Almost since introduction the 5.56×45 NATO has exhibited the qualities of Schrodinger’s Cat, simultaneously being an ineffective battle round while simultaneously being terribly effective to the point where people keep trying to ban it for civilian use. Thousands upon thousands of words, and hours of argument, for both sides have been spent on the subject.
The 5.56 naysayers routinely pull out ballistic tables and show all sorts of lovely numbers about energy in foot pounds remaining at range: http://usacac.army.mil/CAC2/MilitaryReview/Archives/English/MilitaryReview_20120831_art004.pdf and it should be noted that this article makes the “post hoc” fallacy that the 5.56×45 is inadequate because the M14 EBR program was used. The author makes no analysis of what the mix ratio was, and how those rifles were employed, and other tactical considerations. In Afghanistan the SEALs of Seal Team 10 often considered the 7.62×51 inadequate as a sniper rifle platform since they had the 300 Win Mag…
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History and War 
The best solution is having a bullpup in .243 WSSM (Winchester Super Short Magnum) with 64-65cm barrel. It has a little less muzzle energy than .308 but its greater velocity and aerodynamics makes it a great 1km varmint caliber (5.56 is a 300m varmint caliber) : there are less energy/velocity losses, in the end, it’s even more accurate than .300WM.
.243WSSM was studied to allow serious hunting from an AR-15 carbine. Parent case is .300winch-mag. The way powder burns and the smaller caliber makes recoil to be on par to an AK-47.
The only drawback is, although it fits within STANAG magazines, diameter being bigger, consider numbers of ammos will be reduced to two 3rds/magazine BUT as there are extended STANAG magazines, well, a 60rd casket mag will have around 40 .243WSSM rounds.
As there are also huge double-snail magazines around, well, a 64-65cm barrel on a Tavor or X-95 would make it around 90cm long, then you have an assault rifle, a sniper and a “Rambo” medium machine gun for anyone in the platoon.
Well, it’s clear you may not carry as much round as 5.56 but with 5.56, many train a lot to control 2 rounds burst or use 3rd ‘programmed’ burst as, in the distance, calibre is too weak. a .243WSSM puts down a 350kg wapiti @+400m with a single round. Muzle velocity is about +3kJ. It might be feasible to re-design micro-gun and so get a serious Gatling while being much lighter than mini-gun.
Now, there is also a pretty interesting calibre to consider where many have to carry a short barrelled M4 as a PDW (personal defence weapon) while things like H&K MP-7 or FN P90 end not being so great. No surprise : a hot-loaded .22winchester-magnum rimfire performs as well as FN5.7×28 and better than H&K 4.6×30. 5.7×28 overall length is 40.5mm. Now remember what was the WW2 PDW? US-M1 Carbine! And .30carbine OAL was 41.91mm and had a 1300J muzzle energy. Surestrike created a serie of wildcat calibres from the .30carbine. What seems to me to be the most interesting would be .25Garin :1350J is pretty serious : energy is on par with Russian 5.45×39.
Re-utilising the US-M2 mechanism in a MP-7 form factor would be feasible thus, although some study may be needed, the ST-Kinetics CPW being a multi-calibre modular platform may help a lot :
http://modernfirearms.net/smg/sing/stk-cpw-e.html
Now, go for 50 rounds casket magazines, polygonal riffling then you have something as potent if not more as an AK74U. Being modular, a 40 or 46cm barrel can also be considered.
Now, let’s take the .30carbine wildcat caliber one step further. There are 220 and 250gr .300winch mag bullets. Let’s consider a fitting to a .30carbine case with the goal of no more than 320m/s velocity. Knetic energy will be on par with .357magnum with better aerodynamics. We have a perfect client for silencer job.
We can also point that AMT AutoMag-III pistol used to shoot .30carbine.
When you consider how many secondary personnel has to carry a short barrelled not so efficient carbine or a pistol (and you need very high training to be effective with a pistol). In the west, pilots, tank/truck drivers have nothing practical too but there may be serious LE applications : SWATs are not happy with P90, French DGSE had a very bad surprise using silenced MP7 in a hostage liberation case in Somalia (subsonic 4.6×30 has less energy than .22LR!), 2 SOFs and hostages ended killed. Many police forces still rely on much less potent and pretty heavy MP5 or even Berreta M-12. Here there’d be a 1.5kg PDW that can be carried on thigh-holster as a big pistol and may easily be switched to a full carbine or a silenced weapon with as much kick as you may have from a VSS Vintorez.
Some studies could also be considered : I’ve red an article comparing .408CheyTac to .50BMG? .408 may have less energy, it has also longest range, higher accuracy and retains better its energy during flight. .50 riffles are really heavy and will ruin thermal/night vision sights and other system. There are 7.7kg .408 riffles and hit holds the world record of longest target hit.
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Have you considered the .270 Winchester ? BlacktailDefense praised it on an old Youtube video. To quote him:
“Speaking of intermediate cartridges, the one I”m rooting for is a caliber that isn’t even being considered by the authorities — .270 Winchester. It’s such an incredibly efficient round, it’s even more powerful on average than 7.62x51mm NATO, but STILL has less recoil! Consider how the lethality of the 270win compares to an M855 Ball (5.56x45mm), an M43 Ball (7.62x39mm), and an M80 Ball (7.62x51mm) when you apply the Taylor Knock Out (“TKO”, for short) value to all 4 of them;
62 x 3025 x .22 / 7000 = 4.67
123 x 2400 x .311 / 7000 = 13.11
146 x 2640 x .308 / 7000 = 16.95
130 x 3060 x .277 / 7000 = 15.74″
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You still end with cumbersome rifles to withstand the power. Recoil is lessened due to smaller caliber. Cartridge is even longer than .308 (OAL = 84.8mm vs 71.12mm), base diameter is near 12mm. One of the goals of 5.56 was also to carry more rounds. With .270, you’ll carry even less than using .308
.243WSSM has much more potential : the 6mm calibers like these are the real thing in 1km competitions, superior aerodynamics of such modern bullets allow also to have less drop during the flight and retain much better energy, thus, although mule energy being lower, the terminal one ends being higher.
Moreover, .243WSSM case being of a little more diameter (14.1mm) also helps a better burn of powder but the main point is its OAL of 60mm.
This round was created to fit in an AR-15 (M-16/M-4), thus, is compatible with all the western assault-rifles and all STANAG magazines.
For sure, you’re not likely to put more than 20-21 rounds in a mag for 30x 5.56NATO, but there are numerous extended magazines on market, including double-drums.
A Tavor or X-95 wouldn’t be longer than a mere 90-92cm if fit with a 64-65cm barrel, the kind you’d have on some long range hunting riffle and, even without bullpups and extended barrels, all standard assault rifles in 5.56 can be modified for .243WSSM, thus I maintain that bullpup is really the best in such case as you end with capabilities over a .308 sniper rifle in a very compact AR while even converting a .308 in .270winch wouldn’t be possible and it’s pretty unlikely to be used in light carbines while, with .243WSSM, your light carbine ends being as powerful as a M60 machine-gun too, even, again, if muzzle energy’s a bit under.
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