Another way of looking at things is to review the trends during the first half of the 20th century.
In the 2nd Boer War the ranges were quite long. At Modder River the British were pinned down once they got any closer than 1000m, suffering ~450 KIA & WIA. At Colenso the British lost 10 of 12 field guns (the other 2 were eventually retrieved) to long range rifle fire. Modder River would require units with 12+ normal and 24+ long range fire. This was a war where MG were extremely rare and the Boers had relatively little but good artillery. Unlike later 20th Century wars the bulk of British casualties were due to plain rifle fire.
In the Russo-Japanese War, things were not so simple. At the siege of Port Arthur the majority of casualties were a mix of artillery with a good chunk from machine guns. Being closer to WW1 trench warfare I suspect something like 50%+ artillery, 25%+ MGs and the rest a mixture of rifle, grenade and bayonet. The other main land battles were more manoeuvre orientated with longer ranges with artillery dominant, MGs to a lesser extent than PA and rifle fire.
In WW1 the vast majority of casualties, something like 2/3, were due to artillery with MG fire accounting for most of the rest followed by rifle fire. Ranges could be as low as 50m, as long as 500m in places like the Vosges but most in the 100m to 200m range. In the Eastern Front and subsequent Russian Civil War unit density often was much less than the Western Front and ranges opened up, but 500m would be about the limit. Defensive depth usually was much less also.
In the interwar years most nations reviewed WW1 lessons. While all recognised that WW1 battle ranges were in the order of 200m, many militaries felt that WW1 was an aberration and that subsequent wars would be more fluid and long ranged. Hence, combined with enormous stocks of long ranged rifles, any move to replace existing rifles with less powerful ones died. The really physically long rifles were gradually replaced or converted to less clumsy weapons, continuing a trend from during WW1, but they were still quite long ranged.
The one development that did arise was the change in purpose of the rifle squad. In the Japanese and German army the squad became the LMG squad with the riflemen as support for the squad LMG. The Italians started WW2 with a platoon with a LMG squad and a rifle squad, the LMG squad having 2 LMG sections each with 2 LMG and the rifle squad with 2 rifle sections (no LMG), sections being closer in size to other nation's squads. The British and French on the other hand viewed the LMG as a support for the riflemen. The Soviet and US Army were closer to the British and French in doctrine while the USMC experimented with the fireteam concept in the various colonial Banana Wars.
WW2 finally solidified the lessons of WW1. While not as close ranged as the majority of trench warfare, the various combatants found 200m-300m being typical. The Germans first doubled the number of LMG in the infantry squad to 2, starting with Panzergrenadiers (in Panzer Divisions) quickly extending to Motorised Infantry (later called Panzergrenadier Division with the addition of tanks or assault guns) and by '43 to ordinary leg infantry divisions. They also introduced the first intermediate power round (7.92x33mm Kurz) that were used in the first mass produced assault rifles (StG 44).
The StG 44 using 7.92x33mm K was not the first AR using an intermediate round. The USA had introduced the Pedersen Device (
http://en.wikipedia.org/wiki/Pedersen_device) in 1918 that converted a M1903 Springfield (.30-06) to an assault-ish rifle for walking fire (ASL Assault Fire) when 'going over the top' but the arrangement though ingenious was quite impractical on the battle field and was quickly dropped post WW1. The WW2 US M1/M2 carbine was in practice more a greatly enlarged pistol substitute than an assault rifle and lacked stopping power. Prior to WW1 there had been 2 trends in rifle rounds. One was the various 7.5-8mm, the German 7.92x57mm, British .303", Russian 7.62x54mmR, French 8×51mmR, etc which were about the limits for most due to recoil and subsequent accuracy. The other were the various 6.5mm like the Japanese 6.5x50mmSR, Swedish 6.5x55mm and Italian 6.5x52mm. The latter though with not the same stopping power (and recoil) were as accurate or at least pretty close to their 7.5-8mm brethren. The Russian Fedorov Avtomat (
http://en.wikipedia.org/wiki/Fedorov_Avtomat) was designed for a new 6.5mm round but switched to using the existing Japanese 6.5x50mmSR in the world's first manageable automatic rifle that used a less than full power rifle round but greater than a pistol round. At 400m its dispersion was 1.1x0.9m. Due to its complex manufacturing and use of foreign ammunition only about 3k were manufactured until 1924.
Prior to WW1 and more especially in the interwar period various rifle round in the 6.5-7mm class were proposed. The reasons were a mix of better ballistics and the inability of most soldiers to fully utilise the full range and power of the 7.5-8mm rounds. However the inertia and economic limitations of that period meant that most countries were loath or unable to switch to lighter rounds (indeed the Japanese and Italians went or tried to go up).
The Soviets were the first in WW2 to apply the lessons of reduced practical range by their introduction of the Regimental SMG company (2 in Guards Regiments), starting in '42. By late-'43 a platoon in each rifle company might be converted to a SMG platoon, becoming more common towards the end of the war. They also introduced the SKS semi-automatic rifle using their own 7.62x39mm intermediate by war's end and that round was later used in the AK47. The Germans had accepted the reduced range and combined with the requirement to have weapons with longer range than the 9mm pistol round that was still controllable in full auto lead to their 7.92x33mm K round. While much hot air has been expended about how much the German 7.92x33mm influenced the Soviet 7.62x39mm the rifles were internally quite different and it's not like the Soviets were incapable of producing good, reliable war winning weapons.
Post WW2 nearly all nations started to move towards lower power rounds, like the British 0.280". However the US (stupidly, I think) insisted on keeping full power rifle rounds (7.62x51mm, a warmed over .30-06) despite 50+years of battlefield experience only to switch to 5.56mm in the '60s. Today there is bit of a push to move to something a bit more powerful in the 6-6.5mm range due to experience in Iraq and Afghanistan. It is not that the average engagement range has gone all that much beyond 300m, just that there are sufficient number of times when the range gets to 400-600m in the less vegetated terrain, also more firing up hill can be a problem.
With the exception of the US, nearly all other nations had, from their experience of both World Wars, come to the conclusion that an intermediate round was quite sufficient for the vast majority of cases. Though some like the British favoured a little more punch than the Soviets for example, to cater a bit better for the rarer longer ranged fight, such a switch to the 200-400m class of rifle can only have come from hard earned battle field experience. Nations do not switch weapon systems just for the fun of it. I outlined the rifle trends as evidence for the dominance of the short to intermediate range in infantry firefights that eventually overcame institutional and doctrinal inertia and economic restraints.
