But having a weapon that goes longer periods without parts replacement is an advantage.
That sounds like it should be true, but it isn't. By adding a gas system regulator you are adding more parts and a potential point of failure to make the weapon last longer, yet there is no problem of the life of the weapon... in fact they already have 17 million in storage, so it is actually "lasting" too well already.
SLR like the L1A1? I have a SVT-40 and I fired a FAL, and I thought there was a decrease in recoil when I set the gas setting lower. I'll have to fire them again some time. It is a good point you illustrated how a gas regulator can add complication.
The other aspect is that in combat as the weapon gets dirty you will need to increase the gas by adjusting the regulator to keep the weapon firing properly... you can't see what the weapon looks like on the inside and you really can't guess what sort of quality ammo you might be issued with in combat so your first warning will be not enough gas pressure to properly cycle the weapon... which means the weapon fails to reload properly... after doing this a couple of times (ie failing to reload properly) the soldier will try turning up the gas regulator to solve the problem. The point is that with a fixed gas system there is no need for such stoppages and no need to adjust anything while being shot at.
I think my L1A1 is Australian... I am looking at buying a suppressor for it at some stage, it is a very powerful rifle, but it has a rounded buttstock that is nearly inline with the barrel so firing it results in more of a push than a kick in my opinion... it is very comfortable to fire, though I have never tried it in full auto.
The main problem is that its recoil spring takes up the entire butt stock. The mechanism is very similar to the SKS, though the latter does not have a butt full of recoil spring.
I haven't fired a M76, but I've heard they are just as accurate as a SVD. I think the AK system can be made just as accurate as the SVD with the technology we have today.
...that is my point... the M76 is not inaccurate, and both weapons have significant recoil during firing. The SVD is better when properly held during firing and follow up shots are easier, but the M76 is not a bad rifle in terms of accuracy and is not even heavier than the SVD, which is a very light weapon in that class.
Look at the VZ 58 with an overall length of 33.3 inches versus AK-104 32.4, not a huge difference considering the VZ 58 has a 15.4 inch barrel versus AK-104 with a barrel length of 12.4.
I would suggest your logic is a little flawed. Carbines are shorter handier models of standard rifles likely to be carried by either special forces like paratroops (ie VDV, or naval infantry) of for crew that need a more compact weapon but still need to be able to shoot accurately.
The VZ58 was designed to meet specific requirements so its overall length of 845 mm, that is 635 mm with the stock folded meets their requirements with its 390 mm barrel.
The AK-104 with a length overall of 824mm that is 586mm when folded has a barrel length of 314mm. The 5.45mm cartridge is very much like the 7.62 x 39mm round in that barrel length is not critical to its performance... unlike the 5.56mm round when velocity is everything.
The AK-104 is significantly smaller than the VZ58 when both have their stocks folded, which suggests that assuming the buttstocks are the same length that the difference is in the receiver and barrel lengths.
The requirement for the AK-104/105/102 would be folded size for compact carry where the barrel was long enough to be effective as an assault rifle... they clearly think 314mm is long enough... and I have not read of any complaints... unlike with the AKS-74U, which is actually an assault rifle calibre SMG rather than an assault rifle. At ranges of less than 200m the AKS-74U seems to be OK in terms of accuracy, though its round would be lethal to much greater ranges.