Push feed and controlled feed

The main disadvantage with a push feed system is the possibility for a double feed malfunction if the bolt is not fully closed when chambering a round, followed with pulling the bolt back to strip another round from the magazine, resulting in one chambered and one partially chambered round. A double feed in itself is somewhat uncommon as an independent malfunction, but it often manifests itself as a consequence of other firearm malfunctions. A controlled feed mechanism should in theory not be able to double feed if it is set up correctly.

Push feed mechanismens in theory give the opportunity to design a safer firearm when it comes to a case rupture (for example due to overpressure or excessive headspace) since the bolt head can be designed with a smaller extractor claw which in turn gives the rest of the bolt head a bigger surface are and therefore better support for the cartridge during firing.

Most push feed mechanisms use a spring loaded plunger type of ejector situated at the breech face, and this system ejects the spent casing as soon as the case mouth clears the ejection port.[9] Most controlled feed mechanisms use a fixed mechanical ejector attached to the receiver, which results in the bolt having to be pulled all the way back to eject the spent casing. This can give a marginally quicker cyclic rate with a push feed action, since the bolt no longer has to be repeated all the way as far back.[10]

Most controlled feed mechanisms should not be closed on a chamber already containing a cartridge (similar to how a push feed mechanism would operate). On short term this can result in difficulties with closing the bolt, and therefore also difficulties with extracting the unfired cartridge from the chamber. On long term, the extractor may be damaged or take significant wear if so is done, since the extractor claw on most controlled feed mechanisms has not been designed to slip over the rim of a chambered round. As a result, controlled feed mechanisms should always be fed with cartridges which first have been filled into the magazine.[11]

Push feed mechanisms on the other hand have extractor which are designed to move around the rim of an already chambered round, and cartridges can therefore either be fed via the magazine or hand fed directly into the chamber (sometimes called "single loading").