Rapier Batteries are a little unknown gem to many Space Marine players that can potentially add quite a bit to your army.
Coming in units of 1-3 in the HS section, with 2 Marine crewman per Rapier, they have either a Quad Bolter (i.e. 6 x TLed Heavy Bolter shots) or a Laser Destroyer (S9 AP2, Ordnace 1, TLed), and they cost 8 MBs or 11 MBs per model, respectively, so they are an extremely good deal for what they bring to the battle.
What makes them even better is supporting them selected HQ choices. The following are some examples:
1 – Tigurius attached to 3 Rapiers (Quad Bolters) — Tigurius gives the whole battery strong Deny the Witch and Ld 10, plus he benfits from majority T7 and can cast a whole array of powers on them. Best powers include Perfect Timing (gives Battery Ignore Cover), Endurance (gives Battery Relentless, Eternal Warrior, and FNP(4+)), and Invisibility (makes it very difficult to hit the battery with shooting or close combat). Additionally, he can use his Storm of Fire WL Trait to make all 18 shots in the Battery Rending, giving the unit the ability to threat Heavy Infantry, MCs, and even medium vehicles.
2 – Tarnus Vale attached to 3 Rapiers (Laser Destroyers) with another 3 Rapiers (Quad Bolters) within 12″ of him, all behind an ADL with a Quadgun — Vale gives his Rapier Battery Ld 10, Stubborn, and Tank Hunters to make them excellent vehicle destroyers (S9 AP2 Ordnance with re-rolls to failed glancing/penetrating rolls makes them even better than Railguns). Vale can also use the same Storm of Fire WL trait that Tigurius has to buff the nearby Quad Bolter Rapier battery and give them rending, in essence buffing the shooting of two different units a turn. Finally, Vale himself can man the Quadgun with his BS5 and Tank Hunter to shoot down enemy flyers and skimmers with Interceptor firepower.
3 – Ahazra Redth attached to 3 Rapiers on top of a Imperial Bastion with Comms Relay — Redth gives Ld and Deny the Witch bonuses, plus can cast Mirage on the Battery to give them a 2+ cover save (4+ from the Bastion wall + Shrouded). He also can draw on Divination, possibly getting Perfect Timing (for Ignore Cover shooting for the unit) or Misfortune (for making all attacks against a designated enemy target Rending). Finally, his WL Trait gives the Rapier Battery Night Vision and Interceptor, making them a potent “counter-punch” unit against any enemy units arriving from Reserve within 36″ of them.
4 – Ordo Hereticus Inquisitor with Psyocculum, Servo-Skulls, the Tome of Vethric Relic, and PML(1) attached to 3 Rapiers (Laser Destroyers) — Inquisitor gives the unit Ld 10 Stubborn, plus makes them BS10 when shooting at anything with a Psyker in it and can roll on Divination table to try and get Ignore Cover or Misfortune. He also uses his Servo-Skulls to keep enemy Infilitrators/Scouts at bay and, depending who you are fighting, uses the Tome of Vethric to give Rapier Battery one of the following:
*Eldar: Split Fire
*Dark Eldar: Night Vision
*Necrons: Tank Hunter
*Tyranids: Monster Hunter
5 – Land Raider Excelsior and Rhino Primaris next to 3 Rapiers (Laser Destroyers) — Each turn, the LR Excelsior can buff the Battery with either Tank Hunter, Skyfire, Interceptor, Split Fire, Preferred Enemy, or Fearless, all of which make the Rapier Laser Destroyers measurably more effective. The Rhino Primaris can, once per game, make the unit Fearless and, also once per game, make it shoot Snap Shots at full BS. A great combination of these two is to have the Rhino Primaris give the Rapier Battery snapshots at full BS and also have the Excelsior give the battery Tank Hunters during the same shooting phase. This means the 3 x TLed Laser Destroyers can hit Flyers on re-rollable 3+s, get two die a piece for penetration rolls (take the highest) with re-rolls for any failed attempts due to Ordnance/Tank Hunters. Even against heavy flyers like Stormravens or Heldrakes, this combo has a very solid chance of killing them in a single volley. Finally, if you want to use the Rapiers as a form of “heavy sniper,” then grant them Preferred Enemy from the LR Excelsior and shoot them at Terminators, Obliterators, Centurions, Tau Broadsides, MCs, or some other kind of high value enemy infantry. With re-rolls to Hit and to Wound and S9 AP2, they should be able to snipe down several wounds on these tough enemy troops (or even kill multi-wound models, as long as they are T4 or below, due to the Instant Death rule).
As you can see, Rapier Batteries supported by these HQ choices (I am sure there are other good ones out there as well, such as the Cult Mechanicus Tech-Priest Dominus) are a very affordable and effective unit on the tabletop.