The next generation of small arms coming to the top, a lot are using a bull pup configuration. Since it utilizes a rifle length barrel (about 18-20") in a carbine sized package (about 28-35" give or take) no performance or accuracy is lost in compacting the size of the rifle. However, this design is not as perfect as it sounds.
With the exception of the FN F2000, bullpups can only be fired from one shoulder safely because of the ejection next to the user's head. Also, many are back-heavy and it is more difficult to balance than a standard setup. Also, it is reportedly slower in magazine change time, and it is a lot more awkward.
Overall, there's a reason that most special ops units from countries whose conventional forces use a bullpup, instead chose a standard rifle and/or carbine. The SAS uses the M4 or the C8, GIGN (though occasionally using the FAMAS) often operates with the SIG 551, and the Australian SASR also utilizes the M4.
Depending on the means of attaching the sight and the optic employed, the bullpup design has the potential of being more mechanically accurate than a conventional weapon the same length because of barrel length and the potential for added barrel rigidity (For example the Walther WA-2000 and Bushmaster M17S).
Additional problems are practical accuracy for most designs because of extended trigger linkages, greater difficulty creating an ergonomic stock, and (without optics) limited sight radius. There is also a practical limit to conventional magazine length without going to an unconventional design parallel to the bore line like the Calico-series, FN P90, or HK G11. This means, in most weapons, the inability to use a high capacity drum for use in the AR/SAW roles.
Belt feed is actually less problematic in bullpup or semi-bullpup designs because it makes it easier to put the ammunition closer to the point of balance (like the M60/M60E3, MG42, and some of the Colt LMG prototypes.)
Modularity seems to suffer on many designs because of a combination of decreased mounting spaces (ergonomic requirements and competition with necessary optics) and balance issues from adding mass forward of the trigger.
Many bullpups have the ability to have their ejection port switched between sides simply and quickly. Examples are the FA MAS-series and the Steyr AUG. A select few eject straight down and are essentially ambidextrous like the FN 2000, FN P90, and the Calico-series of pistol/machine pistols. A few designs, like the L85/L85 series and the Bushmaster M17S's are exclusively right handed. It's not terribly reasonable to expect to halt combat to reconfigure your ejection port. This limits even these platforms to shooting from one side or the other. Not a big deal for all, to me, it's one strike against the bullpup that I don't have to worry about with a conventional rifle.
The Bullpups offer only one real advantage - compact size - full length barrel. Everything else about them is 'bog standard', even though the FN2000 has definitely got potential to be a very workable platform.
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