Looks like FW just released another Rapier Battery “variant” for use in both Horus Heresy and regular 40K:
Looking at the rules, you can take 1-3 Quad Mortars in a single unit, with each one costing 12 MBs and coming with a crew of two Space Marines (so that is between 4-12 x T7 wounds, all protected by 3+ armor saves). The mortar/crew get ATSKNF and Chapter Tactics, just like any SM non-vehicle units, and each Mortar can fire with one of the following two profiles:
1 – Frag Shell — 12″-60″ S5 AP5 Heavy 4, Barrage, Blast (3″), Pinning, Shell Shock
*Shell Shocks makes target enemy units take pinning check at -1 Ld
2 – Shatter Shell — 36″ S8 AP4 Heavy 4, Sunder
*Sunder gives re-rolls for failed armor penetration rolls
As you can see, right out the gate this unit is quite versatile, being able to engage light infantry with multiple S5 Small Blasts or take on light-medium vehicles with lots of S8 shots (with re-rolls to glance/penetrate). Given than a battery of 3 Mortars only costs 36 MBs total and takes up a single HS slot, this is a great deal from an “economics” point of view when building an army list. It is also available to Space Marines, Blood Angels, Dark Angels, and Space Wolves, so plenty of options for Astartes armies (GKs of course are the exception).
Where this units really shines, though, is when you force-multiply its capabilities with other elements of your army:
1 – Storm of Fire WL Trait — If you use Calgar, Tigurius, Tellion (all who have Storm of Fire automatically) or another SM HQ that rolls it on the WL table, then you can cast it on a Quad Mortar Battery, making all of its Shooting Rending. This means that Frag Shells suddenly become deadly to even heavy infantry and MCs and that the Shatter Shells even have the ability to deal with AV14 armor, if they roll well.
2 – Psychic Buffs — Prescience for better accuracy, Perfect Timing for Ignore Cover shooting, Shrouding for a 2+ cover save behind an Aegis Line, Endurance for FNP(4+), Relentless, and EW, Levitation for redeployment of the battery an extra 12″ each turn, Fire Shield for a 4+ cover save in the open and enemy moving in Dangerous Terrain nearby, Aversion for neutralizing enemy shooty units that could threaten the Quad Mortars, Living Lightening for additional shooting support, Shield of Sanguinus for a 5++ save, etc.
3 – Chapter Tactics — UM tactics for re-rolls of 1s in shooting, WS tactics for Hit and Run (to get out of combat, which Artillery units definitely do not want to be in), IF tactics for increased damage against buildings, BT tactics for Adamantium Will, IH tactics for FNP(6+) (or FNP(3+) if you successfully cast Endurance on the unit), and RG tactics for first turn Shrouded.
4 – SM Command Tanks — The Land Raider Excelsior can “cast” one of the following on a friendly Quad Mortar battery: Counter-attack, Fearless, Hit and Run, Interceptor, Preferred Enemy, Skyfire, Tank Hunter, and Split fire. Of these, Interceptor, Preferred Enemy, and Skyfire all stand out, with Interceptor giving the unit the ability to smoke check DSing enemy vehicles or massed infantry as it comes it, Preferred Enemy increasing the damage of Frag Shells and the accuracy of Shatter Shells, and Skyfire allowing the battery to become an exceptionally potent anti-flyer unit for a turn (i.e. 12 x S8 shots, all with re-rolls to penetrate). Alternately, the Rhino Primaris can make the unit Fearless for a turn or allow it to shoot its Shatter Shells at full BS for Snap Shots, further increasing the versatility of the unit.
5 – Attached ICs to “buff” Quad Mortars — Ulrik gives the battery Preferred Enemy, Lias Issadon gives it Shrouded, Grimaldus/Servitors give it Zealot and FNP, DA character with Eye of the Unseen Relics gives it Preferred Enemy, Sanguinary Priest gives it FNP, Ahazra Redth gives Interceptor and Night Vision, Tech-Priest Dominus with Autocaduceus Relic gives the unit IWND, Coteaz gives unit Stubborn and “I’ve Been Expecting You,” Inquisitor with Psyocculum gives Stubborn and BS10 when shooting at Psykers, etc.