Monthly Archives: May, 2010

Springs for BAM B40

Let me share some technical details:

Piston ID: 21.25mm

Guide ID: 12.93mm

Allowable Solid height of spring: 96.28 mm (I will discuss Solid height and allowable solid height in another post)

Considering the Spring guide ID, a supplier of mine has sent me a spring whose spec is as follows:

Wire ID: 3.048mm

Coil ID : 13.43

Number of Coil: 31.5 and almost 10 inch long

Seems to be a good fit for the spring ID and very easy to cock the gun.

I was trying to insert it inside the gun…and oops…very hard. I could not do it by hand. Like most GAMO’s, the B40 has almost nil preload length and you don’t need a spring compressor to dismantle the gun. The spring was hard because the spring was not set.  So I decided to SET the spring by using a spring compressor.

This is what I did. I inserted the spring in a tube and then inserted a rod almost the same size as spring ID behind the spring and put them under compression, in solid height state for almost an hour.

The picture below shows the spring under compression.

Spring Compressor

Spring under compresson

Once the spring was removed from compressor chamber, I found the spring reduced by 20%. This is absolutely normal!! If you do not preset and try to inset the spring directly in to the gun, there is a high chance of accidents, and moreover greater chance of structural defect of the spring. Therefore “setting” spring is very important and has to be done very carefully.

After setting the spring, I found it  is very easy to assemble the gun. Once assembled, I found the gun to be very easy to cock and very less recoil. And power??? Yes the power has increased. 

The picture below shows the impact.

On wall from 8 meters

On wall from 8 meters--Closeup

Yes, that’s true; the pellet is IN a wall made of solid concrete. BAM!!!

21.25
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Main Spring Busted

It was two weeks ago. A few of us went plinking and the agenda for the day was “shoot, shoot and shoot” 🙂 . After a lot of casual pinking came the serious part. I was very determined to hit the bulls eye and at a very descent distance. I leveled my rifle….set my sights….was very confident that the result of this shot would be the bench mark to follow for the day. As I squeezed the trigger, apart from my finger, my whole body froze momentarily to ensure the least possible deviation of my aim and ……Bang!!! A loud noise from my gun just like a .22 hornet. That wasn’t right. I knew it was not supposed to sound like that. I instantly knew for sure that this sound spelt bad news. This bang was very abnormal unlike the others before it. Everyone’s face turned to me with a big question mark. On opening the breech area of the BAM B40, the rifle in question, I found the pellet stuck close to the breech washer which caused a blank fire. Intriguing indeed! I still don’t have a clue how on earth this happened.

After removing the pellet, I went ahead and shot a few more rounds. I noticed that the firing cycle not as smooth as it was earlier. While cocking the gun, one thing was obvious. While pulling the under lever to cock the gun, the immediate resistance had reduced by about 3 to 4 inch. I could also feel some metal friction. It dawned on me that the only explanation to this sound was a broken spring. What was interesting however was that the Point of Impact of the pellet had not changed.

Back home and a week later, I at last fond time to dismantle the gun over the weekend. And yes, exactly what I thought. The spring was busted. What surprised me though was that the spring wire was fat and one could never have guessed that one blank shot could have caused this. Perhaps the spring was highly stressed due to buckling as the stock guide was not a good fit for the spring (please refer to the topic “Spring Guide! What is it” for a better understanding on this subject), or the stress was not properly removed during the spring manufacturing process.

Spring Busted

The Seal seemed to be okay. Not a single scratch could be found.

Seal Profile view

I have now replaced the seal with an Apex seal but could not find a replacement stock spring. I’ll soon have one custom made for a perfect fit. The existing stock spring has a 3.32mm wire dia. The spring I have designed will have a 3.2mm dia to get more tensile strength while simultaneously softening the cocking effort.

How to remove rust

This is what I am going to restore.

Rusted object

Rusted Object Anothe view

Now going to Add Cooking salt 1 spoon in a mug filled with water so much so that the object submerged it fully. Then I add equal amount of vinegar. Mix it well and the put the object.

Salt

Vinegar

submerged in water

Keep it for 1 hr. and then remove it and then wipe it off with a wet cloth. See the result..

Rust is removed

Rust removed

Understanding airrifle Barrel.

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 →

Spring Guide! What is it?

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. 🙂