How much SCHV is too much SCHV?

[LostCosmonaut]

There are many who feel that the 5.56 NATO is a superlative rifle round. Much has been said about larger alternatives to 5.56, such as various 6.5mm and 6.8mm rounds among others. Less has been said about smaller rounds. Off the top of my head, I can recall that there was a German 4.6x36mm round, used in the HK36, and the British 4.85x49mm round. Neither of these rounds managed to gain widespread acceptance. My knowledge of the voodoo that is ballistics is somewhat limited, so I'm uncertain as to whether these failures were caused by flaws with the rounds themselves, or because they were below some lower limit of effective bullet size, beyond which performance decreases rapidly. Could we see a resurgence of these concepts in the future, or do they represent an evolutionary dead-end?

[Sturgeon]

Daniel Watters' theory on this is that one of the limiting factors is how thin the cleaning rods can be made without being too fragile for grunts. I don't know to what degree he was joking in saying this.

Very early on during SCHV research, calibers of .18, .22, .25, and 27 were tested, and .18 was determined to be unfeasible. I'll have to take another look to recall why, exactly.

In theory, calibers smaller than .22 offer lighter weight while retaining acceptable ballistics, but micro-caliber bores do come with a host of problems. Sabot ammunition is a possible solution, though that invites issues of its own.

[Collimatrix]

In terms of accelerating the bullet, it's bore volume that matters, not barrel length.  Since bore volume is a function of the square of caliber, you're effectively cutting your barrel length way down if you go too small.

[Xlucine]

IIRC sturgeon mentioned once that the british 4.85 had the same muzzle velocity as the 5.56 of the time, since it had less of the swept volume goodness

[Sturgeon]

According to Assault Rifles and Their Ammunition, it was 55gr at 3,120 ft/s, compared to 55gr at 3,280 ft/s for M193 and 62gr at 3,100 ft/s for SS109.

[Donward]

At what point does a caliber stop becoming small and the velocity stops becoming high?

Just wondering of there was an agreed upon definition. And if not, it's the perfect chance to whing over minutiae.

[Sturgeon]

I am not sure what you're asking. Could you clarify, Donward?

Below this post, I will write up a small bit on what SCHV actually is and why it's a good thing, which may answer your question.

[Donward]

Is 6 mm still small caliber? 6.1? 6.2? 6.3? 6.4? 6.5? Nope that's GPC! Will await the write up.

[Khand-e]

Is 6 mm still small caliber? 6.1? 6.2? 6.3? 6.4? 6.5? Nope that's GPC! Will await the write up.

 

 

 

[Donward]

Yes. Yes. Yes.

But at some stage the fulcrum tipping point is reached. 45-70 Government is certainly not "SCHV".

[Khand-e]

Yes. Yes. Yes.

But at some stage the fulcrum tipping point is reached. 45-70 Government is certainly not "SCHV".

 

I'm sure it would've been very SCHV by the standards of the late 1700s if it somehow existed back then.

 

Checkmate!

[Sturgeon]

Hey guys, I appreciate enthusiasm for the topic, but let's avoid trite back and forths and image macros. We don't want this forum to become SA.

 

Khand, you're right that at one time (the 1870s), .45-70 was considered small caliber and high velocity.

[Sturgeon]

A Decidedly Brief And Altogether Inadequate History And Description of The Small Caliber High Velocity Ammunition Concept: A Treatise On Extended Nomenclature And Extraneous Verbosity For The Individual Highly Valuing The Immediate Temporal Period

(THIS ARTICLE ASSUMES FAMILIARITY WITH BASIC SMALL ARMS TERMINOLOGY ON THE PART OF THE READER)

"Small Caliber; High Velocity" is nothing more or less than what it says. Relative to the contemporary norm, ammunition and the guns that fire them should be smaller caliber, while also achieving higher velocity; doing both will increase performance while decreasing weight, recoil, and materiel usage.

Why is this desirable? In brief:

 

1. The bullet weight of a cartridge is a major driving factor in that cartridge's characteristics. A lighter bullet weight is, from the perspective of the shooter, better than a heavier bullet weight. Lighter weight bullets will induce less recoil, all things being equal, than heavier ones. The bullet of a rifle cartridge also typically makes up a third or more of the total weight of the round; reducing bullet weight is a good way to reduce the weight of ammunition.

 

2. A faster bullet produces a superior trajectory to a slower one. Less adjustment is needed for distant targets, and the error possibilities for incorrect range estimation are lower. Contrary to popular belief, 5.56mm produces an excellent trajectory for its size; in fact, I have a hard time noticing a difference between it and 7.62mm NATO until about 800 yards. Ballistic tables bear this out; along its supersonic flight range, 5.56mm has a very flat trajectory. PEO Soldier sets 5.56mm's Maximum Effective Range when fired from an M4 at 500m. Compared to that, 7.62x39 lags behind by about 120m, despite having a very similar bullet shape, sectional density, and ballistic coefficient. Note that 7.62x39 produces 80% of the velocity of 5.56 when fired from the same barrel. I created a graph to illustrate the relationship velocity and ballistic coefficient have in terms of informing trajectory. It plots the ballistic coefficient necessary to produce the same drop at 500m as a .151 G7 BC bullet fired at 2,970 ft/s. Note that below 2,700 ft/s, a tremendously high BC is needed to achieve this level of performance. In short, muzzle velocity is the overwhelmingly primary agent in producing a better unadjusted sight trajectory at normal and even extended ranges. If sight adjustment is assumed, this changes, but that is not relevant for the discussion of modern military small arms.

 

3. For a given trajectory and specific energy*, a smaller-caliber round will be lighter and smaller than a larger-caliber counterpart. This is only true within a certain bound of performance, but the performance needed from modern military rifle ammunition lies well within this bound.

4. Sectional density, a major factor in both penetration and drag, can be maintained as caliber is reduced relative to length. Sectional density is calculated, in common discussion, as mass/diameter^2. Imagine a cylinder .5" in diameter that is 1" long, made from steel. Now imagine another cylinder, also made of steel and still 1" long, but now .25" in diameter. Both cylinders have the same sectional density, but the second cylinder is one quarter the weight of the first.

*Specific energy, .5*mass*(velocity^2)/(diameter^2), is an important metric in approximating target penetration characteristics when assuming homologous bullet design.

 

Now, how is this bounded? Well, in a couple of ways:

A. In conventional rifle ammunition, the projectile diameter and bore diameter are both coupled together. Change the diameter of the bullet, and you need a wider or thinner bore to accommodate it. Because of this, you reach a limit where so much powder is being used to try to attain a higher muzzle velocity through such a small bore that the fluid flow rate is limited by the extreme necking down of the cartridge case. Therefore, for normal modern rifle ammunition, muzzle velocity does not typically exceed 4,600 ft/s. However, if bore diameter and bullet diameter are decoupled (as through a sabot), or unusually light projectiles are used, velocities at the limit of nitrocellulose's capability are possible, out to around 6,000 ft/s.

B. Military ammunition has other concerns that limit it further. One is the barrel wear of the ammunition; militaries are economical organisms, and they do not want to be throwing away barrels too quickly. Because of this, and because trajectory and weight gains are minimized beyond around 3,500 ft/s, velocity is limited. Another concern is weapon overheating: Beyond a certain temperature, rifles lose a great deal of their accuracy; thus ammunition is maintained below the heat flux threshold for a desired sustained rate of fire for the rifle. This doesn't limit velocity as much as it limits over-boredness, the ratio of the caliber squared over the chamber volume. A small caliber, high velocity round will want to be as overbore as possible, so this does limit the concept for military use.

C. Spin-stabilization only works out to a point, and that point is "about" a 7 caliber length-to-diameter ratio. For military use, the limit is closer to 5 calibers due to stability being absolutely essential from a wide variety of barrels and conditions. Remember our dieting cylinder example up there? Once it gets down below about .18" in diameter, it will begin to have problems.

Now, if one decouples the bore diameter from the projectile diameter, and drag-stabilized, instead of spin-stabilized the projectile, they would run into very different limits. Research into this area, which began in the early fifties with the SALVO project, resulted in micro-caliber finned flechettes fired at velocities up to 5,000 ft/s:

 

 

These high velocity flechette rounds weighed very little, while producing exceptional penetration and flatness of trajectory. So, they can be considered the ultimate evolution of the small caliber, high velocity concept.

 

A detrimentally brief history of SCHV:

SCHV has been around for a while. .45-70, .30-40 Army, and 5.56 NATO were all considered "small caliber" and "high velocity" for their day; all of the principles I describe above were known well before the 1870s. However, the term "small caliber, high velocity" doesn't really get used in official literature until the 1950s when experiments with .22 caliber military rifles really took off. In the context of today, small caliber high velocity refers to a cartridge optimized for low weight and a flat trajectory out to 300m, typically with a caliber under .24" and a velocity above 2,800 ft/s. However, it's important to remember that caliber does not inform performance; A 6.5mm cartridge may perform much the same as a 5.56mm one, if the velocity is high enough and the bullet weight low enough. It's better, then, not to group ammunition sharing a bullet diameter together, but rather group together cartridges that share similar performance characteristics.

I've covered the GPC before, several times on my blog. Feel free to click through these links and read that material, too:

The Case Against a General Purpose Cartridge

 

The New Caliber Mafia

The General Purpose Cartridge Revisited

[Toxn]

Since this is the place for it, I was wondering how often if the small arms equivalent of APCR has been developed. I konow that sectioned projos are a thing, but these all seem to be homogeneous rather than cored...

[Collimatrix]

The Russian 7N21 and 7N31 9x19mm cartridges have bullets that essentially amount to scaled-down APCR with a plastic wrap around a hardened steel penetrator.

[Sturgeon]

Since this is the place for it, I was wondering how often if the small arms equivalent of APCR has been developed. I konow that sectioned projos are a thing, but these all seem to be homogeneous rather than cored...

 

M995 and M993 are good examples. The biggest problem is that tungsten is too rare to use for small arms ammunition that often.

[Toxn]

TFB (read: Sturgeon) covers a modern attempt to make APCR-esque rounds (ie: very light high velocity) using pellet gun pellets.

 

Notably, high velocity seems to improve penetration, radically improve energy deposition and deformation and partially solve the transonic instability problem that pellet guns have.

[T___A]

Since flechettes have fins is there any reason SPIW, AAI's ACR, and Steyr's ACR all have rifling? The Steyr IWS 2000 also fires a flechette yet it is smooth-bore unlike other flechette firing weapon.

[D.E. Watters]

The spin imparted by rifling can help with sabot separation.

[Khand-e]

Since this is the place for it, I was wondering how often if the small arms equivalent of APCR has been developed. I konow that sectioned projos are a thing, but these all seem to be homogeneous rather than cored...

 

 

DM131 Tungsten cored AP for .300 Winchester Magnum, That white "Cup" in the back is to hold the core in place.

[Sturgeon]

 

DM131 Tungsten cored AP for .300 Winchester Magnum, That white "Cup" in the back is to hold the core in place.

 

Very similar construction to M995 and M993. Both of which, I am given to understand, still have classified penetration characteristics.