Gamo Hunter 440 AS Tuning guide

 Tuning guide for Gamo Hunter 440 AS.

If you have arrived here, then you likely own a Gamo Hunter (or similar). This is mine, and I do love it. Shoots consistently, it's fairly accurate and the build quality for a cheap gun, is to my mind pretty damn good.

But! Now here is the but. These guns are mass produced, and although most gun manufacturers have some smidgen of quality control, some are better than overs. For this price you can't expect too much. Having said that, I own two Gamo rifles and neither are particularly bad.

 

You will find that the imperfections are inside the gun. They don't make it unusable, but the small imperfections can have a effect on accuracy.

Take the trigger for example. The trigger on both my Gamo rifles, are the Gamo SAT (smooth action trigger) trigger. And although they work well and have some adjustment, they can be better.

In this tuning guide, I have put together for the Gamo Hunter (and other similar Gamo rifles), I show you how to polish the trigger, clean up the piston, change the piston seal, smooth out the cylinder and put a new spring in. All these things can be done at home, you don't need to be a gunsmith, you just need to be in possession of a little common sense and a handful of tools.

Before I get started, I do want to be quite clear that the idea of tuning an air rifle, is not to make more power. It can bring back the power the gun had originally, especially if it is old and worn out. But it is to make the gun more efficient and smooth in operation, making the gun a pleasure to shoot.

Also I will list the tools that I have used, in the form of Amazon links.

First job is to remove the stock from the gun, this is an easy task. Just three torx screws, two at the front and one at the rear of the trigger.

Get your own set of Torx drivers here: https://amzn.to/3btyztB

 

Before you get all excited, and rip off the stock. Roll it onto it's left side, there is a piece that is likely to drop out and roll across the floor, never to be found again.

 

 This little piece (not sure of it's name) I call the cocking roller, it runs in the stock, when you cock the gun.

 

Put this piece somewhere safe, you'll need it when you put the gun back together.

Now the stock is off, you will have just the barrel and cylinder assembly. Normally, at this point I remove the barrel. But this gun wasn't giving it up that easily. My Big Cat barrel, comes undone nicely with one screw, but on my Hunter the screw was so tight, I decided to leave it on and take it apart slightly differently to normal.

 Before taking out the barrel pivot bolt, you need to remove the bear trap from the trigger. This is fairly easy, just pry up the spring in the picture, that holds the plastic bear trap in position. Now the bear trap should slide forward, with the metal rod moving forwards on the cocking lever. Twist the rod through 90º, and then slide it out of the gun. Now take out the barrel pivot bolt, and you should be able to remove the barrel and slide out the cocking lever from the cylinder.

Next up, your going to need a spring compressor of some kind. I have a makeshift one that I use, it is just a sash clamp. You can find them here: https://amzn.to/3CMpU1e

It is best to wrap your cylinder in a towel, and probably best to strap the cylinder to the sash clamp. This time, I managed to compress the spring by hand, by pushing the rifle down onto the top of a 14mm socket, and the wife pushed out the pin. There isn't too much preload on the spring, but enough to send things across the room, if your not paying attention.

 Once you have removed the pin from the trigger assembly and released the tension of the spring, you should have something that looks a bit like this

Now you can pull out the trigger, main spring and remove the cylinder.

Here is the piston, with the seal. This gun is only a year old, with about 500 pellets shot through it. You might be able to tell from the pictures that the seal doesn't look that great, it has some nicks in it. This is normally caused when they are assembled in the factory, being pushed into the cylinder that has some very rough edges on it.

So our first bit of tuning is going to be on the cylinder itself. On the openings, you will notice that the edges have quite sharp burrs on them.

For this I got out the trusty dremel https://amzn.to/3pWoVrP you can do this job with a file or even a bit of sand paper. Just take off the sharp edges, as that is what the new seal is going to have to get past, when you put it back together again.

While you have the dremel plugged in, I would smooth off the spring guide that goes into the piston. On this gun, it was already quite smooth, so I didn't worry too much about it. However on my Gamo Big Cat, the guide was quite rough, with fairly large casting groove's on the sides. I smoothed those off, which should allow the spring to move nicely.

I then turn my attention to the piston and the seal. I have a new seal to fit, and getting the old one off is as simple as getting a screwdriver under it and popping it off. Putting a new one on is another story.

With the seal removed, check the piston to see if it is smooth. On this gun it was quite good, the main points of contact are at the front and back. I normally just clean these areas up with some wet and dry paper, ready for some grease later on.

 

Getting the new seal on can be a bit tricky, here I have an original Gamo seal. It is fairly hard and not very pliable. Best way I have found is to pop the new seal in a cup of very hot water, and leave it for about five minutes. It seems to soften it up enough to be able to get the seal onto the piston. Make sure it is seated correctly all the way round, and then I grease up the ends of the piston, ready to go back into the cylinder. 

Be very careful not to get grease onto the front of the seal, it will cause dieseling (NOT GOOD)

All set for putting the gun back together. I normally do some trigger polishing at this point, but I have a trigger assembly that I have already polished, and that is going into this gun. If you are interested you can see the video of that job here: https://youtu.be/dUgxlS8yK8w

The first thing I do when re-installing the piston into the cylinder, is to find some non flammable oil. This is to lubricate the seal, as it goes into the cylinder. Non Flammable, is used to prevent dieseling. Dieseling your airgun from this side of the seal is not good, and will only cause damage.

Slide the piston assembly into the cylinder, making sure that the seal doesn't catch on the sides of the cylinder. Use a small screwdriver to tuck in the seal when it reaches the cut outs.

Put some grease on the spring (I am fitting a new spring), and make sure you put in the spring guide, which goes into the piston first.

Once you have those back in the cylinder, place the trigger assembly on the spring, and compress the entire contents together and then push the pin through the trigger to hold it all together.

Slide the cocking lever back into the slot on the cylinder, replace the barrel pivot, and then the gun is almost back together. 

Last thing to do, apply some grease to the cocking roller. That little bit I told you about at the start. I put some grease on the post, put the roller on and then some grease onto the roller. Then to make doubly sure of a smooth cocking action, I put some grease on the inside of the stock where the roller runs.

Now, put the end cap on the cylinder, just behind the trigger assembly and replace the stock. Make sure you get all three screws started before you tighten them down.

And then, last of all! I try to pump any excess oil from the gun, by pulling the barrel back, but not letting it cock. Just keep going backwards and forwards with the barrel, you will hear oil coming out of the transfer port. Wipe any excess oil and grease from the gun and then it's happy shooting.

After all that has been done, it is normally good practice to fire a fair few shots through the chronograph. This way you can tell if any of your improvements have taken the gun over the legal limit for the country that you reside. If it comes in a bit high, shoot some more, as the gun will settle down. But if after all that, it is still too high, you can strip the gun down again and remove some coils from the spring.

Hopefully, if you have got this far, you might have found this blog useful. I'm not stating that this is the best way to tune an air rifle, just that, this is how I do it, and it seems to work.

My guns are smooth in operation, which makes for a more accurate gun.

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