How An SLD Works

The key feature of a Hush Puppy® Project pistol is the incorporation of an internal Slide Lock Device (SLD) to prevent a semi-automatic pistol from self-loading, thus turning a delayed blowback, semi-automatic action into a locked breech, manually operated action. In short, the SLD prevents the slide from cycling.

By arresting the slide's travel, the slide remains "locked" to the barrel and receiver. In a delayed blowback system using a Browning-style tilting barrel lockup, the barrel, slide and receiver become "locked" when the three parts are mated "in battery" (their locking lugs or locking block seated, ready to fire). When a Browning pistol is "in battery," the lockup between the slide, barrel and receiver contains the force of the powder's combustion and resulting pressure wave that propels the bullet down the barrel.

In normal operation, the barrel would drop out of battery and the slide would continue to travel rearward on the receiver, only to bottom-out and then slam forward, propelled by the recoil spring.

This is how a pistol works. The SLD itself does not "withstand" chamber pressure at all. The SLD simply blocks the rearward travel of the slide which does not commence until after the bullet has left the muzzle! Chamber pressure has dropped precipitously by then.

It may seem that the SLD must bear the brunt of the slide's force, but it doesn't. The lockup does. The purpose of a pistol locking into battery is to contain chamber pressure. The three mated parts are designed to contain pressure— that's why John Browning invented the system! The SLD does not need to be made of angle iron, just a little tab will do nicely to hold the slide shut, just for a moment, until the pressure is fully released at the muzzle as the bullet exits. Until then, the lockup takes care of the pressure. This is why a Hush Puppy® Project Slide Lock Device is such an unobtrusive, minimalist design.

Please note that an SLD does not interfere in any way with a pistol's fire control system. An SLD is not a safety! A Hush Puppy® SLD equipped pistol will fire in either locked breech or semi-automatic mode depending on if the SLD is engaged (locked breech mode) or disengaged (semi-automatic mode).


The SLD for a Smith & Wesson M&P is fitted to the fire control system but does not affect the trigger action— it is not a safety! When in the "up" position, the SLD is engaged (pistol fires in locked breech mode) but in the "down" position the Slide Lock Device is disengaged (pistol fires in semi-automatic mode).


The barrel is "in battery" (left) and fully locked to the slide and receiver. In order for a Browning-system like this to unlock and come out of battery, the barrel must "tilt" or drop down (right). The only way the barrel can come out of battery is if the slide moves rearward. An SLD works by keeping the pistol in a locked breech mode, preventing the slide from moving, thus preventing the pistol from unlocking.


The SLD for a Glock is a sliding bar that's machined into the slide. "Up" is disengaged (pistol fires in semi-automatic mode) while "down" engages the Slide Lock Device (pistol fires in locked breech mode).