well, depending on the situation and the weapon the bursts can be pretty long (i've seen m249 gunners belt out like 20-30 round "bursts" without much care in war footage). but suppressive fire is kinda hard to replicate, and just a cone aoe attack doesn't really do it. i can think of a couple ways to help this.
- make it a burst aoe within the weapon's range (maybe at long range the burst increases in size but cover is more effective?) that lasts until the start of the gunner's next turn. anyone not in cover rolls the save, anyone in cover rolls the save if they leave cover
- as above, but instead of lasting until the start of the gunner's next turn, anyone not in cover rolls dex vs damage, anyone in cover rolls wis vs some sort of demoralize effect
i think i prefer the first one just because it's cleaner