Like many other guided missile weapons, swarmers have a longer flight time than their default range, so firing them from beyond normal range works well (can sometimes flame out vs very fast enemies, but oh well).
As an aside, I was looking into this, and it seems like it's because the missiles actually use flight time (in weapon_data.csv) not range to determine how long they last and thus how far they can travel, whereas range just specifies at what distance they'll fire. (This makes sense, and is probably what the flight time field is for in the first place.) And so, depending on their acceleration, i.e. how quickly they reach their top speed, they can actually travel much farther than their range in some cases.
So, taking the missiles in weapon_data.csv, we can multiply the projectile speed with the flight time to get their actual maximum range (assuming they start at full speed):
range speed launch time spe*tim % diff name
500 50 20 7 350 70% Stinger-class Proximity Mine
1500 20 20 60 1200 80% Cluster Bomb Bay
2000 170 20 25 4250 213% Standard Bomb Bay
1200 400 100 3.5 1400 117% Reaper-class Torpedo
1200 400 150 4 1600 133% Atropos-class Torpedo Rack
1200 400 150 4 1600 133% Atropos-class Torpedo (Single)
1200 500 40 4 2000 167% Hammer-class Torpedo
1200 500 40 4 2000 167% Hammer-class Torpedo (Single)
1000 300 100 5 1500 150% Swarmer SRM Launcher
1000 300 100 5 1500 150% Swarmer SRM Launcher
1500 400 50 3.75 1500 100% Annihilator Rocket Launcher
1500 400 50 3.75 1500 100% Annihilator Rocket Pod
2000 400 50 15 6000 300% Salamander MRM
2500 300 100 12.5 3750 150% Harpoon MRM
2500 300 100 12.5 3750 150% Harpoon MRM (Double)
1500 400 50 5 2000 133% Breach SRM
1200 150 50 8 1200 100% Sabot SRM
1200 150 50 8 1200 100% Sabot SRM (Double)
1200 150 50 8 1200 100% Sabot SRM (Single)
1500 300 75 6 1800 120% Gorgon DEM SRM
1200 150 25 5 750 63% Gazer DEM SRM
2500 300 100 12.5 3750 150% Harpoon MRM Pod
2000 150 0 12 1800 90% Dragonfire DEM Torpedo
1500 300 75 6 1800 120% Gorgon SRM Pod
1200 150 25 5 750 63% Gazer SRM Pod
1500 400 50 5 2000 133% Breach SRM Pod
1200 150 50 8 1200 100% Sabot SRM Pod
2000 400 50 15 6000 300% Salamander MRM Pod
1500 400 50 3.75 1500 100% Annihilator Rocket Pod
4000 125 100 30 3750 94% Pilum LRM Launcher
1000 100 400 4 400 40% Proximity Charge Launcher
2000 100 200 12 1200 60% Proximity Charge Launcher
1200 500 40 4 2000 167% Jackhammer
1200 400 100 3.5 1400 117% Typhoon Reaper Launcher
1200 400 400 3.5 1400 117% Cyclone Reaper Launcher
4000 125 200 30 3750 94% Pilum LRM Catapult
2500 250 50 12 3000 120% Hurricane MIRV Launcher
2000 150 0 12 1800 90% Dragonfire Torpedo Pod
2500 300 75 7 2100 84% Hydra MDEM Launcher
2500 600 100 4.75 2850 114% Squall MLRS
1400 400 100 6 2400 171% Locust SRM Launcher
1200 500 40 4 2000 167% Hammer Barrage
1000 200 0 100 20000 2000% Reality Disruptor
1200 1000 200 3 3000 250% Antimatter SRM Launcher
2500 400 200 10 4000 160% Resonator MRM Launcher
2000 200 100 15 3000 150% Rift Torpedo Launcher
In the above chart, "speed" refers to their max speed, i.e. "proj speed" in weapon_data.csv, while "launch" refers to their initial speed, i.e. "launch speed" in weapon_data.csv. "spe*tim" is just speed * time, while "% diff" is the ratio of speed * time divided by the range -- in other words, how much the max possible range is compared with the range that the weapon will fire.
Some of the numbers don't work out, such as the Proximity Charge Launcher since that depends on the ship's initial speed, and Omega weapons also don't seem to work out (they pretty closely match their firing range). However, it's obvious from the above that quite a few of the missiles can actually go much farther than their stated range. In particular, the Salamander can actually fly up to 6000 su before flaming out. This is probably to account for that it needs to maneuver not just to the target but also around the target to the back where the engines are.