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BERETTA Model A 300 Semiautomatic Shotgun

The automatics, or self loading shotguns, according to their principle of operation fall into two main groups, namely:

- recoil operated and,
- gas operated arms.

When a recoil operated gun is discharged both the bolt and the barrel, locked together, recoil back for approximately three inches. At the rearmost position, the bolt is caught and held back while the barrel returns to its original position. Thus, two separate springs are needed... one to return the barrel to its forward position, and one to return the bolt, since both are separate operations. When the barrel and bolt are moved to the rear at the time of discharge, two springs are compressed: then, white the bolt is held in the open (rearward) position, the barrel return spring pushes the barrel forward until it releases the bolt locking latch, allowing the bolt to come forward, feeding a new shot shell.

The design of such a system is necessarily a rather complicated one, and in most recoil operated shotguns the gun must be adjusted to handle the particular load being used. In addition, the large weight and mass of the barrel and bolt moving under recoil results in the so called «double shuffle» effect, a combination of the clatter and noise of these parts, and the separate inertia of their movements.

The basic principle of the gas operated mechanism, on the other hand, is the utilization of part of the gas pressure in the barrel to move a piston which in turn operates the action bar and the bolt.

The Beretta Model A 300 uses this system.
When the gun is fired, the shot charge propelled by the expanding gases, travels up the barrel, which is stationary. As the shot charge passes the gas port in the barrel, a small, scientifically measured portion of the gas (corresponding to less than ¾ of 1% of its total volume) goes through the gas port into the expansion chamber. The pressure of this gas against the piston which is in turn attached to the action bar drives both the piston and the action bar back to unlock the bolt.

The Beretta Model A 300 features a built in load adjuster, making ft possible to shoot interchangeably all 2 ¾ " shotshells, from the lightest target loads to the heaviest magnums, without adjustment. The front of the piston is built in such a way as to be able to expand against the walls of the expansion chamber in direct relationship to the pressure of the gas on it. The heavier the load, the more the « Lips » of the piston, press outward to apply extra braking power to retard the piston's rearward motion; the less pressure, the less braking action, so that the rearward movement is accomplished at the same rate of speed regardless of the power of the shotshell used. Thus, before the bolt is unlocked and carried to the rear, the shot charge is well clear of the muzzle, and pressures, within the barrel have dropped to a safe level.

As the bolt travels back, it extracts and ejects the fired case, and a fresh round is released from the magazine. As the bolt and action bar travel forward, the bolt brings forward the new shell, chambers it, and locks itself closed, ready for firing. After the last round has been fired, the bolt remains open.

Because of the ring shaped grooves milled into the outer contour of the piston, the piston actually scrubs the walls of the expansion chamber, resulting in a self cleaning action that eliminates the need for frequent cleaning.

Roger D. & A390 in Action