Operation successful. Patient died--and revived!

[Biren]

Having acquired 2 good air rifles I decided it was time to use the gyan I have gained to operate on my 20 plus year old SDB 50

I removed the screws holding the steel to wood - two on the sides and two holding triggger guard..

I then detached the cocking lever.

Using techniques that I saw on a YouTube video on operating on a Weihrauch rifle I removed the breech cap and gently tapped out one pin holding the trigger and then went on to the last pin.

I was prepared. So I was safe. As soon as the pin was tapped out the entire pre-compressed spring and guide rod shot out like an artillery shell - sliding along the floor where I had placed the rifle. The piston was tight inside. I extracted it. The 1 inch washer looked sick, but I decided to ignore that for now. I cleaned everything with surgical spirit. The tip of piston was very tight with a diameter of 2.6 cm. The hind part was a close fit at 2.55 cm. I sanded the front bit down to about 2.57 to 2.58 to make it a snug but smooth fit. Lubricated with gun grease. I do not have Moly grease and did not want to buy the industrial quantities available on SJP road.

Put everything back and struggled like hell and took the help of others to compress the barrel and breech on a 1" water pipe plug while the holding pin was inserted. This is not something I fancy doing often.

On test firing
1. Power was down with muzzle velocities of 130 to 140 mps (140 to 150 earlier) - not a problem for me
2. The trigger had become dangerously light - almost an auto-fire. Too dangerous for me. Or anyone.

The rifle will be taken to a rifle hospital today for repair.

Almost hilareous Bene.. I tried the same while putting new spring in my IHP.. Remember calling two extras to help me when caught midway... wonnt forget..g8 you tried & then had the courage to bring it open..

Cheers
Biren

[bennedose]

LOL Biren. The tech required to open that air rifle is rudimentary and the only knowledge required is curiosity. I needed only one screwdriver and a rod to tap the pins out. A little care will ensure that nothing happens. I needed one strong male to help me compress the spring and replace it - again it was rudimentary tech at least with the SDB rifle.It is a gross exaggeration to describe it as highly dangerous. Maybe it is for someone low on common sense or if you are living abroad and need to cover your ass with warnings or get sued. It is no more dangerous than operating a power drill or climbing a 10 foot ladder. Or crossing some roads in India.

I would hesitate to open a more expensive new rifle - but here I was simply enjoying myself and I am still having fun. the damn thing has a different personality and I am trying to see if I can train the gun or the gun will train me.

[bennedose]

The performance (power and accuracy) of my re spinged SDB is so pathetic that I decided to take it apart simply to see if I could clean up its innards.

Here is a report,

The photo below is my spring compressor. Corridor and car jack. The walls of the corridor support the barrel on one side and the jack on the oher. Stone slabs keep the assembly level. I retained the barrel in place to suit the width of the corridor The jack could be tightened/"raised" by hand - a far cry from the effort required to raise a 1.5 ton SUV on the same jack.

spr-compressor.jpg

The plate of the jack itself cannot push in the spring retainer fully because the latter is recessed a couple of mm inside the breech. I found a suitably sized plug (1 in water pipe plug) that would slide inside the breech tube to push in that extra 2-3 mm. (see last image at bottom for a better view of this)

inch-plug.jpg

This arrangement allowed me to take the rifle apart in 10 minutes. This shows the spring completely released and the jack is in the "down" position, After this I cleaned up the insides and the spring and spring guide.

spr-released.jpg

I then replaced the parts and re compressed the spring. This is where the plug came in useful. I insert a screwdriver into one of the spring retaining pin holes. This prevents the spring from springing out accidentally even if the jack-compressor arrangement slips out while manipulating and tapping the pins back in position. In this image the trigger assembly pin has been placed in position, and all that remains to be done is to remove the screwdriver and tap in the second and last pin. The 1" pipe plug between the jack and the breech can be seen clearly. The plug just enters the breech and enables the assembly to be pushed in that extra 2 mm required to align the holes to replace the spring retaining pins.

safety-screwdriver.jpg

Will test the rifle thoroughly tomorrow. I fired two shots tonight and it still looks bad. I might have to do some drastic things to the spring. It is probably too powerful for this rifle. But since dismantling, cleaning and replacing took only 1 hour this time, I will do that soon if the performance remains bad

With basic care this procedure is no more dangerous than changing a car tyre and its difficulty and danger should not be mindlessly exaggerated.

[bennedose]

> The Moral of the Story = Practice Makes One Perfect... Now you know the DOs & DONTs
> Everything comes with a "Price" attached
> Take a quick rewind of the entire procedure you went through - What went right & What went wrong / Follow the right & forget the wrong
> Soon you'll be a DIY Tuner
Briha

Thanks for the words of encouragement. They worked and boy how they worked. I will report what has become a success story.

After I reopened the rifle using my DIY spring compressor and cleaned up the oil, the performance remained worse than sub par. It was pathetic. I was getting muzzle velocities anywhere between 120 m/s and 200 m/sec but worse than that, one pellet would go 10 cm above target and the next would go 10 below completely at random. In a group of 10 shots not even one would hit the circle. With this performance, simply shouting at the target to make a hole in itself would have been equally effective.

Out of frustration I re opened the rifle for the third time and looked at the new spring which was clearly too powerful. The old spring had 40 coils and was 11" long. The new one had only 36 coils and was 11.5 inches long. Pre compression was pressing it into a compact and power loaded 8". Every time I fired the rifle it appears that a supersonic shock wave was simply dislodging the pellet to a variable distance down the barrel followed by a blast of air that sent the pellet out at variable speed and a loud report after that which was causing echoes and interfering with the Chrono Connect software.

I had to do something to reduce the power of the sping so that the compression was slower (and not supersonic) allowing the pellet to be pushed out by air and not a shockwave. Using guesswork, I simply took the spring to a workshop and had about 2 inches cut off so that the new shorter 9.5 inch spring had a pre compression of just 1.5 inches

spring-cut.jpg

That did the trick After reassembling the rifle cocking effort has been easy. Cocking is smooth and the rifle has regained its accuracy. It has also gained some power and is now averaging 150 t0 160 m/sec (500-525 fps)

Lessons for me.

• A spring compressor makes life so much easier that one can simple disasemble and assemble a rifle at will if you have one. A DIY one such as the one I have illustrated works perfectly.
• Cocking effort is the most useless way of judging the power of an air rifle
• A monster spring does not mean monster power and can greatly degrade the performance of the rifle
• All that the pellet needs is a subsonic jet of air to continuously accelerate it down the barrel. Not a supersonic explosion in the piston chamber caused by a spring loaded piston closing in like a cannon shell.

I am thrilled because my old SDB is now better than it was even when I bought it new and should be good for thousands of shots of plinking pleasure.

[Basu]

Dear benne,
So there is resurrection of patient.
Major Gladiator rightly said that the patient was in coma.
Your observations are quite significant.It raises the question as to why more potential energy does
nöt convert into kinetic.
Grand effort !!


Basu

[essdee1972]

Basuda, potential energy is getting converted into kinetic, the issue being that the kinetic energy released is lot more than required and also being released at a faster rate!! You want the KE to be released along the axis of the barrel, and it's getting dissipated in all randon directions!!

Bennedose, good idea with the car jack! Where did you get the new spring, BTW?

[dev]

Very well done Benne.
Love the spring compressor design.
The other one is where you cut a c clamp and attach it to a length of wood and all that.
I have also had a spring inserted by a friend who did it using body compressor.
The only difference being that he knew when things would go sproing. Long as you know what you were up to.

Regards,

Dev

[bennedose]

Your observations are quite significant.It raises the question as to why more potential energy does
nöt convert into kinetic.
Grand effort !!

Thanks. I suspect that the over-powerful spring raises pressure in the piston chamber so high and so quickly that air tries to escape down the exit hole at a supersonic speed. (over 330 meter/sec). This speed cannot be exceeded by air - and a shock wave like a solid wall develops that I suspect simply taps the back of the pellet and moves it down the barrel with a jerk. The accelerating pellet clearly cannot reach the speed of sound and as the shockwave dissipates a much slower blast of compressed air comes and pushes the pellet down the remaining few centimeters of barrel. The energy is dissipated as a loud audible crack and as superheated air that cools on expansion.

For any bullet or pellet, constant acceleration down a barrel is needed. In air guns a subsonic squish of air can do this better than a powerful spring that acts like a small bomb going off in the chamber. Even propellants for guns do not simply explode at high velocity like RDX - they burn at a constant rate producing gases to propel a bullet faster and faster down the barrel.

[bennedose]

Very well done Benne.
Love the spring compressor design.
The other one is where you cut a c clamp and attach it to a length of wood and all that.
I have also had a spring inserted by a friend who did it using body compressor.
The only difference being that he knew when things would go sproing. Long as you know what you were up to.

Thanks. I was toying with the idea of getting a 1 meter long 1/2 inch diameter threaded steel rod with c- clamps that could be tightened from either side until I realised that my car boot had a contraption designed for a similar job

[bennedose]

Bennedose, good idea with the car jack! Where did you get the new spring, BTW?

Thanks, Spring from Bangalore Armoury.

When I decided that the spring needed cutting I initially tried to use a heavy duty wire cutter I have, but no the spring steel is too hard and I did not want to damage my barbed wire cutter. A local metal workshop chap impressed me by doing it 15 seconds using a rotary power saw

[bennedose]

I have one more report to make in what has turned out to be a happy story for me

My tuned up 20 year old SDB 50 is now as accurate as my IHP 35

sdb-tuned-target.jpg

Power as measured by Chrono Connect Mobile exceeds all expectations. I did not expect a jump from 400 to 450 fps to 600 feet per sec

sdb-tuned.jpg

Power remains marginally lower than both my IHP and the Orion. But on the plus side the SDB is quieter. It fires with a hush and startles me with the rapidity with which a target at 20 meters jumps.

The rear sight of the SDB is crude and there is not a lot I can do immediately. But it has a peep sight that was placed off center at the factory. I will remove it and replace it dead center to finish the tuning work

Thanks to everyone who was encouraging.

[airgun_novice]

What an experience bennedosse. Happy that your SDB did not end up like Humpty-Dumpty.

Thanks for the Pyramid Air link FF2003 - that was an eye-opener and darkly enough, a comic read.

Quite tough for lay man like me to see where experienced gunsmiths are coming from. After experiencing so many differing difficult situations, it's quite natural for you guys to take a few basic precautions for granted; which newbies like me miss at the start. I remember having asked you what all the 'dangers' I might come across before beginning to open my Chini AR and later on the IHP and you having re-directed me to the Mack The Knife thread on lubing of internals and especially asking me to test the spring compressor first. Finally I must confess that I though at first I brought up my car jack and purchased two #3 C-Clamps, I ended up using my tummy compressor. But of course that does not mean newbies can simply push the responsibility on experienced folks, without requesting advice. After all, the first line that comes with any air gun is "Exercise caution. It is not a toy". And that too when the AR is boxed and unopened from any perspective.

[Basu]

Dear benne,
You seem to have achieved more than what you expected.
When I saw SDB first , got a feel that these guns have something to deliver.
A 8.76 fpe is góod power and accuracy ,as noticed with creepy trigger, is quite good.
Enjoy plinking.....

Basu

[Biren]

I have one more report to make in what has turned out to be a happy story for me

My tuned up 20 year old SDB 50 is now as accurate as my IHP 35

sdb-tuned-target.jpg

Power as measured by Chrono Connect Mobile exceeds all expectations. I did not expect a jump from 400 to 450 fps to 600 feet per sec

sdb-tuned.jpg

Power remains marginally lower than both my IHP and the Orion. But on the plus side the SDB is quieter. It fires with a hush and startles me with the rapidity with which a target at 20 meters jumps.

The rear sight of the SDB is crude and there is not a lot I can do immediately. But it has a peep sight that was placed off center at the factory. I will remove it and replace it dead center to finish the tuning work

Thanks to everyone who was encouraging.

Nice Bene.. specially that JACK thing... But now donnt write a blog or post a video:)

Cheers
Biren