A rifled barrel means a few helical grooves around a hole. Usually the grooves are all in even numbers viz. (4, 6, 8, 10, 12 etc).
The diameter of the head of a pellet is usually the same diameter as that of the groove, and the diameter of the pellet skirt about .2mm more. This allows the pellet to sit firmly on the breech and its travel path through the helical groves gives it a twist in it’s axis which in turn gives stability while traveling a long distance. Continue reading →
For any one who opens a Springer, one very familiar sight greets them. It is an object like a giant bolt that holds the spring from the back. They may vary in length and thickness but their shape is remarkably similar as you can see from the various photos shown.
Now the question is why do we need them and what roll do they play in there??? Does it not unnecessarily increase the weight of the gun? Does the spring really need it? Isn’t the tubes inner wall enough to act as a guide (outer guide)?
Well for one, if the tubes inner wall acts as a guide, significant increase in friction can be expected thus reducing the spring force etc.
Another frequent question thrown to me often is, “does the guide contribute to the increase in velocity of the pellets or does it enhance the accuracy”?
Interesting question indeed. Let’s me try to answer the above question.
Many air rifles I have seen (and I have seen many) have springs that feature a few common characters between them. These include
- The length is significantly longer than required.
- The wire diameter is thin
- The coil OD (outer Diameter) is small
The picture below shows what happens when the spring is getting compressed. This is known as critical deflection.
The picture above shows the deformity of the spring in non-axial direction. Once a spring buckles, it no longer provide the intended force. Typically, buckling deformation accelerates rapidly and the spring fails, resulting in the frequent necessity to change the spring due to non performance.
Theoretically a guide should arrest Buckling.
In most Indian air rifles and for that matter some foreign ones, the spring guide rod is much smaller in diameter than the coil ID. Compression of the spring causes it to buckle. This in turn allows for strong friction between the spring and inner piston walls. So what happens when the gun is fired? The spring gets uncompressed with the heavy friction we just talked about, causing less force output than expected, with lots of torque. This will turn your gun in it’s axis, right or left. Under certain conditions, a resonance may occur resulting in a very violent motion, when the spring actually jumps out of contact with the end plate, often resulting in damaging stresses and accuracy.
If maximum output from a spring is to be achieved, then buckling of the spring should be arrested. This should be done inside the gun resulting in minimum friction. This is only possible if the guide and spring design is synchronized. Let me avoid mathematical formulas to keep the topic simple. So simply put, maintaining a 0.5mm gap between shaft and coil should be able arrest buckling.
It has to be ensured that the guide is perfectly straight in the assembly. I know of IHP guides that fall a bit short while engaging with the piston sheer, it bends almost 10degree which is not good.
It is very important that the guide should be of a perfect parallel cylindrical shape that maintains a perfect gap from every side of the spring ID. Let me give a simple example to illustrate what I mean. Imagine you wearing a perfect fitting bangle (my apologies to men). The bangle sits comfortable around your wrist. If you try to push the bangle to your elbow, you find out it get stuck. When you want to pull the bangle back to your wrist you find it’s stuck and does not sail smoothly back to the wrist. It wouldn’t have been so if your arm was perfectly parallel. This illustrates why the guide has to be of a perfect shape. Some guns like the IHP , old Diana and some Chinese makes use sheet-metal and is not perfectly cylindrical and parallel, causing friction.
Ensure there in no friction between the guide and the spring. The guide in the first picture is that of an IHP. Take a closer look a the surface and near the rest. Looks like it is designed to stop a compressed spring from bouncing back; just like a bangle in the arm. 🙂