I don't own SM:P so this will probably be info about as useful to you as a chocolate coffee pot. It's a long post, so feel free to let your eyes glaze over and move to the next relevant post. But this is how SM2 handles them:
Grenade Rifle is a specific weapon requiring a skill to use. This falls into the "Launcher" weapon category which also includes Portable Missile Lauchers and Rocket Propelled Grenades. This is distinct from Support Projectile or Support Energy weapon categories. The grenade rifle is the least powerful launcher, and can only fire grenades up to mk 2, but the range is far superior to a thrown grenade, especially if aerodynamically modified grenades are used.
SM2 is less clear about PMLs and Quad PMLs (bazookas, essentially) but since my players discovered them I have allowed them to go up to mk 5 grenades. Which are dangerous, even in the hands of unskilled users because of the way grenades work.
On the attack table, instead of being determined by weapon mark, maximum results are determined by proximity to the point of explosion. The diameter of the blast radii is determined by a combination of grenade mk and wither they are point defence, standard, or enhanced. A point defence has blast radii of 1m per mk, and an enhanced is 3m per mk - so a mk 5 enhanced has a ground zero of 15m radius, 2nd blast radius if 30m, out to 75m for the last one. This is a large area and I'm considering making that radius a diameter as it can take out some large armies.
There are result bonuses based on mk of explosive, which blast radius they are in, whether they are touching the grenade when it blows. All hit results are multiplied by the mark of the grenade.
Example time.
Player shoots a mk 2 enhanced grenade at a mook, who unknown to him is being followed by some more mooks, spaced out at 10m apart. The player scores 100 on his attack roll (easier math).
Mook A in the lead is wearing AT12. The result of 100 is modified by +10 (for grenade mark), for a total of 110. The way I do this is look that up on the small projectile chart to see if it hits - and 110 is enough for a 8A result so the grenade does actually hit the target. However, this means that Mook A's troubles are just beginning. The 110 is further modified by +40 for him being in ground zero, and +35 because it actually hit him when it exploded (the 110 result earlier). This means that the final result os 110+35+40, and 185 maxes out the table for a result of 15E, which becomes 30E because hits are multiplied by the mark. If he had a buddy with him at point, the result would have been 150 - since he would still be in ground zero, but not touching the grenade. Still bad news though.)
Mook B, is also wearing AT 12. He is 10m behind Mook A, and the grenade has a blast radius of 6m per radius (mk 2 x 3m for being enhanced) so he is in the 6-12m range of the second blast radius. from the roll of 100, it gets 110 for mark number, +30 for second blast radius. Nothing for touching, as that is a problem for the fine red mist that used to be mook A. But he's still looking at a total of 140 - second blast radius tops out at 135 though, which is 10E, increased to 20E for mark.
Mook C is at 20m from the blast. This is far enough that is is not in blast radius 2 or 3, but in blast radius 4. the roll of 100 +10, is modified by +10 for the fourth radius - for a total of 120. 4th radius tops out at 105, and this result is a 2A, modified to 4A.
Mook D is at 30m from the blast. The maximum extend of the 5th blast radius is 30m. I would let him have a luck roll to see if he is caught or not. If he is unlucky, he takes 2 hits - 4th radius tops out at 90, for a table result of 1 which is then doubled for the mark.
Of course, if the shooter is 30m or less away from the target, they may be caught in the backwash. I had this happen to someone shooting an enhanced mk5 plasma grenade from a PML at a target 60m away. Sure it looks a long way, but when your blast radii are all 15m, those add up fast. Character was a telepath so not that built, took a triple A critical (plasmas do heat, radiation and impact), rolled pretty high (in the 80s I think) and came within about 3 hits of actually dying due to hit loss and burning hits/round.
For vehicular rather than anti-personnel, it's worth remembering that even todays RPG Lauchers (which are rocket propelled grenades - they don't shoot roleplaying books out to concuss the opposition
) are designed such that when they hit they shoot a wad of molten metal forwards and high velocity. This has no trouble punching through significant armour plating - I've seen the results of test firing and they are capable of getting through at least an inch, probably more, giving all the squaddies on the inside a molten copper bath. Which will get you clean but will smart a little. The point is for vehicular, there will be grenades designed specifically to get through armour plating - so they will remain very effective anti-tank weapons even into the far future.
