I'm copying this over from the Space Marines forum to combine references.
Deathwatch Frag Cannon vs Space Marine Grav-Cannon
The Grav-Cannon has been a top xenos killer when it comes to fighting Monstrous Creatures (or worse) such as Riptides, Wraithknights, Stormsurges, and the like commonly seen in competitive armies. Strangely, the Imperiumís top Space Marine xenos hunters donít use Grav-Cannons in their arsenal. Although it is listed in the Deathwatch Codex Glossary. Just, no one can use it. Instead, the Deathwatch has their own version of the Frag Cannon which is much improved over the Blood Angelsí Furioso Dreadnought Frag Cannon.
So, how does the Frag Cannon compare to the Grav-Cannon as a xenos slayer?
For this article, Iím comparing the basic units which can use these weapons: Deathwatch Veterans for the Frag Cannon vs Space Marine Devastators for the Grav Cannon. That way they have the same basic stats (Vets have more attacks but thatís not critical to this comparison) and cost the same number of points after being kitted out with four heavy weapons. That seems intentional on GWís part, unlikely that itís merely a coincidence. Handily, that makes the comparison a 1 to 1 comparison (yes, I used comparison twice, now thrice).
The Frag Cannon has two fire modes: Frag which fires two S6 Rending Templates and Solid Shot which is 24Ē S7 AP3 Assault 2. Essentially a slightly weaker but also safer Plasma Gun with two shots. The Solid Shot improves at closer range, becoming S9 AP2. A very short ranged Lascannon if you will.
The Grav-Cannon has only one fire mode: 24Ē Grav AP2 Salvo 5/3. Pretty sure everyone is familiar with this weapon so weíll move on to the comparisons.
Since itís an assault weapon, the Frag Cannon fires at full effect even when moving, a strong advantage. The Grav-Cannon, being a salvo weapon, drops shots and range when moving. This can be solved in several ways (Skyhammer, Cataphractii Terminator Captain, Endurance) but requires investing more points. 1 VP for the Frag Cannon.
Chances are, if you are using the Grav Cannon, you are using one of those methods of retaining the full range and fire power of your Grav-Cannons. Skyhammer and CatCap have their own advantages so their additional cost isnít always a disadvantage. In that case, the high volume of shots from the Grav Cannon is a serious advantage. At 20 shots vs 8 for four models, thatís over double the firepower. 2 VP for the Grav Cannon.
On the other hand, if youíve run into a Daemon army with nary an armor save to be found, those 20 Grav-Cannon shots arenít looking so great. Itís like youíre fighting a horde of miniature vehicles with Invulnerable saves. Emperor help you if their Invul has been boosted to 4++ or better. The Frag Cannon at range is much more likely to do damage against poorly armored foes, particularly if you can drop a template on them. 1 VP Frag Cannon.
Similarly, the Frag Cannon can double out foes while Grav cannot. Also, the Frag Cannon has the potential to negate Feel No Pain while Grav Cannot (little typing joke there). 1 VP Frag Cannon.
Versus vehicles, itís a bit trickier. At 12Ē, the Frag Cannon has some chance of getting an Explodes result since itís AP2 but the Grav Cannon always nets an Immobilized result, often leading to the swift demise of the vehicle or at least diminishing its utility. When it comes to AV14, Grav doesnít care while the Frag Cannon would have to close within 12Ē for a chance to pen. When you add in Grav Amps and their re-roll to pen to the high rate of fire of the Grav Cannons, you are very likely to get the 2 or 3 sixes needed to kill off a vehicle. Slight edge to the Grav Cannon. 1 VP Grav Cannon.
Overwatch. If theyíre on their own, Grav Devs will get a high rate of fire for overwatch, likely netting 3 hits. At AP2, that can be pretty devastating vs many attackers, depending on the armor they are wearing (since that effects the roll to wound). But if theyíve been granted Slow and Purposeful by a CatCap, they arenít overwatching at all. Frag Cannons, on the other hand can use their Templates to net 4d3 automatic hits. 6 hits on average with 1 Rending. Against many opponents (T3 or T4) theyíre wounding on 2s, maybe 3s if theyíre on Bikes (not to mention doubling out T3 models). Point being, youíre getting more Wounds. How many get through will depend on the opponentís armor but Frag Cannons have an advantage here. Particularly if the Grav Devs have S&P. 1 VP Frag Cannon.
As things stand now, the Frag Cannon is up 4 VPs to 3. But letís get to the real question. Whoís better at killing Riptides and Wraithknights, the alphas of the xeons?
Hereís our Scenario. The xenos scum are milling about in a forested region (5+ cover save). Our heroes are dropping in via Drop Pod. Weíll further assume that they hit their mark and donít scatter to keep the math simple. The Grav Cannon touting Devastators are part of a Skyhammer Annihilation Force so they have Relentless for this first turn. The Frag Cannon Veterans donít but they donít need it. The Space Marines wonít use Chapter Tactics since the case could be made for different Chapters being used. Just keep in mind that Ultramarines CTs granting re-rolls to the the Grav Devs would increase their effectiveness. Deathwatch, on the other hand, will be using the Aquila Doctrines to re-roll 1s to Wound since this is the most versatile option. Their mission is to eliminate a key enemy model. Weíll assume that due to sacrificial models and cover saves, the Heavy Weapons models survive to shoot any Intercept
Letís start with the Riptide. Since heís almost certainly going to be cowering in cover, letís grant him a 5+ cover save. Keep in mind, this is a Turn 1 alpha strike. If the Imperium goes first, the Riptide will have no chance to use its Nova Reactor to improve its Invulnerable save. If the Tau go first, the Riptide has a good chance of improving its save and the number of Wounds passing through would be much lower.
Grav-Cannons. 20 shots hitting on 3s. About 14 hit. Since the Riptide has a 2+ armor save, the grav shots are Wounding on 2s. About 12 Wounds, maybe 13 after re-rolls due to Grav Amps. Drop 3 Wounds due to Cover saves (or Invul, same difference). 9 pass through. Riptide is very dead.
Since they canít stay out of the Riptideís Intercept range anyway, the Deathwatch Frag Cannon Veterans drop within 12Ē so they can use the Impact rule. Also, weíll grant Deathwatch the Dominatus Tactic since thatís almost certainly what theyíd employ. 8 shots hitting on 3s, re-rolling 1s should net about 6-7 hits. S9 vs T6 Wounds on 2s re-rolling 1s nets about 6-7 Wounds. 5+ Cover or Invul saves drops that to 4-5. The Riptide might be dead, might be alive.
Deathwatch needs more dakka to be sure of a kill. Hellfire rounds will get the wounds with Poisoned but not the AP, so it would take a lot of shots to burn through the Armor saves. Some plasma and/or grav guns could do it since they have the S and the AP at some additional cost. Bring a Terminator and you might get the Rends from an Assault Cannon. Or maybe get lucky and the Riptide will fail a save vs a Cyclone Missile Launcher. Off hand, it seems you should go with at least two plasma and/or grav guns.
Next, letís look at the mighty Wraithknight.
The Grav Devs open fire with 20 shots hitting on 3s. ~14 hits again. The Wraithknightís 3+ armor save makes Grav slightly less effective so 9-10 Wounds are dealt, ~13. Since itís a Gargantuan Creature, no Cover save. If itís the dirty D, no Invul save. But letís go with the Sword and Board for the moment and grant her a 5++ Invul. This drops the Wounds down to 8-9. Then Feel No Pain drops the wounds further to 5-6. Maybe dead, maybe not. Without the Invul, the Wraithknight is very dead (8-9 wounds after FNP).
This is why it is almost mandatory for Skyhammer to use Ultramarines CTs. They need those re-rolls to hit to insure a hard kill vs a Wraithknight w/Invul (as well as mitigate bad rolls vs the dirty D). Then the math changes to 20 shots, ~18 hits, 16 Wounds after Grav Amps, 10-11 Wounds after Invul, 6-8 pass through after FNP.
Frag Cannons can open up at range since they donít need the AP2 but should still close in for Impact to deal with T8. So, 8 shots hitting on 3s and re-rolling 1s gets 6-7 hits again. S9 vs T8 gives us 3s to Wound so 4-5 get through. If thereís an Invul, that drops to 3-4 and then 2-3 after FNP. Without the Invul, 3-4 could get through but itís not good enough.
Similar to before, the Frag Cannons need some help. This time, adding some additional weapons to the squad probably wonít be enough. Youíre going to need another unit to do some heavy lifting too. Maybe a Psychic Shriek, an Assault Kill Team, or a second Frag Cannon Kill Team.
Letís look at another common scenario, the Imperial Knight. Similar to the Wraithknight, he gets no cover save. But he does have the Ion Shield. So, weíre going to assume that you drop pod into his feet and arrange your unit so 2 heavy weapons are hitting the front and the other 2 the side. Iím figuring that you canít get to the back because he has that against a board edge.
The Grav Devs open fire with 20 shots hitting on 3s. ~14 hits again. Hereís where the Grav rule is a pain, we now need 6s to glance although we do get the Grav Amp re-rolls. This gives us 4 glances, dropping 1 due to the 4++ Ion Shield. So 3 HP lost.
Again, Ultramarines CTs are huge here. That would improve the odds to ~18 hits and 6 glances, 1 or 2 of which get blocked by the Ion Shield. Ideally, youíd have two grav units so they can triangulate and guarantee one gets a clear shot so no saves can be taken. and all 6 HPs are removed to destroy the Imperial Knight. But thatís a tactica for another day.
Now the Frag Cannons. 6-7 hits from 8 shots. S9 needs 3s to glance vs AV12 and 4s vs AV13. Math gets tricky at this point. So weíll average it out as probably 3 pens and 1 glance overall. The pens are unlikely to convert into Explodes results which net extra HPS so itís most likely going to equate to 4 HPs, Ion Shields block 1 of those resulting in 3 HPs most likely. Triangulating would be helpful but probably not enough to snag a kill. Similar to the Wraithknight, I think having a second unit would be better than trying to load more firepower into the same Kill Team.
Clearly, 4 Grav Cannons are better than 4 Frag Cannons at killing the worst of the worst. But that should come as no surprise. Weight of fire is massive. The upshot is that a Frag Cannon Kill Team can do real damage, just not on their own. They need support whether from other units or added Special Weapons in their own unit. Also, keep in mind, the Frag Cannon is a better all-around weapon since itís highly effective vs the lightly armored opponents which Grav Cannons are almost comically bad at dealing with.
For myself, I think itís still worth it to have a serious Grav Dev Squad or two, preferably two, to deal with the largest of Xenos threats and some Frag Cannon Kill Teams to deal with the middle ranks. For the lesser foes, Deathwatch Special Ammo for their Boltguns should prove highly effective. Maybe with an Infernus Heavy Bolter/Flamer to add some weight. Rest assured, Iíll be tinkering with this on BattleScribe for weeks to come.
Also, a brief thought concerning Deathwatch Assault Kill Teams. Those Heavy Thunder Hammers look great. My concern is how to get them there alive in a timely manner. A Corvus Blackstar wonít get them to assault before Turn 3. Land Raiders might get them upfield by Turn 2 if they survive but how well has that been working for everyone? Running them across the field seems as bad or worse. Youíd need Electro Displacement or Shifting Worldscape to really get much use out of them. Two powers which are almost universally banned or nerfed. So, I wouldnít make an Assault Kill Team but a Heavy Thunder Hammer tucked in here or there could prevent opponents from happily charging in. And if youíre already up against their gun line due to drop pod assault, you can fire off your assault weapons and then . . . assault! Gets pricey real fast though.