It's what the Sentry Guns have in 'Aliens', so it's what they have on my models.
In the film, Hudson and Vasquez literally bolt them into the floor; these guns are the same. They're not just resting on the dirt. The idea is that they're quickly assembled, and a Guardswoman uses a heavy-duty nailgun to drive a twenty inch long barbed retaining spike into the ground so the weapons have a stable platform to fire from. It's also worth remembering, Bolters fire gyrojet ammunition, so recoil, while present, is minimal. Heavy Bolters have far less recoil than other solid projectile weaponry.
Of course, if they were lascannon, there'd be none.