Tuning the FX Impact for slug ammunition

Long-range hunting expert Lee Watts explains how to tune the FX Impact for optimum performance with slug ammunition.

Everyone is talking about tuning airguns to shoot slugs. But what do you do if you want a slice of the action but don’t know where to begin? It isn’t a simple process, but it needn’t be prohibitively complicated either. Read on and I’ll do my best to share some of the tricks that I have picked up during my own experimentation with airgun tuning.

There are plenty of airguns that can be tuned to shoot slugs remarkably well at long range. My first choice is an FAC-rated FX Impact, so that’s what I’ll be focusing on here – mine is a MkII, but the principles are the same for the new M3 model. Tuning Impacts can be a daunting task for anyone trying it for the first time, but once you get your head around what each component does it becomes an enjoyable process which can bring some remarkable rewards.

Before making a start with any airgun tune, the most important thing we need to ask ourselves is what we want to achieve. Are we shooting long-range or close? Is it even safe to shoot slugs in the area we would like to use them? What is our quarry? Although the tuning process is broadly the same for all applications, the power of the gun and the design and weight of the slug we choose will influence what we are trying to achieve and how we do it.

Not only are the above questions very important for the obvious safety reasons, but they will also save you a lot of time and money by avoiding having to keep changing ammo or re-tuning your air rifle again and again to suit different projectiles or shooting scenarios.

Most of my shooting is done on a large farm with a lot of open fields. This makes it hard to get close to my quarry – which is made up mostly of skittish rabbits and wary crows – and windy weather can also be a real problem. 

With this in mind, I have tuned my Impact to work with a fast, heavy slug that gives great long-range performance out to 120m and also really combats the wind. This setup is great for open ground, but wouldn’t be at all suitable for use near buildings or public rights of way. 

One myth about slugs that I am eager to dispel is that you have to shoot them at extremely high speeds to get them to shoot accurately. This is not the case at all; a well-balanced tune, for the right situation, using the right slug or pellet for your barrel is essential, but it is more about finding a sweet spot than trying to make projectiles travel at outrageous speeds.

What You Need

You need a few things before you can crack on with a tune, and a very important one is the right barrel or barrel liner because slugs don’t shoot to their best through all barrels. 

FX’s Superior liner, either in standard format or heavy for big slugs, comes as standard with FX barrels now and is what I use on my .22 calibre setup.

Another very useful tweak is a slug power kit. This aftermarket kit made by FX and other companies includes a specialised spring and washers to provide more compression and more power to match higher regulator pressures, although it is not essential for pellets and lighter slugs. Do not attempt to fit these components if you’re not confident about it – your local gun shop should be able to do it safely and effectively.

A chronograph is an essential accessory for airgun tuning as you need to know how fast slugs are travelling and whether they are doing it consistently. You will also need a decent set of Allen keys, some Loctite Blue and a variety of slugs – I take .217 and .218 for .22 and make sure I also have them in a selection of weights. All barrels are different and finding the correct size and weight of slug is a matter of trial and error, but it is well worth making the effort.

It’s important to have all essential kit to hand before you start your tune – it’s a lot to take into the field, but you’ll be glad you’ve got it

The Tune

I start by having the hammer spring dial on the Max power setting and also maxing out the dial’s adjuster screw. Using a 1.5mm Allen key, I turn the screw inside the cam until it just stops at the end of its travel. This can sometimes cause the gun to not cock; just back it off half a turn at a time until your rifle cocks as normal if this happens. The front valve is wound out to notch number 4 so I’m also getting maximum power and have the full range of adjustment to use.

Regulator pressure needs a little more thought. Mine is set on 120 when using my favoured 30-grain .22 Wildman Dish Base slug, which I like to shoot at around 990fps. I know from my own testing that this speed is accurate, but you will need to find out what works best for your slug/barrel combo by doing your own experimentation.

Let’s assume that we are using an FAC-rated .22 calibre Impact. We have the regulator set at 120bar and have maxed out the hammer spring and front valve, but are not getting near the speed we want. We will try increasing the regulator pressure by taking a 2.5mm Allen key and turning the regulator screw anti-clockwise one quarter of a turn until we reach 130bar. The gun should be unloaded and uncocked for this.

The regulator adjuster screw on the FX Impact is positioned behind the trigger blade

It is now time to fire five or six shots over the chronograph – this is the minimum number of shots required for a reliable reading as each adjustment needs time to settle. If the speed is still below what we want, we simply up the regulator pressure to 140bar and repeat the chronograph test. Do not increase regulator pressure beyond 150bar on a standard setup.

Now we are getting 1,020fps. That is quite some power, but I’m happy with that as I like to go 20 or 30fps over my desired speed so I can use the front valve to bring the speed down and improve consistency – but first we need to balance the hammer spring tension to the regulator pressure.

Take your 1.5mm Allen key and loosen the hammer spring screw one turn and then fire over the chronograph. Repeat the process until you notice a dip in the speed on the chrono – remember to take five or six shots after each adjustment to allow it to settle in from each alteration. Once the speed dips, retighten the hammer spring screw until the power just peaks. This is the perfect hammer spring tension for the regulator pressure and everything is beginning to work in harmony.

The Final Stage

Now on to the front valve. Starting from the fourth notch setting we selected at the start, turn the knob in a quarter of a turn at a time and shoot five or six shots over the chronograph each time. 

Repeat this process and you will notice the velocity of the slug slowly decreasing to where you want it to be – shot-to-shot consistency will tighten in a sweet spot. Once you find that sweet spot around your desired speed, it’s time to put some rounds on a target and confirm that the rifle and slug are working together in harmony.

It’s important to fire a string of test shots to assess the changes brought about by each adjustment you’ve made

As we have tuned the rifle with the power wheel on maximum, we still have the whole range of its settings to try if accuracy isn’t perfect from the outset. 

Shoot test groups at each setting and stick with the one that gives the best results. After making this selection, you may even want to do some fine-tuning with further small adjustments of the front valve.

When you are 100 percent happy with your results, add a dab of that Loctite Blue to the threads on the hammer spring adjuster screw. This will stop it from working loose and undoing all of your hard work. If you then decide that you want to re-tune later on, it will still work free under the simple turn of an Allen key.

Tuning isn’t a quick fix, but the time and effort will be rewarded with great performance and I am sure you won’t look back. The process of tuning for pellets is exactly the same, you just don’t need to change any of the factory rifle parts. Good luck and enjoy! 

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