Showing posts with label Tuning Spring Air Rifle. Show all posts
Showing posts with label Tuning Spring Air Rifle. Show all posts
A first look at the NPSS Nitro Piston by Jim House
Posted On Wednesday, 9 December 2009 at 07:27
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.
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Spring gun tuning part 2 : Building a mainspring compressor
Posted On at 06:21This is the most important tool a spring gun tuner owns. It may not be used for every job, but working without one when you need it is like walking a tightrope without a net. I now use a B-Square compressor, but for many years I used a homemade rig that did everything I asked of it. The plans for my compressor came from Tom Gaylord's Beeman R1 book. I've seen simpler compressors, but I've never seen one that was easier to make.
Mainsprings are under tension
To get maximum power from an airgun, the mainspring is usually under tension (compression). In modern spring guns, the trend is toward more compression than in the past. A few rifles such as the TX200 are under almost none - but they are the exception.
You can't contain it!
Never think you can contain the force of a mainspring. Eventually, you'll be able to do so with certain guns you have disassembled many times or even with certain gun models you may have learned very well; but the first time you work on a spring gun, you need to use a compressor.
Simple design
All a compressor does is restrain the rifle while relaxing (or decompressing) the mainspring with control. The task would be simple if all spring guns were built alike - but they aren't. I will address several different methods of gun design in a later posting. For now, just take my word that the compressor has to be very adaptable.
This compressor is built on a 2x8 piece of wood. All the parts attach to a plank.
The headstock, the bridge and the tailstock
The headstock contains the moving ram that compresses the mainspring. You can make a rugged one from a bench vise. There's really nothing to build! The vise is bolted to the plank and used in reverse. The tail of the vise puts tension on the end of the gun holding the mainspring.
The bridge is a tunnel through which the body of the gun passes. It keeps the body of the gun from moving sideways when the mainspring is under tension but not restrained by the gun.
The tailstock is a block of wood with the grain end exposed. The muzzle is pressed against it and the gun cannot move.
The bridge and tailstock are adjustable to accommodate different guns. They can also adjust for a gun that has the barrel on or off.
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Spring gun tuning part 1
Posted On at 06:17I'm going to show you the fundamental steps to tune a spring gun. Although I won't show every kind of gun, I will talk about how underlevers and sidelevers differ from breakbarrels. If you're clever enough to do this kind of work, you'll be able to figure out the particulars for yourself. I'm just going to show you the important points to get you started.
We'll begin by looking at the tools and supplies needed to work on spring guns. I assume you have a standard set of tools and all the screwdrivers and Allen wrenches you need for any job. If you don't have them all now, get them as you need them. Never try to make one tool do the job of another - that's how accidents happen and mistakes are made.
Pin punches
You need a good set of these, because there are hundreds of uses for them when working on spring guns. We will use them from the beginning, so get a small set of punches right away. Airgunsmiths need small punches, because most of the pins encountered are small. I bought my set at Sears, and it has four punches - 1/16", 3/32", 1/8", 5/32", plus a 5/32" alignment tool.
Plastic and rubber-headed hammer
This is also a general tool that's used all the time. Get one that's small and handy to use. This kind of hammer is essential. You can buy one at Sears or Home Depot; if you like shopping on internet
A vice
Vices are not as useful as you might think. Spring gun tuners have very little need for vices, save one. A fine, inexpensive mainspring compressor can be made with the right kind of vice. I will give you the plans for how to build a compressor, and I'll also tell you where to buy one if you don't want to build it.
Dowel rods and rubber bands
Get at least one half-inch hardwood dowel. You'll use it to lubricate the mainspring cylinder. If you have a screwdriver with an 18" long blade, it can take the place of the dowel. Also, get a supply of thin rubber bands, to use with the dowel. I will explain what to do when we get there. A dowel 36" long is more than enough. All you need for most jobs is about 18".
Lubricants
Lubricants are a major part of a spring-piston tuner's bag of tricks. Ten years ago, the market had the products you needed and they were easy to find. For example, Beeman sold M-2-M moly paste that was wonderful stuff. Well, they don't sell it anymore, so you now have to buy your lubricants elsewhere. I want you to at least get black tar and moly paste. I'm sending you to Air Rifle Headquarters for these things. They are also an excellent source for replacement mainsprings, piston seals, spring guides and other important parts you will need to tune a gun. They have already created drop-in tuning kits for many popular spring rifles like the Benjamin Legacy and the FWB 124/127.
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