 Pre-charged pneumatic PCP air rifles are powered by compressed air contained in either an integral air cylinder or bottle. The design incorporates a hammer that strikes a valve that releases some air to propel the pellet out of the barrel. The hammer always strikes with the same force, so the amount of air released by the opening valve changes as the rifle's onboard air pressure behind it drops. This impacts the power of the rifle as it progresses through its air charge, resulting in what's known as a power curve. Generally, an unregulated PCP's power output will rise to a peak before dropping off to the point it needs recharging. A power output that's dependent on the air rifle's pressure is not ideal because the pellet's downrange point of impact, POI, will alter accordingly. Because of this, many air gun manufacturers incorporate an air regulator into their guns. In a regulated PCP, the power output stays consistent throughout its entire air charge. When an air regulator is fitted to a PCP, the volume of air released is far more controlled for the entirety of the rifle's charge, and there is therefore no power curve. A regulated PCP offers what is known as a sweet spot that extends from the first to the last shots of the usable shot range. In some cases, because a regulator deals with air delivery more efficiently, the rifle also returns more shots between fillups. A regulator system incorporates a secondary air chamber, the plenum chamber, which is designed to operate at a constant pressure. This means that as the rifle's air pressure drops, the main firing valve always delivers an exactly metered volume pressure of air. The benefit of this is that the rifle's power output is the same, no matter where it is within its charge cycle. Thus, the power curve is eliminated, and the point of impact is not altered. It is important not to let the pressure in the rifle's main air reservoir drop below that of the regulator's operating pressure. Otherwise, the regulator effectively becomes redundant in the airflow chain. Many manufacturers now incorporate an additional gauge on their regulated PCPs to specifically indicate the regulator air pressure status.