This is an extract from 'Assault Rifle: the Development of the Modern Military Rifle and its Ammunition' by Max Popenker (of the guns.ru site) and myself (it's due to be published later this year):
"There are certain disadvantages to bullpups. In most cases, fired cartridge cases can only be ejected to the right-hand side of the gun, which means that they cannot be fired left-handed as the cases would hit the firer's face (most can be adapted for left-handers, but that takes time). This means that users can't switch shoulders to fire round the corner of a building, for instance. Magazine changes may also be more awkward. The necessarily straight-line stock means that the firer cannot sight along the top of the barrel, so if iron sights are used they have to stick up high above the barrel and the firer therefore has to expose more of his head 'above the parapet'. Proponents of bayonet fighting will also point to the shorter length of the weapon, which means that you have to get closer to the enemy. Bullpups have the action by the firer's head, which some find uncomfortable, and short-barrelled versions have the muzzle quite close to the firer, which means that muzzle blast can be more of a problem.
There are of course counter-arguments. The lack of ability to switch shoulders may be more theoretical than real, as this may in practice be very little used by ordinary soldiers as opposed to special forces. Most soldiers in combat have enough trouble hitting the target when firing from their usual shoulder, let alone from their 'wrong' side, so many armies train only in shooting from one shoulder. The magazine change is not necessarily more difficult, and some users prefer the 'inboard' location as it makes it easier to change magazines when travelling in an open vehicle, for example. Military rifles are also increasingly being issued with optical sights, so the iron-sights objection is less important. In any case, military rifles of traditional layout also have high-mounted sights nowadays, because they generally have straight-line stocks, in which the top of the buttstock continues in a straight line from the barrel, instead of being angled downwards as it is in most older rifles. This is because the recoil thrust in a straight-line stock goes directly into the shoulder, whereas in an angled stock it goes over the shoulder and hence tends to rotate the gun upwards. Bayonets are now too irrelevant to modern combat situations for their length to matter.
Most significantly, bullpup proponents will point out that the increasing deployment of troops in cramped helicopters or armoured vehicles, together with the needs of urban combat, put a premium on compactness. Traditional rifles can only match a bullpup's short length by using stocks which can be folded alongside the barrel, or sometimes over the top of it, giving the choice between a long weapon or a short one which can't be fired accurately. Their only other option is to reduce significantly the length of the barrel, to the detriment of ballistics and effectiveness, especially at longer ranges. These folding stocks are commonly of the 'skeleton' type (i.e. they consist of an open framework) and may be made of metal or plastic. They are usually less rigid and comfortable to shoot with than fixed stocks. Not all rifles are able to use folding stocks anyway because the action may extend into the stock (e.g. the M16). In such cases telescoping stocks may be used instead, but these do not deliver such a reduction in length as a folding stock, and cannot match the compactness of a bullpup.
Finally, firers used to the traditional layout often criticise the different, more rearward, weight balance of a bullpup, but that is, of course, a matter of what you are used to.
What is certain is that the debate between proponents of the traditional and bullpup layouts can become heated and rely more upon emotion than logic. It is also worth noting that the use of bullpup rifles has been gradually spreading, with the majority of recent assault rifle designs being of this type, and that the latest of them – the Belgian FN F2000 – overcomes the principal objection by being genuinely ambidextrous without any modifications or adjustments being required."
Tony Williams: Military gun and ammunition
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