The SR5 rulebooks says that suppression fire involves firing 20 bullets in a pass. Does that mean that only firearms capable of full auto can lay suppressive fire? Or is the intent that burst-fire weapons (e.g., machine pistols and most SMGs) can also provide suppressive fire?

Suppressive Fire requires Full Auto Capabilities.

As such, machine guns are the way to go. Sure you have assault rifles but most run out of ammo by 3 suppressions, but belt fed mg? Rock N Roll!

i'm not sure i consider running out of ammo after 3 full rounds of suppression is really that big of a concern for a shadowrunner team. definitely a concern for full scale war scenarios, but that's not what you hire shadowrunners for most of the time.

Meh, just need a Bigger Clip

this is one of those things where the game mechanics and the reality sort of don't mesh well. in theory, i have no problem with someone performing suppressive fire with anything that's at least semiautomatic. it's quite possible to throw the whole clip of a glock or AR-15 downrange in a few seconds by hammering the trigger, though your accuracy is obviously shit.

the real advantage of machine guns for this purpose is the higher ammo capacity and heavier barrel, which works to reduce recoil and keep the gun from unpleasantly warping the barrel after a few hundred rounds of extended fire. unfortunately, it's hard to emulate this in shadowrun except possibly by either some type of bonus to suppressive fire from true machine guns and/or mercilessly enforcing glitch=jam or even worse problem on non-machineguns that are firing suppressively. i would go so far as to say glitch=critical glitch for SA suppressive fire. one could also reduce the number of ones needed for glitches on non-machine guns.

Advanced rules allowed for suppressive fire with other firing types in SR4, I'd expect the same in SR5.

We will just have to wait a year or two to get them all.