Back in the 1860’s after the Civil War the U.S. adopted the .50-70 cartridge for the trap-door conversion of the Springfield rifle. Latter they switched to the .45-70. Keep in mind that the .45-70 weighs 9.375 pounds per 100 rounds and is bulky.
Understanding Bullet Grain
Fast forward to WW1. A 100 rounds of .30-06 weighs about 5.75 pounds. A 100 rounds of 7.62 NATO is about 5.33 pounds whilst the 5.56mm NATO is 2.64 pounds. A soldier can carry twice the number of rounds of 5.56 NATO versus the WW1 and WW2 .30-06, a consideration. Loadout with the old Garand was 80 rounds though most carried more, just like today. The standard load out of 5.56 NAT0 is 300 rounds for ten 30 round magazines (actually it is 280 rounds as they only carry 28 rounds to prevent spring binding of the magazines). But most will have up to 600 rounds available in vehicles and such. For comparison the Russian 7.62x39mm M43 round for the AK series weighs in at about 3.81 pounds per 100 rounds. The .50 Browning Machine Gun ammo is about 35 pounds per 100 rounds.
Infantry Combat Range
Point two: WW1 through Korea most rifles had sights marked out to 1000 meters, though most combat took place at 300 meters or less. The Germans took notice and introduced their 7.92x33mm Kurtz to Eastern front while the Soviets took note and introduced the 7.62×39 M43 cartridge used is the SKS carbine and the AK-47 rifles. The US introduced the 5.56x45mm into the Vietnam conflict with most of the soldiers where carrying 7.62x51mm NATO in their M14 rifles. These constitute most calibers along with the Russian 5.45x39mm encountered on the modern battlefield. The Russian 7.62x54R is comparable to the .30-06.
How Much Weight in Infantry Loads Do Soldiers Carry?
The modern intermediate rounds, or so-called assault rifle cartridges are all good out to about 400 meters or so, are lighter so the loadout is more rounds. Weight when your forces are burning through about a million rounds a day is important. A lot more 5.56 NATO rounds per pound, and shipping cost depend on weight. Aluminum cases, polymer and caseless ammo are all being experimented with to save weight. Less weight, cheaper to ship more rounds carried by grunts. Infantry loads are up to around 80 pounds for some specialists, so any weight savings is appreciated by those who must hump it across the desert. Operating in the desert you need a lot of water that weighs in at 8.34 pounds per gallon. Keeping the weight below 50 pounds goes along way in keeping the individual solder efficient and effective. Ammo is a consumable. In Vietnam the western forces were burning through a million rounds a day. Over 45,000 rounds were expended per enemy killed. Ammo is one of the single most critical expenditures for a combat unit engaged in operations. You run out of ammo, well you do have your bayonet, right!
Are Full Auto Guns Practical?
In the U.S. we admire full auto guns. But at 600 rounds a minute, that equates to 10 rounds per second or about 3-5 seconds per 30 round magazine. Most GI’s state they seldom use full auto, well the M16 weapons and their derivatives have a 3 round burst limiter, so most combat entails semi-auto fire only. Given the above full auto for civilian carry, along with accuracy problems means that full auto arms, hand or long arms just aren’t practical.