Hallconnen

The Hellsing ARMS Anti-Midian Cannon 'Harkonnen', often referred to as simply "The Harkonnen", is the preferred firearm of Seras Victoria for much of the Hellsing series. She is the only one who uses it, and is probably the only one who can because of its weight; Seras' sharp precision and superhuman strength are ideal for using it.

Profile
Weighing 120 lb. (approx. 60 kg) unloaded, the 30 mm, single-shot breech loader cannon uses various types of shells, including depleted uranium with silver for piercing armored targets and incendiary napalm rounds for destroying large groups of opponents both on land and in the air. Carrying this weapon, the vampirically-enhanced Seras serves perfectly as long-range fire support. Her immense strength also allows her to use the weapon while moving; sometimes, especially in the manga, she wields it with an extraordinary ease, even firing over her shoulder.

Usage
Walter presents her with the Harkonnen as a weapon for her personal use in Volume 2.

Trivia

 * Before gaining the Harkonnen, Seras used a relatively smaller rifle, firing 13.7mm rounds, as is seen written on the gun in the OVA. In the manga this gun is unnamed but the letters BAERLKS are spelled out on its side; in the OVA, however, the gun has Harkonnen written in the same place.
 * The Harkonnen's large size sometimes makes it hard to transport, a feature wryly remarked upon by Seras. Nevertheless, it continues to be her preferred weapon until it is destroyed.
 * Seras' signature weapons are most likely named after the main antagonist of Frank Herbert's novel Dune, Baron Vladimir Harkonnen. This is further reinforced by the appearance of the gun's "spirit" during dream sequences in the manga (and thus OVA) and episode previews in the anime; the spirit in question bears a strong resemblance to Baron Harkonnen from David Lynch's film adaptation of the novel.