Steyr AUG A1 rifle, one of the first successful bullpup designs.
Image licensed from Steyr Mannlicher GmbH & Co. under the Creative Commons Attribution 2.0 License/
Since the bullpup design has the trigger in front of the action, this means that the action sits close to the back of the buttstock and closer to the user's face. That means the overall length of the weapon is reduced. Also, since the stock contains part of the barrel and the action, the stock is much smaller and hence, a bit lighter than a conventional rifle. For example, the Steyr AUG rifle that you see in the image above has a 20 inch long barrel and uses 5.56 mm. NATO caliber cartridges, but is only 790 mm. long and about 3.6 kg in weight. By comparison, a conventional configured rifle, such as an M16A2, also has a 20 inch long barrel and uses 5.56 mm. NATO caliber, but is 1010 mm. long and 4 kg. in weight, which means that we save about 25% in length and about 10% in weight with a bullpup configuration.
The original bullpup design was invented in England in 1901. The Thorneycroft carbine was chambered for a .303 rifle cartridge and held five rounds in an internal magazine. It was 7.5 inches shorter and 10% lighter than the standard Lee Enfield rifle, which was the standard infantry arm of the British military and also fired a .303 cartridge. The Thorneycroft rifle suffered from recoil and ergonomics and was not adopted for military service. Later inventors tried to improve the design, such as a couple of French inventors in the 1920s and 1930s, but were not widely accepted. The Enfield factory came out with the EM-2 after World War II, but since it was not designed for NATO caliber, it faded from use very soon after. However, the British did not forget the concept of a bullpup configuration and later designed the L85 assault rifle in 1985, which is the current standard British military firearm.
Public domain image of a British L85A1 assault rifle
However, it wasn't the British that came out with the first successful bullpup configured rifle. That honor goes to the Steyr AUG which came out in 1977. The Steyr AUG was adopted by Austria initially and was later adopted by over 25 countries. Unlike previous attempts, the Steyr AUG was highly reliable, light and accurate and showed off the advantages of a bullpup configuration. France followed soon after with their new standard infantry rifle, the FAMAS, in 1978. This was followed by the UK's L85A1 and L85A2 models in the 1980s and early 1990s. Soon, many other countries followed suit, such as the Chinese QBZ 95, Singapore's SAR-21 and the Israeli Tavor TAR-21.
While bullpup designs are shorter and lighter (and therefore easier to maneuver in confined spaces), there are a few disadvantages as well. One of the early ones is that the ejection port of the rifle sits a lot closer to the user's face. Since most rifles eject spent cartridges to the right, left handed shooters have to shoot right-handed style to avoid getting a hot cartridge to the face. Some assault rifles (such as the Steyr AUG and FAMAS) get around this issue by allowing the user to easily swap the bolt and ejection cover around, so that left handed users can make the rifle eject to the left. Other rifles solve this by ejecting the spent cases forwards or downwards. Other issues with bullpup configurations are that the noise appears louder as the firing chamber is closer to the user's ears and magazine changes take a little longer, due to the ergonomics of this configuration. On the other hand, with the success of the Steyr AUG, many other militaries have also adopted rifles with bullpup configurations, because of the lighter and shorter nature of such designs.