Friday, December 6, 2013

Hammerli 850 Trigger Job

The Hammerli 850 is a great air rifle nearly ruined by a god-awful trigger.  Previously I posted about making a steel trigger for this rifle.  That was all about personal preference; it did nothing to improve the trigger pull.  The stock trigger pull isn't that heavy, it breaks at a reasonable 2.25 lbs.  But the pull is a very long and very mushy single stage that feels much heavier.  Mine was adjusted with no pre-travel, followed by 4 mm of movement that felt like you were pulling the trigger through chewing gum, and then another 11 mm of over-travel after the sear released.  There was absolutely no indication when the sear would release making the trigger very unpredictable and distracting.  And the over-travel essentially guaranteed that you jerked the trigger every time.  There is a useless adjustment screw but it only changes the amount of pre-travel.  It does nothing to change the pull weight or length of pull.  Time to work on the trigger.

Manual Safety

Before I worked on the trigger I had to do something about the safety.  It automatically engages whenever the gun is cocked.  This is not only a nuisance, but it breeds bad habits.  Engaging the safety should be a conscious decision.  Since 99.9% of the guns in the world have manual safeties, you do not want to get into the habit of assuming the safety will engage by itself.

Part that needs to be removed

Luckily this is an easy fix.  The safety block has a plastic tab that the bolt pushes on when cocking the gun.  Simply cut off the tab and you have a manual safety.

As an added bonus the bolt operates a bit more smoothly now without having to push on the safety.  Now to do something about that trigger.

Over-travel Adjustment

The worst aspect of the trigger is the over-travel.  Eleven millimeters may not sound like much, but it's huge!  To eliminate the over-travel I added a screw that limits the movement of the sear.  I drilled and tapped a hole for a 5-40 screw right where the sear spring is located.  I cut the head off the screw to make a threaded rod.  Then I carefully ground off a little bit of the screw and tested the trigger.  I repeated the grinding and testing until all of the over-travel was removed.  Then I used some Loctite to secure the screw in place.  I ended up with the screw protruding 5.5 mm above the sear.

screw added to sear

Adding a Second Stage

As I mentioned, there is 4 mm of movement needed to trip the sear.  Unfortunately, much of that movement is the sear pushing the hammer back against the hammer spring.  That accounts for most of the pull weight and trigger creep.  There is nothing that can be done about that unless you are willing to change the geometry of the sear and hammer faces.  Since they are die-cast in soft metal I didn't feel comfortable messing with that.  Luckily, someone came up with a way to add a second stage to the trigger.  What this does is, it allows you to put most of the pull weight and "mush" into the first stage, then just a little more movement is needed to trip the sear in the second stage.

I checked my supplies and found a length of 2.36 mm stainless steel drill rod.  I cut it to length and just placed in under the trigger to test it out.  It looked like it would work so I cleaned the parts in acetone and epoxied the rod to the trigger with PC7.

rod epoxied to trigger

During reassembly I burnished some molybdenum into the trigger and sear contact points and oiled the pivot pins.  The results were amazing.  Now the trigger pull is as follows.  There is no pre-travel, a very smooth 5 mm pull through the first stage, followed by a definitive stop before the second stage, then < 1 mm of movement to trip the sear with absolutely no over-travel.  Pull weight is now just 1 pound.

the finished trigger and sear

The original adjustment screw now determines how much of the trigger movement is in the second stage.  Turn the screw all the way in and you end up with a long second stage pull.  Turn the screw out and you transfer the trigger movement to the first stage.  Turn it out far enough and the sear trips in the first stage, giving you a single stage trigger again.  I have mine adjusted with the screw about one and a quarter turns out.

The trigger is so much better it makes the rifle much more enjoyable to shoot.  Now I can concentrate on the sights and not lose focus wondering when the gun is going to fire.  Don't get me wrong.  This is no match trigger, but I certainly can't blame any bad shots on the trigger anymore.

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