Sorry for the history lesson,
But If we are talking real world, most LSW's were designed before the purpose built SAW (the minimi).
The early LSW's couldn't take the rate of fire and the barrels would heat up too quick and melt.
Later models like the Colt automatic rifle (C.A.R.) did have heavier barrels. But apart from being able to go full auto, it never actually brought anything new and useful to the party so to speak.
It still had the same range as lighter rifles, still had the same stopping power and would need to stop more for regular cleaning/maintenance than its normal counterparts.
Other LSW's like the L86 or the Galil did have longer barrels and better range, but ended up better as DMR's than LSW's.
Yet with belt feds, say the Minimi.
Same calibre as the Colt automatic rifle, but has;
- A better rate of fire
- Can touch out to around 800-1000m (C.A.R. is 600m)
- The Minimi only slightly heavier than a C.A.R. with a full C-mag. (C.A.R (with C-Mag) = 7.88kg, Minimi (with 100 rounds) = 8.5kg
- More reliable bolt system
- The barrel is specifically designed for sustained fire.
- Quick change barrel (where LSW's would have to allow for cool down)
- The para version is more compact than the C.A.R when folded.
I know I've just stated about range.
But if you put a C.A.R. up against a GPMG for range, the GPMG would piss all over it, as the GPMG has a range out to 1800m when on a tripod.
That's why GPMGs are used with tripod as perimeter defence. So you can reach out and touch them before they can get you.
Same as reloading, its easier to reload a belt fed by clipping on more link so it has a continuous flow of rounds. A LSW would have to stop and reload.
Plus the space a C-mag takes up, I could easily fit 150-200 rounds of link into that pouch.
Now as for airsoft.
LSW's of a sort isn't that frowned on. But M4's with box mags are!!!
You will find that all support weapons and genuine LSW's will have bipods (Like galils, RPD's, L86 etc). So when used, you tend to have to deploy and use it as it should. Yet a box mag on a M4 is just an excuse to spray and pray.