Don't even nominally 30 caliber Kalashnikov type assault rifles have shorter, less powerful rounds than their WW II analogs? I'm not really a gun guy so I could be wrong about this, but it seems like firing a burst of full auto with the type of rounds used in single shot bolt action rifles would be a challenge.
I also wonder about the casualty averse nature of the "modern" sides in asymmetric warfare - you're rather safer firing at a 'technical', improvised bunker, etc. from anti-material rifle range than from LAW ranges. Might be a factor?
Yup,
BarkingMonkey, the AK in 7.62x39mm has a lower muzzle velocity (715 m/s = 2,350 ft/s) than their Tsarist era 7.62x54mmR (840 m/s = 2,755 ft/s), see physics section at end. You would need a strong man indeed to control full power power cartridges in full auto.
Yeah, you would have a range advantage with a rifle over a LAW but that could just as well be said about a tank gun or IFV auto cannon or even an artillery strike. The anti-material rifle round is more surgical, cheaper and might generate less political blowback from the local residents. Less enraged locals might save you further casualties later on.
While I have a better than knowledge than average person in an Irish street (average US street might be a different matter, bloody US gun nuts
) about arms, my knowledge is from (2nd hand) battle reports and simple physics. So I might as well outline what some of the above figures mean.
Terminology:
A cartridge is often specified like the above "7.62x54mmR". A round is both the cartridge (which contains the propellant) and bullet. The 1st number obviously the calibre, the 2nd number is the cartridge/casing length, including the overlapping bit that holds the bullet in place. There are other measurements like the angle of the bottleneck in high power cartridges that allow the round to seat properly in the chamber that manufacturers have to meet, but something like 7.62x39mm is usually enough to uniquely specify the round. The 2nd, cartridge length, gives a good idea of the powder amount and the relative power of the round.
You will sometimes see "R", "B" or "SR" after the 2 values. All cartridges have some way that allows an extractor claw to grip one side of the cartridge when after firing the bolt opens to cause the cartridge to be flipped sideways (or up or down), whether manual bolt or (semi-) automatic. No letter means a depressed ring/trough near the base for the claw to slip into, "R" or Rimmed means the base forms a raised ring that the claw catches, "SR" or Semi Rimmed means a bit of both, a depressed ring with a small raised rim at the base and "B" means Belted which involves a short but fatter section of a cartridge just forward of the extractor rim or recessed ring. Belting has more to do with reinforcing the cartridge and provide sealing against gas escape than extraction and belted rounds typically have rather hairy power.
So the 7.62x54mmR is a bottleneck cartridge (the body is fatter than the bullet) but has an protruding Rim at the base for extraction (like many common .22" varmint rounds). Rimmed rounds are fine for small magazines and bolt actions but can cause problems in large magazines and auto weapons. Compare the Czech ZB-26 (7.92x56mm) straight magazine with it's British .303" variant, the Bren, with its curved magazine for 7.7x56mmR. The rims can interfere with each other causing feed problems.
History:
Between WW1 and WW2 there was a lot of discussion about rifle rounds. From the introduction of rifles in the 1850s+ the idea was to get maximum velocity and range. That was great for colonial warfare and indeed the Boers reinforced that idea with the British when they often out-ranged them (1000m+). However the experience from WW1 showed in high intensity warfare that battle ranges were in the order of 200-400m, often much, much less. In addition mass conscript armies were mostly incapable of achieving accuracy much beyond moderate ranges. The heavy recoil didn't help either.
So between the wars there was a lot of talk of replacing the old full power rifle rounds with somewhat less powerful and more manageable rounds, especially in light automatic weapons. None of those designs got anywhere because there were enormous stocks of rifles, MGs and ammunition and most of the world was pretty well broke after WW1 and the Great Depression. The Germans in WW2 depended upon large numbers of auto weapons and wished to replace bolt rifles with semi and ideally full auto weapons. The pistol cartridges used in SMG were just too weak and normal rifle rounds caused sidearms to climb, dive or go sideways off target in auto mode. So they developed a 7.92x33mm
Kurz (= Short) intermediate round for the StG 44, to replace their 7.92x56mm standard rifle round. There is some debate as to whether the Soviet 76.2x39mm was developed in parallel or based off the German 7.92x33mm, but the Soviets were just as good as the Germans in arms design and the idea of an intermediate round had been around for decades. Basically the same weight but lower velocity bullet.
Meanwhile in NATO the US insisted on retaining a full power rifle round that emerged as the NATO 7.62x51mm, somewhat based upon the old .30-06 (7.62x63mm) round (more powerful modern propellants, less volume needed). Eventually they did go intermediate but with a smaller calibre 5.56x45mm. That resulted in a much lighter bullet but kept the velocity high, indeed MV was as good as if not higher than the WW1 era 7.62/7.92mm/.303" cartridges. The Soviets eventually went that way with their 5.45x39mm cartridge.
Light but high velocity bullets give nice flat trajectories out to 300-400m but loose velocity at range and are more easily deflected by light cover. I have read in a few places that there is some talk of going to something more like 6.5mm with muzzle velocities between the 5.56x45mm and the 7.62x39mm due to experiences in Afghanistan. That would be a quite ironic full circle as the world's first Assault Rifle was the
Federov Avtomat, a Russian 1915 design using the (Japanese)
Ariska 6.5x50mmSR round with a MV of 2,145 ft/s, 654 m/s.
Physics:
Now for a little physics. Bullet energy is (M*V^2)/2 where M is mass and V is velocity. So doubling the velocity gives 4 times the energy and the destructive effect on the receiving end. That doubling of velocity means the flight time is halved and the distance the bullet falls under gravity is quartered (drop = (g*T^2)/2 where g = earth's gravity = 9.81/M/s^2 and T is flight time). However that doubling of velocity doubles the drag force on the bullet and lighter bullets will slow quicker than heavy bullets. Recoil impulse is simpler at roughly M*V. So halving the bullet weight but doubling the MV gives the same recoil but delivers twice the energy and needs twice the propellant. That's the simplified physics but you can see there are trade offs between high MV light bullets and slower but heavier bullets and these trade offs keep designers in food, lodgings and drink.
Excuse the length but that should give you a reasonably solid basis for further exploration of the subject whether in books, Wiki or Google and may fill out other's gaps in their knowledge. Sometimes having a grasp of why some armies developed something is as important as what and how.
There are a fair few service people here in GS who will be much better able to educate you on the strengths and weakness of the various weapons that they have used, but you should have a starting point.
EDIT: I forgot one quite important advantage of intermediate rounds, whether high MV, light bullet or medium MV, heavy bullet; They are smaller and lighter so more can be carried by the soldier and shipping is easier. Duh!