Correcting the danger of air rifle "Auto Fire"

[bennedose]

My 25 year old SDB 50 air rifle has been used to shoot over 5000 pellets and for the past few weeks it had been showing a tendency to "auto-fire". The cocked barrel would suddenly release itself with an almighty jerk and it actually auto fired twice as soon as the barrel was shut.

Auto fire is very dangerous because
1. Sudden jerking jump of barrel can hurt someone - either by crushing a finger of breaking a tooth
2. If the pellet fires off after the barrel is shut anyone in the path can get hurt
3. The sudden jerk of uncontrolled barrel closure can make one lose one's grip on the rifle and it can fal. This actually happened to my rifle and the rear sight got bent. I was lucky to be able to straighten it satisfactorily.

Today I removed the trigger and found that the sear was worn and an earlier repair effort had chipped the sear (see white triangular chipped metal in the higlighted rectangle)

sear01.jpg

I took it to a local metal workshop and got a blob of metal welded on to the sear.

sear02.jpg

This is what it looked like after the weld

sear03.jpg

It took me a lot of trial and error filing after that to get the trigger just right. I had to file, fix the trigger, re mount the receiver on the stock, trial shoot and undo everything several times before I got it right. I filed off bits that I thought were causing the trouble.

sear04.jpg

The trigger is holding much better now but I need to shoot at least 100 to 150 shots to be sure. If the problem recurs I will remove the piston and do a similar thing with the piston sear.

[pratik_mahale]

good job
but I think it will not last long because you need hardened metal for trigger sear

[essdee1972]

My SDB, post tuning, has the same issue. In fact the first "auto fire" went through my finger! (my bad, I was holding the barrel at the muzzle end due to the excessively high force required for cocking). Is that a congenital issue? Bad metal or bad HT of the sear can cause higher wear and tear than normal.

Love the Orion - two fingers very close to the stock are enough to cock and close the breech.

[mercury]

if that "yellow blob" is brass.....I will go with Pratik's observation. brass is easy to work on to get a good finish , but will not have the strength of MS. it will also tend to wear off with the continuous friction. getting all that brass off to re weld another bit on to the block...another head ache !

depending on how frequently shoot....you will have an "auto fire" repeat in the near future.

[bennedose]

The yellow discoloration is flux on steel, not brass, because filing turned it grey-white, but both of you are right. The problem recurred after about 50 shots. So much for this solution...

My poor old SDB may have reached the end of its life. The rear sight issues were barely tolerable only because of smooth cocking and pleasing and consistent accuracy. No complaints. I even used and banged up a Weihrauch till it was no longer any pleasure to me - so I need to kill a different air rifle now

[bennedose]

Love the Orion - two fingers very close to the stock are enough to cock and close the breech.

The Orion is also the most powerful of my air rifles - and I have tested accuracy and useful power at 25 meters.

[pratik_mahale]

Weld a piece of drill bit to the trigger sear

[brihacharan]

[quote="pratik_mahale"]Weld a piece of drill bit to the trigger sear[/quote]

> This is a good solution....
> However if the tail end of the piston rod which has a "Sear" is also of a poor quality of steel, then taking care of the trigger sear could be a short lived affair...
> IMHO - Weld a drill bit on to the piston sear & get a new trigger piece machined out of high quality steel...
> Make sure the angle of engagement of both sears is adequate to hold on till the trigger is released for firing.
Briha

[bennedose]

Weld a piece of drill bit to the trigger sear

Interesting idea. I bent a drill bit while drilling through a clay pot last Diwali. Maybe I will use that - but this time i will have both the piston sear and trigger sear welded and buy me a new file. It should take me hours of labor - so this is becoming a "major project"

[pratik_mahale]

You can’t file a drill bit with any file
use sharpening stone or 60/80 grit metal polish paper

[bennedose]

You can’t file a drill bit with any file
use sharpening stone or 60/80 grit metal polish paper

LOL. Thanks for telling me. This looks like a project that I must not take up without adequate preparation - phew.

[essdee1972]

Bennedose, if I may give a suggestion ............ GET A NEW AIRGUN!!!!!

[Basu]

Auto-fire takes place when the engagement between piston sear and trigger is not in 90° angle.
All you need to do is to get the angle back for both the parts.
you can do the by rubbing it on 220 grit ,kept on a
flat file.
The sear will not take much time to come back to exact angle but trigger will consume longer time.
While doing that ,one need to be very careful to restore the 90° angle by putting appropriate pressure to to required angle.

Basu

[bennedose]

The interesting thing that I discovered while looking for various grit sizes of sandpaper is that they are imported from the US. It it such a high tech affair to make it in India - or am I mistaken?

[brihacharan]

The interesting thing that I discovered while looking for various grit sizes of sandpaper is that they are imported from the US. It it such a high tech affair to make it in India - or am I mistaken?

> Various grits of sandpaper & emery (paper backed & cloth backed) are made in India & are available in most Hardware stores.
> The manufacturer's name may sound 'foreign' as these are parent companies who have set shop in India to manufacture them.
> Two of the major companies are:
1. Carborundum Universal (The Chettiar Group of Tamil Nadu are major stockholders)
2. Grindwell Norton
> These companies are involved in manufacturing abrasive products for the 'Metal working & Wood working' industries,such as:
1. Grinding wheels
2. Sanding discs
3. Various grits of sandpaper
> Besides these there are other local abrasive material manufacturers - however the above two are well known for their quality .
> Now coming down to the "Interface Angle" between the sears (Piston rod & Trigger Sear) - 90*(deg) is the optimum angle for a perfect engagement...
> If so "Too Perfect" an angle of 90* in all probability may not result in easy "dis-engagement" when the trigger is released...
> Hence it is desirable to have 88* - but then creating this angle is totally "Skill Based"... and there is no way to measure this with precision!!!!
> My experience has been to use a "Jeweler's File" to create this minuscule difference, which I have accomplished successfully in my Samurai 3G - The trigger pressure being around 2 1/2 lbs (Crisp & Smooth)
Briha