Small Arms Design Proposal

 Let me just start by saying that I may not have the perfect answer, but it is the best one I can think of at this time. Throughout much of military history, armies have been plagued with the logistical nightmare that comes from trying to equip (and feed) as many soldiers as possible in a streamlined and straightforward manner. The Romans did a very fine job of standardizing training and equipment to maximize the effect on the battlefield. Their swords were not of an unwieldy length, and the instinct to hack and slash was drilled out of the fighters as strictly as possible. Because each soldier used the same equipment, rather than bringing their own weapons to war, it insured any man could grab weapons or supplies from a fallen comrade and be utterly proficient with them. With the advent of muskets, it was soon realized that all infantry weapons should be of a common caliber, with each nation often using a slightly different sized ammunition. The realizaton that captured musket balls were not always inherently useful, the way arrows might be, did affect a very few battles.
 In regards to the current US (and NATO) armory, one sees a relatively few cartridges of various pedigree and power. Though the list of stocked calibers has been pared to a useful minimum, each retains a seperate origin and specific set of measurements that makes parts commonality between weapon sets a practical impossibility. For the purposes of this essay, I will address the three most common NATO cartridges; the 5.56x45mm and 7.62x51mm rifle, and 9x19mm handgun rounds. The .50 caliber machine gun round is not commonly found in individual arms, and then, only in specialized sniper weapons.
5.56x45mm: Adopted by the US for the M-16 series of rifles, this round was developed from varmint rounds in .22 caliber. Often disputed as too weak for deer-sized game, it was still recognized as a lighter and handier round when recoil and magazine capacity were considered. It shares a similar rim diameter and case taper with the 9mm pistol cartridge, although not close enough to make a simple barrel change the easiest solution for cross-caliber weapons.
9x19mm: Originally created as a less costly alternative to the 7.65mm Parabellum, due to the case bottlenecking step being removed from the manufacturing process. This cartridge has become the most widespread pistol and submachine gun round in the world. While often displaying less than satisfactory lethality, it does have a lower recoil impulse that is manageable by just about every shooter on the planet. Rim diameter of this case is sufficiently similar to the 5.56 that some assault rifles have been re-chambered as submachine guns, though gas operation is usually bypassed for the smaller pistol round.
7.62x51mm: A shorter and slightly less powerful evolution of the 30-06 battle cartridge, this round was pressed into NATO service with heavy pressure from the US. Used for medium machine guns, and sniper rifles, it provides a decent range and knockdown ability. The recoil and muzzle blast are often considered too great for a burst-fire or assault weapon, but the larger bullet does have a much better reputation for lethality and penetration.
 The weapons used to fire the various ammunition are even more myriad, with several different styles of operation found among the auto-loaders.
Gas operated weapons are generally preferred, although these may use a gas-impingement tube (M-16), or a piston (FAL) to drive the bolt back.
Gas pistons are considered more rugged, and tend to show an increased tolerance for debris and grit; these are things found in abundance on any battlefield. Direct gas impingement is actually cleaner, but does make tighter tolerances necessary. Ammunition may be fed from a magazine (internal or external, rotary or box-type), or from linked belts (whole or disintegrating)
Firing may be from an open or closed bolt; with closed bolts being more accurate, but at a higher chance of overheating. Following, is a proposed list of cartridges and firearms, with special consideration toward parts and accessories being somewhat interchangeable.
 Pistols have been a staple of the battlefield since shortly after the US Civil war, usually as an answer to arming Officers and other non-infantry troops. In recent years, there has been growing interest in the PDW, or Personal Defense Weapon. A PDW offers greater accuracy and power than a pistol, with an ideally negligible increase in size and weight. Several different companies have come up with designs, usually with a proprietary round. I cannot readily endorse pistols as battlefield issue, but they are appropriate for police use. A pistol cartridge having a rim diameter of 10.5mm and case length of 21mm, with slight taper and a moderate neck would push a 9mm bullet at velocities that allow good terminal wounding effects. The recoil level from such a round would vary, as the case allows a wide range of powder charges and bullet weights to be used. An unnecked case using a 10mm bullet would be comparable to the .40S&W, which derives much of its performance from high chamber pressures in comparison to the 9x19mm. Lower chamber pressures are generally desirable if one wishes to reload ammunition cases.
 For a PDW or tactical assault weapon, I propose a 6.5mm caliber, using the same case rim and taper as the aforementioned pistol cartridge.
The case length is increased to 32-35mm, allowing a greater range and power level to be achieved. The common taper and case rim means that many submachine gun designs could be chambered for either cartridge by swapping out magazine, barrel, and possibly a spring. Bullet selection could vary as widely as a saboted penetrating round, to a full-caliber frangible bullet. From an 8-10" barrel, the muzzle flash would be relatively low, while giving enough velocity to remain effective to 300 meters.
 Assault rifles and light machine-guns need enough power to reach at least 500 meters, but a light ammo load and high magazine capacity give the infantryman a chance to be more effective without immediate logistic support available. Following the above cartridge dimensions, but stretching the case to a full 45mm means that a 6.5mm bullet would be very well-utilized. The bullet diameter is sufficient to provide an incendiary or tracer payload, and full-caliber bullets would carry the necessary energy to reach past the effective limits of unmagnified optics. Assault weapon designs rebarreled and with a magazine adapter could very easily become tactical platforms for either of the two smaller cartridges. Scopes or enhanced optics would allow squad-level marksmen to engage targets when regular sniper support is not available, while still using a common and effective ammo load. This doctrine is already in use by some military units, although the US main battle round is woefully underperforming.
 Further stretching the dimensions and taper angle gives us enough room to use a 55-60mm case in a caliber of 7.5-8mm. A 12mm rim would give better case capacity with a shorter overall length, but would only be logistically efficient if two or more cartridges are using this same bolt size. For simplicity, I suggest a 55mm case pushing a 7.5mm bullet. This allows easy scaling of machine guns and battle rifles to use common bolt heads with other weapons, making field repairs much more possible.
 So, now that we have our basic cartridges, let's get a basic idea what we shall make to fire them from.
9x21mm:Standard duty pistols would have 12-15 rounds available, with carbines and sub-guns more likely using extended magazines of ~25rds. Helical or drum magazines increase capacity beyond 50, but these are usually specific to one weapon design. (Calico and PPSh come to mind)
6.5x35mm:Most likely too long for conventional pistol designs, this would be limited to sub-guns and PDWs. Due to the common taper and rim diameter, any weapon in this category could use a magazine adapter and barrel change to become a 9x21. Magazines would have to differ only in front-back depth. Suppressed carbines would be very effective in either chambering.
6.5x45mm:Slightly more case capacity and overall length than the current 5.56 NATO round, very useful with a modular AR-type weapon. There is at least one company making an AR receiver with a swappable magazine well. This allows the same trigger group and buttstock to be used with multiple calibers. Current SAW doctrine applied, so that the machine gun can run standard magazines if linked ammo runs out.
7.5x55mm:Switch in a larger magazine well and heavier bolt carrier to let any assault rifle or machine gun become a heavy-hitter. Same magazine width and bolt face means that every member of the squad can help the machine gun keep running. If barrel outer diameters are the same, A machine gun could even drop down to a smaller caliber if barrels or ammo supplies fail.