A first look at the NPSS Nitro Piston by Jim House

The Personality of the Nitro Piston
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If you have ever watched a dog wag its tail, you have doubtless noted that the back part of the dog also wags slightly. Moving the tail requires some force and there is an opposite force moving the dog. In the case of airguns, compressed air forces the pellet down the barrel at the moment of firing, but there is a huge difference in how the air is moved toward the base of the pellet.

In a break action (spring piston) air rifle, pulling the barrel downward forces a piston to the rear against the pressure of a strong spring. When the spring is compressed and the piston is in its rearmost position, the sear is engaged to hold the piston in place. At the moment of firing, the piston is released and the compressed spring forces it forward at high velocity. This action compresses the air in the compression chamber behind the pellet causing the pellet to be moved down the barrel. One problem with this type of rifle is that the piston and spring have considerable mass and when the piston reaches the forward end of the compression chamber, it jerks the rifle forward. All of this takes place before the pellet leaves the barrel. Consequently, achieving high accuracy with this type of rifle requires practice and consistency of shooting form.

Another problem with the spring piston rifles is the fact that leaving the rifle cocked for a considerable period of time causes the spring to lose some of its elasticity. As a result, many shooters who use break action rifles cock them just before taking a shot. This is inconvenient is hunting situations.

A significant improvement over the traditional spring piston rifle could be made if instead of using a strong, heavy spring a gas were compressed behind a piston in the compression chamber as the rifle is cocked. In that way, there is less mass jiggling around in the rifle at the time of firing. Equally important is the fact that a compressed gas does not lose its elasticity so the rifle can be left cocked for a long period of time. This type of propulsion system is generally referred to as a gas ram or gas spring break action rifle. A gas ram rifle has many of the advantages of a spring piston model but fewer of its drawbacks.

Crosman has continued the development of powerful air rifles to include a new break action rifle using new technology. This rifle, known as the Nitro Piston or NPSS because the gas utilized in the sealed cylinder is nitrogen, is something special. Because only a gas is compressed in the compression chamber behind the piston when the rifle is cocked, the Nitro Piston is lighter than many spring piston rifles that employ steel springs.

The Attributes of the Nitro Piston Rifle

The Nitro Piston has several interesting features. First, the barrel has no sights. This rifle is intended to be used with an optical sight, and in most cases it will be a scope. In fact, the Nitro Piston is supplied with a 3-9X AO CenterPoint scope in a very robust mount. Second, the barrel has an aluminum sleeve surrounding it that has a uniform diameter of 0.875-inch so that it is essentially a bull barrel although it is not one solid unit. The sleeve is attached by a long threaded section at the muzzle, and the end cap has a hexagonal opening for using an Allen wrench to attach or remove the sleeve. The sleeve makes the barrel serve as a convenient, easy to grip handle when cocking the rifle. Third, the Nitro Piston has an usual stock. Not only is it a thumbhole style, but also it is made of a polymer that is easy and comfortable to grip. Not only is the soft polymer stock of the Nitro Piston pleasing to the touch, it serves as a shock absorber to reduce vibration. Both the gas ram and the synthetic stock result in a rifle that has much less vibration and noise than a break action rifle that uses a steel spring. The dimensions are such that the Nitro Piston is easy to use. Fourth, the stock has a nice cheek piece that folds over the comb of the stock so is comfortable when shooting from either side of the stock.

The Nitro Piston has the styling that really attracts attention. During the first tests I conducted, another shooter at the range saw the Nitro Piston and came over to look at it. There were unconcealed expressions of approval of this sleek rifle. Weighing just a shade under seven pounds, the Nitro Piston is convenient to carry. It will be available in .177 and .22 calibers.

When I began testing the .22 caliber Nitro Piston, I was surprised to find that it cocks very smoothly. Because there is no spring grating along in the compression chamber, pulling the barrel of the Nitro downward to cock it requires a uniform pressure.

The performance of the Nitro Piston

After cocking and loading the Nitro Piston, I wanted to see how it would perform so I prepared to fire. When firing any new gun you never know exactly what to expect from the trigger action. In the case of the Nitro Piston and other break action air rifles, movement of the trigger has two stages. The first is a rather long, light pull (know as the take up) that moves the trigger back to the point where firing is about to occur. The actual firing motion is short and crisp and usually requires considerably more force than the take up motion. In the case of the Nitro, the take up motion required a slight but noticeable force, and the actual let off required a harder pull . However, because the let off was crisp and predictable, I found it easy to control the trigger when shooting from a bench.

Because another shooter was firing a high power rifle, I was wearing hearing protectors when I fired the first shot from the Nitro Piston. I thought the gun had misfired somehow until I looked at the chronograph which showed the velocity of the pellet! The Nitro Piston is significantly quieter than most if not all of my other break action rifles. Moreover, there was a noted absence of the twang and jump that accompany firing a spring piston rifle.

Two aspects of the performance of the Nitro Piston were evaluated. First, it was necessary to determine pellet velocity, but here a problem was encountered. My wife and I travel in the western mountain states during the summer. This year, one of my special traveling companions was the Nitro Piston so the testing was conducted at high altitude, and the range where the Nitro Piston was tested is at an elevation of 5,500 ft. At this altitude, cocking any break action rifle draws less air into the compression chamber in front of the piston so when the gun is fired the velocity is lower. Previous testing has shown that at 5,500 ft the velocity is approximately 94% of what it is at an elevation of a few hundred feet. The .177 Nitro Piston gives velocities up to 1000 ft/sec and the .22 caliber gives up to 800 ft/sec. These velocities would be obtained at low elevation with pellets of light weight. However, these velocities will not be realized at high elevation with pellets of normal weight. With the chronograph in place, I fired a string of Crosman Premiers across it and got an average velocity of 631 ft/sec. When this value is corrected for the elevation factor, the velocity would be 675 ft/sec with a pellet weighing 14.3 grains. With pellets weighing about 11 grains, the velocity would be approximately 770 ft/sec which is close to the advertised value. Keep in mind that this was with a new gun and that performance generally improves after a break in period.

With Crosman Pointed pellets the average velocity was 625 ft/sec, with the Crosman wadcutter it was 623 ft/sec, and with Crosman domed it was 637 ft/sec. Keep in mind that at low elevation these velocities would be 40-50 ft/sec higher. As a result, the Nitro Piston would generate about 14.5 ft lbs of kinetic energy which means that it is a “magnum” airgun that is entirely suitable for hunting small game and pests.

Because accuracy is of paramount importance with an airgun, the Nitro Piston was tested with several types of pellets by firing three or four 5-shot groups at a distance of 25 yards. The average group sizes obtained are as follows: Crosman Premier, 0.91 inch; Crosman pointed, 0.77 inch; Crosman wadcutter, 0.89 inch; and Crosman domed, 0.80 inch. Keep in mind that these groups were fired outdoors at a range where there is always a prevailing breeze from a rifle that did not have an extensive break in period. Under better conditions, these groups would shrink to perhaps 0.5-0.7 inch. Therefore, it is clear that the Nitro Piston has plenty of accuracy to be an effective game and pest rifle.

The Crosman Nitro Piston represents a significant advancement in airgun technology that has resulted in an exciting, effective air rifle for a wide variety of uses. In these tests it was found to be significantly quieter, easier to cock, and to give less recoil than any break action rifle of comparable power that this reviewer has tested which uses a steel spring.