FAL stands for Fusil Automatique Legere: LIGHT Automatic Rifle. It is just about as lightweight as a rifle can be built for a full-power cartridge and work for a reasonable time without beating itself to pieces. Even then, it is NOT truly Automatic but SEMIautomatic, what the British call "SELF-LOADING".
In its original 7.92x33 cartridge, it was capable of automatic fire. When converted for test to the British 7x45 EM-2 cartridge, it was barely within limits of controllability for automatic fire. The Americans just HAD to have their 7.62x51. Converted yet again to the 7x49 Second Optimum round, the rifle was uncontrollable in automatic fire. When it appeared as a 7.62x51, more than 95% of rifles were built WITHOUT automatic-fire capability.
Yes, there WAS a full-auto variant. We called it the C2, the Aussies (who used it in combat in Viet Nam) called it the "Bang-bang-jam". It had a bipod, heavy barrel, weighed about 14 pounds, was prone to jamming and parts failure. It was too delicate for parachute operations, sometimes 20% of rifles being inoperative after hitting the ground. It was BARELY controllable and not accurate. The Americans had similar problems with their M-14/15 program, the little-known M-15 being the selective-fire, heavy-barreled version of the normal M-14, itself supposedly a selective-fire rifle which was locked onto semi-auto fire.
The M-31, on the other hand, has none of these problems. Guns today are more than 80 years old and, if they haven't been dicked with, are as reliable as rocks.
Also, consider that your FAL has a bolt which weighs about a quarter-pound. When the rifle fires, the bolt is LOCKED in place and the entire rifle takes up the recoil. The bullet passes down the barrel, some gas is bled off and operates the mechanism to reload the rifle. With the old M-31, there is a 2-pound bolt balanced on the end of a spring. The weight of the bolt keeps it closed until pressure drops to a point at which it is safe to open the breech. Simple? Yes. But it adds weight..... and reduces complication, machining, tiddy little parts that break. You get a different KIND of recoil but the weight of the gun helps to fight it.
No point going any farther. It's comparing apples to fish or something.
Get a couple of good books on firearms design and read them. For a start, I would recommend HATCHER'S NOTEBOOK, followed by George M. Chinn's 6-volume series THE MACHINE GUN. Digest that and you will have no questions..... ever. You can download all 7 over at milsurps dot com, absolutely free.
Hope this helps.
