabilize these excessive BC bullets, MCD cartridges are spec-ed with quick twist charges. Whereas the .243 Win. has a manufacturing facility 1:10 twist, the 6mm Creed and 6mm GT have 1:7.5 twists to stabilize bullets as much as 115-grains. In the 6.5mms the distinction is much less pronounced. The .264 Win. comes with a 1:9 twist (as does the 6.5-284 Norma), whereas the 6.5 Creed, 6.5 PRC, and the vast majority of 6.5×55 Swedes are 1:8.
The 7mm Rem. Mag. has a 1:9.5 twist whereas the 7 PRC is a 1:8. The outdated .30-cals, just like the .30/06 and .300 Win. Mag. and most .308s are 1:10, whereas the .300 PRC is a 1:8.5 and the .300 Norma Mag. is a 1:9.2.
Running a quicker twist charge permits for a cartridge to make use of a greater variety of excessive BC bullets. It may also imply the distinction between a given bullet working properly in a single cartridge/twist mixture, whereas delivering poor ends in one other.
A possible draw back to quicker twist charges is that some older bullet designs may be pressured when spun too quick. That can result in poor terminal efficiency and in excessive circumstances have the bullet come aside in flight, particularly when shot at excessive muzzle velocities.
Velocity as Performance Fallacy
For the longest time shooters and hunters have worshiped on the altar of velocity. It was the principle—and sometimes solely—yardstick by which we measured cartridge efficiency earlier than the arrival of as we speak’s aero-ballistically environment friendly excessive BC bullets.
Faster equaled higher. So, if a .308 Win. taking pictures a 165-grain bullet at 2680 fps didn’t blow your hair again, you can ship it out of a .30/06 at 2800 fps, or a .300 Win. Mag. at 3100 fps, or a .300 Weatherby Mag. at 3350 fps, or a .300 Rem. Ultra Mag. at 3400 fps. But does the .300 RUM actually “outperform” the .308 Winchester? Rifle and ammunition gross sales don’t help that assertion, however many hunters and shooters nonetheless revert to this mindset when debating the deserves of outdated cartridges versus the brand new.
This perspective has its origins within the pre-laser rangefinder period of looking and taking pictures, the place hunters typically needed to guesstimate ranges to their targets. A cartridge that shot a lightweight or medium weight bullet actually quick was useful for taking pictures to 300 or 400 yards since at these ranges they have been fairly forgiving with vary estimation.
Many hunters nonetheless assume this fashion and demand their basic cartridge outperforms MCD cartridges based mostly on muzzle velocity alone. By this logic, the very best racecar on the planet is a prime gas dragster since nothing is quicker—however anybody with a lick of sense is aware of there’s extra to efficiency than velocity, whether or not we’re speaking vehicles or cartridges. Handling, recoil, consistency, wind forgiveness, retained downrange efficiency, and precision all matter too—and that is the place Modern Cartridge Design has the sting.
We Don’t Make ’Em Like We Used To
Bring up the oh-so-trendy 6.5 Creedmoor, as an example, and also you’ll set off the 6.5×55 Swede and .264 Win. Mag. followers who will make the velocity-is-king argument. It’s like dangling a worm on a hook in a pond filled with ravenous bluegills. They can’t assist however take the bait.
Now don’t get me fallacious. The Swede and the outdated .264 are each nice rounds and greater than succesful for many looking and taking pictures duties. But if neither existed nobody would design them as we speak. The identical holds true for all our legacy cartridges. If we have been ranging from scratch with what we learn about cartridge design, we’d by no means find yourself with the .30/06, .270 Win., 7mm Rem. Mag., .300 Win. Mag. or another of our iconic cartridges.
Just like there’s no evaluating earlier era Ford F-150s to these constructed as we speak, it’s foolish to say the outdated cartridges are nearly as good or higher than what we’re making now. We know much more about how one can construct good vans and cartridges within the 2020s than we did within the Fifties (or 60s or 70s).
And as essential, by way of the revolution in CNC machining we’re in a position to maintain actions, barrels, bullets, brass, and ammunition to tighter tolerances than gun and ammo makers a era or two in the past might even dream about. These developments make the tenets of Modern Cartridge Design achievable in a means they weren’t in a long time previous.
The Value of Moderate Velocities
When it involves muzzle velocity it’s straightforward to have an excessive amount of of a very good factor. The instance above evaluating the 165-grain bullet from a .308 Win. (2680 fps) to a .300 RUM (3400 fps) illustrates the purpose. The historic accuracy of the .308 Win. is well-known. No one has ever accused the .300 RUM of that high quality. The recoil and muzzle blast generated by the RUM compounded with the challenges of getting a cartridge designed for speed-at-all-costs to shoot persistently makes it a nonstarter within the precision dialog. (To say nothing of its old-school 1:10 twist, which precludes it from utilizing a few of the simplest excessive BC .30-caliber match bullets.)
We’ve discovered that there’s a candy spot on the subject of muzzle velocity (MV) because it pertains to precision. When we drive excessive BC match and looking bullets in MCD cartridges someplace between 2700 and 3050 fps, actually good issues occur downrange.
Not solely will we get persistently tight teams, whether or not we measure them at 100 yards or 600, we additionally get constant muzzle velocities, represented by small shot-to-shot normal deviations (SDs) and excessive spreads (ESs).
Controlling the Waterline
The purpose we care about low SDs and ESs is that it helps us management bullet drop at longer ranges. When we plug information into ballistic calculators, we use a particular muzzle velocity and the nearer to that muzzle velocity our pictures are in the actual world, the higher that calculation will probably be. When our MVs are constant, we are saying the rifle and cargo have a very good waterline, that means our elevation corrections at longer ranges will probably be spot on.
For taking pictures at average distances, say inside 600 yards, we are able to tolerate a good bit of variation in MV from shot to shot. But past that inconsistent MVs can flip hits into misses.
Improved Marksmanship
Moderate MVs additionally equate to average recoil. The advantages of a spherical with average recoil on the subject of marksmanship are tough to overstate. They assist the shooter preserve appropriate type, enhance the power to name pictures—which is without doubt one of the most crucial expertise for any rifleman to develop—and all issues being equal, rounds that generate much less recoil are inherently extra correct than hard-kicking cartridges.
What About Killing Power?
Terminal ballistics is commonly misunderstood, largely as a result of when a bullet strikes an animal it’s an inherently chaotic act that doesn’t lend itself to black-and-white conclusions. Velocity actually performs a job in how efficient a spherical is on sport, however extra essential is bullet choice and shot placement.
Every looking bullet on the market has a efficiency envelope outlined by its minimal and most affect velocities. If it hits the goal going too slowly (reminiscent of at excessive ranges) it would fail to mushroom and pencil by way of. If it hits it going too quick, the bullet can grenade on contact, leaving a horrible floor wound that fails to achieve the animal’s vitals.
But when used inside its efficiency envelope—and positioned correctly on the animal—a high quality looking bullet will upset and penetrate deeply, damaging tissue and leading to a fast, moral kill.
Historically, hunters favored to level to kinetic power as a proxy for terminal ballistics. How many instances have we heard {that a} big-game animal like a deer “requires” 1,000 ft.-lb. of power for a clear kill? While kinetic power performs a job with terminal outcomes, this emphasis is misguided.
A greater means to consider terminal ballistics is because the interaction amongst bullet design, affect velocity, and density of the goal media.
We’re blessed to stay in an period with a mess of wonderful looking projectiles. And a few of the most spectacular bullets are the excessive BC bullets present in MCD cartridges. Good examples embody Nosler’s Accubond Long Range, Hornady’s ELD-X, and Federal’s Terminal Ascent.
Final Thoughts on Modern Cartridge Design
There’s a variety of misinformation about MCD cartridges. The detractors dislike them as a result of they really feel they’re overhyped and “don’t do anything better” than older designs. While they’ve a degree concerning the hype—a few of the exhortations by their fanboys is excessive—the second level is simply fallacious.
MCD cartridges do loads of issues higher than older designs. All you need to do is have a look at what’s being utilized in—and successful—hunting-style and different long-range competitions. MCD rounds dominate.
When it involves how efficient MCD versus old-school rounds are when looking, the case is much less clear lower. For the vast majority of big-game looking, our older conventional cartridges work simply in addition to the most recent rounds being trotted out, and vice versa. MCD cartridges are simply as efficient because the older rounds.
That stated, there are eventualities the place the inherent accuracy, improved shootability, and superior downrange efficiency of MCD cartridges outstrips what may be achieved with older designs. Whether that’s related or fascinating to a specific hunter or shooter may be very a lot a person choice. But to disclaim the advantages of Modern Cartridge Design flies within the face of actuality.