If the anti gigatons is so strong, why can't you create a convincing case on Spacebattles?
Also on the topic of orbital bombardment...
Planetstrike page 4
Megatons.
Planetary invasions are urgent, swift and terrible affairs, characterized by deafening noise, bone-shattering explosions and the stench of death. Thousands of reckless and battle-hungry warriors plunge downward upon trails of flame and vapour like vengeful angels, pouring from the drop-craft and low-orbiting spaceships that darken the skies above. Megatonnes of ordinance hammer down around these skyborne warriors, their detonation so devastating that the skies themselves seem afire; red, black and blinding white like the fires of hell.
Fallen Angels
Magma bombs are likened to fusion bombs.
The eight shells struck the target area more or less simultaneously, their magma warheads detonating with the heat and force of a fusion bomb.
At a thirty kilometer distance a shock wave picks up a space marine and tosses him through the air. Crouched, his profile might be a m^2. A very powerful shock wave which likely has much further to go. Probably gigatons (a few maybe). The space marine bit is interesting, worthy of further attention --- it could turn out that this requires biggotons for a shockwave to do that at so far.
Also take note of the space marines reaction - it implies this bombardment is unusally powerful, something he had not seen before (contrast to macrocannons and lances), and was unprepared for. Interesting.
These were no simple meteors, falling in thin streaks of light before vanishing into oblivion. Nemiel counted eight separate streaks of smoke and flame, plunging down in a steep arc and converging on a common point: the heart of the forge complex, some thirty kilometers away. When they struck, the entire northern horizon blazed with terrible, white light.
Nemiel had witnessed more than one orbital bombardment in his time, but those had been blazing trials of lance fire that carved across the ground like a burning blade, or salvos of poorly aimed macro-cannon fire that saturated a target area with huge shells. He'd never been close enough to experience the fury of a barrage of bombardment cannons, and wasn't prepared for what followed.
The eight shells struck the target area more or less simultaneously, their magma warheads detonating with the heat and force of a fusion bomb. His onboard systems registered the over-pressure from the blast and had just enough time to yell, 'Get down!' before the blast wave hit.
He dropped to the ground and pressed his helmet to the pemacrete as a roaring wall of superheated air howled over him. His temperature sensors spiked, pushing into the red zone, and the force of the wind lifted him off the ground and tossed him like a toy down the narrow lane. The thunder of the blast was something he felt through his armour, reverberating down into his bones. His autosenses overloaded and shut down once to prevent permanent damage.
It was over in a matter of moments. One second the entire world felt as though it were coming apart at the seams, and the next everything was almost eerily silent. Nemiel lay on his back, trying to regain his bearings. Icons blinked on his helmet display, informing him that his autosensors and vox-unit were resetting. As his vision cleared, he saw tendrils of smoke rising from his scorched armour.
...
It was only then that Nemiel fully saw the devastation that the bombardment had wrought. An enormous column of ash and smoke rose into the sky off to the North, where the volcano and the forge's centre used to be. The rising sun tinged the climbing column of debris in shades of blood red and fiery orange, whiles closer to the ground Nemiel could see thin veins of pulsing orange, tracks of real magma flowing like blood from the volcano's shattered flanks. Fires blazed out of control from horizon to horizon, consuming the shattered husks of wrecked buildings in a vast swathe surrounding the epicenter of the blast. For all intents and purposes, the forge complex had been destroyed.
Nightbringer
Strike cruiser(?) can at least level all within a 50 kilometre radius. Probably gigatons.
Lord Admiral Tiberius had wanted to level the entire palace with orbital bombardment, but Uriel had resisted such a plan, knowing that the vast forces the Vae Victus could unleash would level everything within fifty kilometers of the palace.
Latest Core Rulebook (the same one which talks about conventional vs exterminatus bombardment, Imperial ships prefer to smash planets through brute force until they break apart)
In all, maybe a multi-hundred meter deep shaft, coupled with a square kilometer of slagged ground around according to RT. Megaton range - perhaps.
The largest of the las-weapons - such as the lance-strike batteries employed upon spacecraft of the Imperial navy - produce beams that can sear away entire hab-blocks, leaving only smoking craters hundreds of feet deep.
Execution Hour
Again hundreds of meters deep - consistent. This time it sounds as if their raking the beams along, making the energy requirements higher, but power remains the same
Armaments that could hurl energy hundreds of thousands of kilometres across space now turned their power on the planet's surface in an awesome display of destructive capability, gouging wounds hundreds of meters deep into the rock and soil of the hills in search of the silos, command bunkers and generator caverns buried there.
Lathe Worlds
Six day bombard depopulates a city world.
An entire battlefleet attended to the world, obliterating any existing life in a cataclysmic bombardment of lance and torpedo that lasted a full six days. The fleet then rested a day, searching for any evidence that anything living survived, before completing the annihilation with a firestorm cannonade that burned any remaining oxygen from the atmosphere. Satisfied with their utter eradication, the fleet departed for other combat operations. Such total devastation is normally done on the order of the Inquisition, but the lack of a more simple cyclonic torpedo attack to accomplish the task indicates otherwise. Other planetary attacks during the crusade were often as destructive, though seeking only to kill off any xzenos or rebellious armies, and not to remove all life no matter the type. Whatever their enemy might have been, the Crusade required its complete extermination and was willing to take valuable time during the war to ensure this goal to the best of their ability.
Plenty of stuff that could easily be megaton range to single gigatons at least in the bombardment cannons case (30 - 50 Km radius which levels "anything" so 20PSI, which might not be enough to throw a square meter ton object at 30Km meters through the air).
Based on those two bits, its arguable a salvo of eight magma bombs is several gigatons, which sets a ballpark for strike cruisers and battlebarges.