So I turn a corner and there's 5 guys with shotguns about 30 meters down this 3 meter wide underground hall. They're in a pretty close formation, definitely within 10 meters of each other.
The hall is empty, no cover

I get the jump on them and want them to not fill me with lead, I activate suppression fire.

What happens?
Do they instantly fall prone (reacting out of order)
Do they get to shoot at me with impunity while lying on the floor?
Is there any chance of them getting hit?

We managed to resolve this as a dodge action for all of them then a free move to try and crawl away, but I'm pretty sure we went with that because the book didn't seem to give a clear path.

How would/do you resolve it in your games?

As I read it, first you roll to see if you hit them.

So Agility+gun skill= 4 hits.
Goons roll reaction+edge(+dodge if on full defense)=3 hits.

You hit with 1 net success. Zippy. They take weapon Base DV. If A SMG I think it is 4DV, a assault rifle is 6DV. This is not modified by net hits. That is your attack. If they went full defense I'd assume they dropped prone but it isn't required of them which puts them out of fire of suppression. If they did not they are just standing there eating lead.

So in your example the dodge action, dropping prone and free action to try and crawl away are all possible. It is up to the GM to determine how they would react. The thing is they don't need to though. They will dodge, but not full defense dodge unless they want to. They can stay standing eating lead, they can return fire if they want to as well. Basically the GM should make a call based on the opponents and what they think there reaction would be to a hail of lead.

Machine pistol is 4, SMG is 5, actually. The question is valid, because the book says something to the effect of 'you can drop prone at any time', and it's true that they're at no particular disadvantage to shooting you while they're prone.

And in a different paragraph the book says that you cannot drop prone if you're suprised. So decide what is more valid.

Also, the main reason of supressive fire is not necessarely to hit people, is to create a wall of high velocity lead that no one would dare try to cross it. This way, your allies can flank them while your enemies are pinned down.

I thought the main reason was, I have a big gun with lots of dv and AP, but I can't handle the recoil. also I can't decide who to target first, I take them all down! rule of thumb: the bigger your gun, the bigger your mag/belt, the bigger the recoil, the better to use suppressive fire

suppressing with an SMG loaded with regular ammo will only work on naked morons

Well, yeah, but since any moron can pull their socks up and avoid EVERY hit just by laying on the ground, it turns the main reason of it to create choke points.

WHy can't I tell the GM that I aim my suppression fire at a downward angle? AIming to either hit their legs, or if they go prone, shoot them in the head.

because you can't aim supressive fire, that would be called a long burst

You can partially aim suppressive fire I get to choose the cone length and width, why can't I choose the height?

Because the rules say that prone people are immune to suppressive fire. Also 6P AP-1(Assault Rifle) is not so dangerous. 19 dice of BOD+Armor is doable. Such people won't suffer often or much from suppressive fire even if they don't duck.

Guess this is the application of Movie-Shooting. If mooks shoot through walls, they always aim at chest height or higher, even though everyone should know that SOP in such situations should be to duck and move to cover (not concealment).

The point of suppression fire, is, as has been said, to create choke points, and divert attention. In reality it does much the same thing. The only real problem to this use is Trolls an Orks, who can run through suppressive fire an not even take a box of stun from time to time. The less then toughest humans and dwarves most runners will meet will usually not have this problem, and even if they remain prone, are sitting ducks. Range attacks suffer a penalty against the smaller targets, but they can`t realistically dodge without gymnastics training.

It is also possible to use suppression fire as a means of hitting a target, since it is assumed to be a thick enough spray to hit anything foolish enough to remain in the area. But this is only really useful if the GM has made a real baddass that avoids almost every attack thrown at him (guilty as charged, but it was helped that only one character was ready for action at that point).

At the very least, this is how I understand it.

There is no such restriction in the rules.

If the character is using a sufficiently compensated weapon (9-15 points RC) it is actually harder to hit someone with suppressive fire than with an aimed full burst, not to mention that the latter will do more damage. Against a targeted attack the defender gets REA+(Dodge/Gymnastics on Full Defense) against the wall of lead he gets REA+EDG+(Dodge/Gymnastics on Full Defense). There is no rule against spending Edge to augment either roll. REA+2*EDG with exploding sixes will most probably negate any suppressive fire directed at the character.

It is more useful as crowd control against many low defense mooks.

The miniguns are interesting as their damage is increased for suppressive fire. They have a lot of other problems though (warm up period, only FA, high recoil, except on vehicles).

If I recall correctly, anyone who is prone gets a -5 to dodge rolls. If you are being attacked by a ranged attack, the fact that you reduced your area, you could be considered a small or even mini target, so the attacker would get a -2 or -4. The fact that being prone also helps on your recoil, I think it is a fair trade off.

it's -2 and only for melee. your melee opponent furthermore gets +3
being prone means giving up a simple action to stand up, so that's another drawback

You recall incorrectly. It's -2 against melee only, unless the shooter is closer than 5m. Prone doesn't give RC or 'reduced target size'.

I am incorrect, you are correct. Suppression fire is useless for the purpose of pinning a target so others can make use of it. It makes sense, to me, however, that being prone would leave one with trouble avoiding range attacks, as it is fairly difficult to move when lying on the ground. Unless the person has practice specifically with moving while prone.

Also, as a mention on modifiers attacking prone characters, under Target has good cover, it says a target may be considered to have good cover (-4 to attack rolls) if the target is prone, and at least 20 m away from the shooter.

AFAIK, the idea is that the smaller-prone-target and the harder-to-dodge are assumed to balance each other out (except for melee combat, and ranged under 5m).

Good catch on the Prone/Cover issue. I forgot all about that. So, Prone is -2 under 5m, 0 between 5m and 20m, and +4 (not -4 to attack rolls, that's the old way) beyond 20m. Whew!