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S410 / S400 trigger tuning by Beer Hunter

The Air Arms S400 series trigger is good out of the box, but with a little effort it can be made great!

Below you will see how I tuned mine to operate better than I could have hoped. However (here comes the disclaimer!), please note that it may not work for you, it may cause your gun harm / fail all together or indeed make it dangerous. So proceed at your own risk!

Basic tuning

Firstly take your 4x0:

and remove the stock by undoing the allen bolt marked to reveal the action:

Next remove the trigger guard (allen bolts 1 and 2, remember to catch the spring which pops out) and the trigger cover plate (2 flat head screws above). I'll come back to the allen bolts A,B and C later.

This will leave you looking at this:

Here you can see the trigger blade, top and bottom sear, hole where the trigger tension spring was (A), rubber o-rings over the sears (B) and top sear spring (C).

Remove all the parts by simply lifting them out:

First thoroughly clean all the gunk from the parts. The numbered areas are the ones which you may wish to polish up to a higher standard than comes from the factory. For which you will need:

Yep, a Dremel with a soft polishing pad, Brasso and toothpaste! First polish lightly with Brasso and Finish with toothpaste. Toothpaste is very good for leaving a glass finish. When polishing the sears, less is definitely more! All you want is a smooth finish, don't start rounding the edges!

Once you have finished any cleaning and polishing, reassemble the trigger blade and sears using a light smear of oil (I use 3-in-1) on all the surfaces - wet is way too much, they should just feel greasy in your fingers.

Just remember to set the top sear spring below the sear. Push back in the main trigger spring and replace the trigger guard. Pull the spring up on the bottom sear so that is slightly clear of the action and therefore does not rub when operated.

Now, before you replace the trigger cover plate, is a good time to have a play with the trigger settings as it is easy to see what each adjustment does.

The final stage of the reassembly is to replace the 2 rubber o-rings (again smeared with oil) and trigger cover plate. Do NOT over tighten the trigger plate - all you want to do is nip the screws. Over tightening just presses the rubber o-rings harder against the sears and can make the trigger feel sticky.

Trigger adjustments:

Looking at the second picture from the top, there are 3 adjuster screws designated A, B, C.
(A) located in front of the trigger within the guard does weight of pull. Clockwise to increase weight. Too much and rifle won't fire as spring becomes coil-bound.
(B) is the foremost adjuster on the trigger blade. It determines the total length of trigger travel. Clockwise to reduce travel.
(C) is the rearmost one on the trigger blade. It adjusts the second stage engagement. If first stage screw (B) is incorrectly set this will have no effect.
First set the trigger to go off on the first stage only by screwing the 2nd stage adjuster (C) well out, then set the first stage for weight (A) and length (B) to your preference. After that, slowly reintroduce the 2nd stage (C) until you can just feel it and then give it a further eighth of a turn for a very light second stage (this may be unsafe!), a quarter turn for a good safe trigger with little or no creep or a full half turn for a very safe “hunting” trigger.
Your last adjustment must be on the 2nd stage as touching any of the other screws stuffs all the settings.

Further tuning:

Several further steps can be taken. Probably the least effective is polishing the bottom of the top sear where the top sear spring rubs against the sear. Any benefit has to be marginal, but you are in there anyway :-) The surface is quite rough though - see the picture below where it has been polished on the left.

Make a spring guide. The spring in the AA S4x0 triggers tends to deform while under tension which causes it to scrape against the action. The deformation can also lead to the trigger pressure feeling less than linear as you pull it - often with what feels like a jump to the second stage at the end of the first. If, like me, you like you triggers light, this can be dangerous and lead to the gun discharging unintentionally.
After playing with several different diameters, I found the best results were achieved with a fairly thin polished picture nail (significantly thinner than the internal spring diameter). This nail has a rounded head which sits neatly on top of the spring acting as a top hat (thanks to "Philip W" on the BBS for the tip).
I also found the hole in the action that the spring sits in to be fairly constrictive and poorly finished, so this was opened up marginally and polished. You can see this on the 3rd picture from the bottom when compared with the 3rd from the top. With the trigger already roughly adjusted, cut the nail down a tiny amount at a time, refit the spring and trigger guard then try it. Get it just right and the guide also acts as a over travel stop.


Now the big one!

Friction on any moving part is the biggest problem in producing a nice trigger. One of the places friction is most apparent is in the sears dragging against the action. Just open yours up, disengage the springs and move them over the surface and you will see what I mean.
So, to eliminate the friction you need to minutely raise the sears above the action using very thin washers under each sear. To get rid of the rubber o-rings which are pressed onto the sears when the trigger cover plate is replaced, also use some on top of the sears. To give the plate clearance you may need to also add some to the screws between the plate and action.
The only problem is finding washers small and thin enough to work. I looked everywhere for PTFE or nylon ones, but no one could supply them thin enough. In the end I used tiny fabric washers which seem to be just fine. These were bought from a model shop.
You must ensure that all the sears line up EXACTLY and that the washers above are level. I used:
- one large above and below the top sear
- 2 small below and 3 small above the bottom sear
- one small below and one small above the trigger blade
Your mileage will vary depending on the tolerances of your gun and washer thickness.
Be warned, even with these tiny adjustments I found that the trigger plate screws caught on the stock during reassembly. This necessitated inletting to the stock - see the bottom pic.

Hope that helps!