EDIT: Of course, that makes me wonder about how one would modify a suppression zone for grenades, since we've already established they're not treated like bullets.
If you're anywhere near the blast radius of the full-auto grenade launcher's impacts, you need to make a Composure test and find a new set of underwear.
Well, I figured that part was obvious.
I was more thinking about the damage to objects in the area being blasted into a crater. For ease of math, maybe just assume 1 at 0m scatter (for the primary damage source), 1 at 1m, and 2 each at 2m-10m? The falloff would be a bit more complicated, but I'd probably go with a -X/m/m... -X (the normal increment) per meter per meter.
So, for a HE volley it would go 10 + ((9 + 16 + 14 + 12 + 10 + 8 + 6 + 4 + 2 + 0) / 2) = DV 51 AP -18 (not -21, since 3 'nades don't reach them) for anything caught in the primary blast zone. At the edge of the blast zone, the DV takes a total penalty of of -1 at 1m, -3 at 2m, -6 at 3m, -10 at 4m, -15 at 5m, -21 at 6m, -28 at 7m, -36 at 8m, - 45 at 9m, and -55 (aka, no effect) at 10m.EDIT: Whoops... got my High-Explosive and Fragmentation Blast increments mixed up, there. It's really 10 + ((8 + 12 + 8 + 4 + 0 + 0 + 0 + 0 + 0 + 0) / 2) = 26 DV AP -9 (only 8 'nades apply to any given spot) , and -2 at 1m, -6 at 2m, -12 at 3m, -20 at 4m, -30 (no effect) at 5m from the edge.
Frags, with their higher DV and lower increment would be potentially much deadlier, but much more random, since they'd work out to DV 72 AP +52 and zeroing out at the 12m mark. WP works out to DV 49 AP -50%
*, DV 25 for 10 turns, zeroing out at the 10m mark.
Using this model, suppressing with Frags will make people dive for cover and cause randomly devastating injuries among armored troops ranging from 'gravely injured' to 'fine red mist' over a 44m diameter area, but not do much against vehicles. Using HE will cause less damage at any one point, but will cause a seriously bad day for just about anything in a 28m diameter. WP grenades will, effectively, create a 38m diameter micro-apocalypse.
*When using OE rules for -50% AP mods, you essentially add -25% to the base for every AP -50% after the first. That means 3 or more WP grenades would totally nullify any armor except fireproofing, making such a strike an inevitable OHK against even heavy tanks. Hence why I'm sticking to AP -50%. The actual result should be -525%.