Despite its obvious advantages in terms of simplicity and low cost, the blowback operation also sports several noticeable drawbacks.
foremost, it is somewhat sensitive to the quality of cartridge case
material and to the smoothness of the barrel chamber to ensure
reliable operation and avoid case ruptures or torn rims.
mass of the breechblock has to balance the strength of the spring and
the length of the breechblock cycle: if spring is too weak or the
bolt is too light, the resulting force will be insufficient to keep
the cartridge inside the chamber while pressure within the system is
high. This could cause potentially disastrous conditions of ruptured
or separated cases. If the spring is too strong, the gun will be hard
to cycle manually; if the bolt is too heavy, its movement will cause
excessive vibrations when fired, disrupting accuracy.
Also, as the
cartridge power grows (in this case, the “power” is given by the
internal pressure level multiplied by the inner area of the cartridge
base), so grows the bolt weight level necessary to keep the cartridge
in the chamber. Normally, sub-machine guns chambered to fire the
9x19mm Luger ammunition sport a breechblock weighting around a 0.5
With an intermediate cartridge – such as the Russian 7,62x39mm
M43 – the necessary weight of the breechblock would rise to about 2
kilograms, and with earlier typical military rifle cartridges, e.g.
7,62x51mm NATO, the breechblock would have to weight something around
four to five kilograms, making a blowback system absolutely
impractical (not mentioning the inherent unreliability due to the
very high pressure levels involved).
This ain't a great deal when dealing grenade launchers, since those systems generate noticeably lower pressures and are usually mounted on tripod or vehicles for stability; the increased dispersion, caused by a heavy breechblock oscillating back and forth inside the gun is also not an issue for weapons which are normally used against “area targets” rather than against “point targets”.
above, the usefulness of the blowback operation principle is limited
by the rearwards pressure level, generated by a cartridge being
fired, which dictates the weight of the breechblock and the strength
of the spring.
For handguns, which are normally limited in weight to
1 kg and even less, the 9mm Luger caliber represents the cut-off line
for the use of a straight blowback system. While some practically
working pistols were indeed built to fire the 9x19mm round using a
blowback system – such as the Heckler & Koch VP70, or the
currently-produced, US-made Hi-Point line – most blowback-operated
handguns are chambered to fire relatively low-power rounds such as
the 9x18mm Makarov round, the .380 ACP caliber, and others even
Shoulder-fired guns such as pistol-caliber carbines and sub-machine guns can accommodate heavier bolts, and are thus often chambered in most major pistol calibers such as 9x19mm, .40 Smith & Wesson and .45 ACP, while retaining a straight blowback operation. No blowback-operated rifle or machine-gun chambered to fire an intermediate or full-power cartridge was ever mass produced.