Frequently asked questions

The GK1 puts the heart of Huben's K1 bullpup rifle into a pistol. It is a semi-automatic, pre-charged (PCP) air pistol, and almost every part inside is the same as in the K1. What changes is the layout needed to fit it into a grip, and where the tube is filled. So if you have used a K1, the GK1 will feel familiar in your hand.

The main figures:

	GK1	GK1 Mini
Barrel	210 mm	150 mm
Tube	85 cc	49 cc
Weight	1.20 kg	0.96 kg

You can fill the tube up to 350 bar. The magazine holds 19 rounds in .177 and .22, 17 in .25 and 14 in .30.

It is a deliberate choice. In a PCP the regulator is usually the part that causes the most trouble, and in such a small tube it takes up a lot of room. So how do the shots stay consistent without one? The secret is the closing valve: it shuts early when pressure is high and a little later when it is low, so it balances itself.

Most PCPs have a hammer that strikes the valve; this one does not. Instead, two valves do the work: the firing (opening) valve right behind the pellet, and the vertical closing valve below it. When you pull the trigger, the opening valve is released. How much power you get and how much air you use is set by the closing valve. One more interesting detail: the gun fires straight from the magazine, so the pellet passes through the magazine into the barrel.

No need; all three do the same job in the same way. The differences are small:

• V1: The magazine lever is at the bottom, as on the K1. It can get in the way of a left-handed shooter's trigger finger.
• V3: The lever has moved to the top.
• V3+: The firing valve is now all steel, and there is a longer spring guide with a softer spring. Thanks to this, the magazine stops turning a little before the valve stops working, which is a nice safety improvement. The magazine lever is bigger and the bottom of the grip is a little shorter.

A small note: according to Huben, the "version" names on the K1 were given by distributors, not by the manufacturer. If you give us the serial number when ordering spare parts, finding the right part is much easier.

350 bar at most. The gun works fine if you fill less; you just get fewer shots and slightly less consistency. If you are shooting at high power, there is little point filling below 300 bar.

The manufacturer asks you not to go below 120 bar (12 MPa), because jams and leaks can appear below that. We suggest stopping at 150 bar and refilling. Because the tube is small, at high power you go from 150 to 120 in one or two shots. The valve stops working a little below 100 bar anyway.

Less than you might think. The GK1 reaches 320-340 bar in 60-80 strokes. Because the tube is small, a hand pump is really practical here; you can go to the range with a pump instead of a tank.

Yes. The screw just below the trigger guard holds a one-way fill valve, and the same screw sets the fill speed. Screw it all the way in and no air gets in at all. Best is to turn it all the way in and back it off a quarter turn, or leave it flush with the surface. If you leave it too loose, filling is faster but the small tube gets very hot, which is not good.

Roughly, but not exactly; it can be off by about 10 bar. A pistol filled to 350 bar may show 340. So we suggest you base your routine on your shot count rather than the gauge.

The official power ranges are: 6-30 J in .177, 6-50 J in .22, 6-60 J in .25 and 6-70 J in .30.

Tests show higher figures too (for example ~118 J on the first shot from a GK1 .25). But those numbers come from special conditions: power fully open, a 350 bar fill, a heavy slug and only the first shot. In everyday use, at the recommended setting, a .25 stays around 70 J through a full magazine.

There is a small hole on top of the grip. Insert the 3 mm hex key from the box and turn: clockwise lowers the power, anticlockwise raises it. With the tube full the screw turns a little stiffer; that is normal. We suggest adjusting in quarter-turn steps.

This is not a fault. The pistols we sell in Türkiye come from the factory with the power reduced and set to 7.5 joules. That is why you may not see a clear difference when you turn the power screw.

Avoid two things and the rest is easy:

• Using heavy, tight-fitting slugs at very low power. This can cause jams.
• Turning the power far up at low pressure.

In short: high power goes well with high pressure.

Your pistol will tell you. If you hear a blast of air even with a moderator fitted, or the recoil has clearly increased, the valve is staying open longer than it should. Turning the power down even a little increases the shot count surprisingly.

It can change a little. The closing valve wears slightly with use and lets a little more air through as it wears, so power and air use rise slightly. Checking with a chronograph now and then is a good habit.

Usually not. It comes set from the factory and is fine for most users. If you still want to set it to your liking, keep the following in mind.

Behind the trigger there are two screws that turn with a 2.5 mm hex key:

• The right-hand screw sets the weight of the first stage. It can be adjusted easily with the safety on.
• The left-hand screw sets the weight of the second stage. You will need to dry fire while adjusting it. First make sure the magazine is empty, fill the tube to 250-300 bar, set the power to minimum and, for the sake of your ears, fit a moderator or wear ear protection.

The pull length cannot be adjusted, only the weight.

The trigger does not reset and the valve starts opening and closing over and over. Since the magazine does not turn, no pellet is fired and the pistol is not damaged, but it is not pleasant and not safe either. If this happens, back the screw off straight away. Our advice is to leave the trigger a little heavy. After adjusting, fill the pistol fully and test it at your usual power.

The magazine does not come out; you load it in place:

1. Lift the lever. The magazine will turn freely in one direction.
2. Push the pellets or slugs in one by one through the plastic retainer.
3. Do not let go of the lever when closing it. It is under spring pressure; lower it slowly with your finger.

Opening the lever with the tube full can feel a bit stiff. The V3+, with its bigger lever, is easier here.

11 mm. A longer round sticks out of the magazine and stops it turning. In .30, the heaviest slug that fits this limit exactly is about 68 grains.

These stood out in testing:

• GK1 .22: JSB Monster (Redesign) is among the most accurate. JSB Beast 34 grain is the longest pellet that fits the magazine.
• GK1 .25: Slugs of 0.252 inch (6.40 mm) diameter sit better in the magazine.
• GK1 .30: The ZAN 56 grain pellet is a good choice.

These are user tests, not Huben's official recommendation. Every barrel has a pellet it likes; we suggest trying a few and finding the one that suits yours best.

No, but leaving the chamber in front of the firing valve empty is a better habit for both safety and jams. Just before shooting you turn the magazine a click or two to bring a round forward, that is all.

One more thing to watch: small-diameter pellets can partly drop into the barrel while loading. If that happens (usually around the 16th pellet), stop loading and start shooting. After loading, it is also a good idea to turn the magazine one full turn and check it rotates freely.

The most common cause is a finger touching the magazine while shooting. When the magazine cannot turn fully, the pellet catches on the edge of the barrel. So keep your fingers away from the magazine when firing.

If it still jams, turn the magazine slightly backwards to line the pellet up with the barrel and fire. If that does not work, do not force it; bring it to us or an authorised service centre.

It is also worth knowing that magazine jams caused by misuse, and parts broken as a result, are not covered by the warranty. Touching the magazine while shooting, using rounds longer than 11 mm and letting go of the lever while closing it are the most common causes.

It is a third-party accessory and we find it very handy. You can load without pressing against the spring-loaded plastic retainer, so the pellet is not crushed.

Because it is powerful, semi-automatic, and its magazine does not come out. Versions sold abroad can go above 100 joules. It is also short and light compared with a rifle, so it is very easy for the muzzle to point the wrong way. A few simple habits bring this risk close to zero:

• Do not store the pistol charged.
• Keep it on safe when not in use.
• Look into each chamber of the magazine to make sure it is empty.

This happens when the pressure drops to the valve's working limit. If the air and the tube are cold, you reach this limit sooner.

There is one thing to be careful about here: the trigger is already pulled and the safety does not fully engage. As the tube warms to the ambient temperature, the pressure rises a little and the pistol can fire by itself a few seconds later. At such a moment:

• Keep the pistol pointed in a safe direction and wait.
• If it fires, the problem has solved itself. If not, after a while it can be put on safe.

The way to never end up here is simple: refill before the pressure gets very low.

In a way, yes. The magazine stops turning at a pressure a little higher than the one at which the valve stops working. So the last shots are most likely fired on an empty chamber. On the V3+ this margin is even wider.

Yes. Every GK1 comes with two muzzle caps: one plain, the other with a 1/2-20 UNF thread. Most moderators on the market fit the threaded one. Huben's own moderator is sold separately: 130 mm long, 26 mm in diameter, 67 grams, and it does not block the open sights.

In .177, .22 and .25 you need a 7 mm hex key (8 mm in .30). This key is not in the box and is rarely found in standard sets, so we are telling you now.

The barrel sits about 5 mm inside the cap. If you push the key all the way in, it can hit the crown. Touch the bottom first, pull back a little, then turn.

That comes from vibration and is normal. Checking its tightness by hand now and then is enough.

The safest way, with no disassembly, is dry firing: make sure the magazine is empty, set the power to minimum and fire in a safe direction. Do not fire back to back; leave a little time between shots.

If you need to empty it completely, there are three ways:

1. Loosening the Torx 15 screw on top. We do not recommend this. The screw has no seal, and removing and refitting it often can wear the thread in the aluminium body over time.
2. Loosening the fill screw under the trigger guard. The O-ring comes out of its seat and the air escapes. The O-ring is cheap; fit a new one if needed. This is the most practical way. When refitting, remember the fill speed setting (see Filling and pressure).
3. Removing the firing valve through the back cover. Lower the pressure first, set the power to minimum and hold a folded towel behind it.

Absolutely not. The tube can only be unscrewed and removed when it is completely empty. If you try to turn it under pressure, it will fly off.

The opening valve sits right behind the back plate. When removing the plate, the pistol must be on safe, or the valve can shoot out.

No, leave them alone. These two screws keep the barrel lined up with the magazine. You only need them for alignment.

You can do it without disassembly, with a cleaning rod from the muzzle end. Stop the rod before it enters the magazine, because the brass-tipped valve is right behind it. If you can remove the barrel, cleaning from the back is better. If your accuracy has dropped, first try changing the pellet or cleaning the barrel.

Pure silicone grease. When reassembling, a thin coat on the valve and the inner surface is enough. Silicone spray also works, but grease is better. Stay away from petroleum-based oils; they damage O-rings. There is no need to keep oiling it in use.

An old method that works: put a drop or two of water with a syringe where you suspect the leak. Where bubbles appear, that is the leak.

The most common leak points:

• Hissing beside the magazine: The magazine indexing pin and its O-ring. This is the most common fault, and a spare comes in the box.
• Air coming from the barrel: The front seal of the firing valve. It has two parts (a metal ring and a plastic one) and is changed with a special tool.
• Air coming from the trigger side: The rear O-ring of the firing valve. Rare, and fiddly to replace.
• Behind the gauge: A very small O-ring; a spare is in the box.

Usually loosening the screw a little and tightening it again solves the problem. Do not overtighten. If needed you can use blue (removable) thread locker; do not use red, or you will not get it out later.

This is a job that needs patience and the right tools. Removing and fitting the front seal needs tools of the right size. If you work with something sharp and scratch the inside of the action, you may have to replace the whole action. If you do not have the tools, we suggest bringing it to us.

Faults usually appear in this order:

1. Most often the front seal.
2. Then the seal between the two parts of the valve. This is not replaced on its own; the whole valve is replaced.
3. Least often the rear O-ring.

Do not start taking it apart without the spare parts in hand.

The barrel's alignment is held by the two screws underneath. These screws are loosened, a pilot rod of the right calibre, preferably brass, is pushed from the magazine into the barrel, and then the screws are tightened in turn, a little at a time.

Good news: with Huben's new polygonal barrels there is no need to line up the magazine grooves with the barrel rifling separately. That was needed on the old Lothar Walther barrels.

As the name says, the Mini is a little shorter and lighter: 150 mm barrel, 49 cc tube and 0.96 kg. The action, box and case are the same as the standard GK1.

The surprising part: although the barrel is about 30 percent shorter, velocity drops by only about 10 percent. The energy loss is about 19 percent.

Because the K1 fires straight from the magazine, the pellet's head can get slightly damaged as it passes from the magazine into the barrel. Two simple steps help:

1. Push the pellet all the way forward in the magazine.
2. Do not use more regulator pressure than you need.

For setting up, you can follow this route: open the power wheel fully, set the regulator about 10 m/s (30 fps) above your target velocity, then come down to the target velocity with the wheel. Fire a few shots after each adjustment; that is the only way the regulator and wheel settle.

These settings were tried on an older K1. Models with the new barrel may need different pressures.

Tested settings:

• About 100 bar for light .22 pellets. Below that the magazine may not turn.
• 120-150 bar for heavy .22 pellets.
• About 130 bar for .25 pellets.
• Around 200 bar for heavy .25 slugs.

The Huben regulator tends to raise its output slightly as tube pressure drops.

Yes, with an end cap with a gauge, sold separately. We can fit it for you if you like. If you fit it yourself, empty the tube completely first. The end cap is removed with a flat tool 27-28 mm wide and 2.5-3 mm thick. Grease the new cap with silicone and screw it in until it is flush with the surface. If you hear hissing for 15-20 seconds on the first fill, do not worry; it is normal.

The K1 has no decock; nothing holds the valve except the trigger and the safety. So before cleaning, do this: turn the regulator all the way down. Past a certain point the air in the regulated chamber escapes beside the screw, while the main tube stays full. In this state the gun does not fire even if you pull the trigger.

Then clean the barrel from the muzzle end with a fixed-tip cleaning rod, pushing it in and pulling it back; there is no access from the rear. After setting the regulator again, check with a few dry shots that the pressure has settled.

This method was shown on the first K1 version. It may not apply if the regulator setting is different on newer versions.

On the first K1 version the regulator could be set between 110 and 220 bar. Below 110 bar the magazine did not always turn and air leaks could appear after the shot. Within this range, no misfires were seen.

In most PCPs, tube pressure keeps the valve closed and a hammer strikes it open. In the Huben it is the other way round: the pressure wants to open the valve and the trigger mechanism keeps it closed. With no hammer, the valve opens and closes very quickly, and more energy comes out of the same pressure. There is a price: decocking is not possible and the trigger is a little heavy.

Huben first used Lothar Walther choked barrels. From 2020 it switched to its own unchoked barrel.

• 2020 .25: Slow twist (about 1:28). Gets on very well with pellets like the JSB King Heavy MK2. Unstable with boat-tail slugs; we do not recommend them in this barrel.
• 2021 .22 (Wood SE): About 12 grooves, 1:17 twist. Very good with turned Altaros 35.3 grain and MP Molds v3 slugs; weak with standard JSB-type slugs.

For JSB King and King Heavy the upper limit is about 900 fps, for King Heavy MK2 800-850 fps. Above these speeds the pellet becomes unstable and accuracy suffers.

Because the Huben fires from the magazine, it wants slightly larger slugs than usual. If you size them yourself, these diameters give good results:

Calibre	Suggested diameter
.22	5.55 mm
.25 (older K1)	6.45 mm
.25 (mould v3)	6.42 mm

We think it is ideal for a small-tube pistol. It fills up to 350 bar, is light and folds, and the body is milled to help it cool. The pump locks while the bleed valve is open, which is very useful in transport. It comes with a Huben fill probe; a Foster coupling can be added for your other airguns. If you shoot fast and a lot, a tank is still more practical.

It lets you make your own slugs: it shapes lead wire in a cold press. The kit also has a guillotine cutter. What you should know:

• It is designed for pure lead; hard alloys can damage the die.
• Using a press instead of a hammer gives more consistent results and protects the die.
• You can make about 150 an hour.
• In testing the slugs grouped under 2 MOA, but did not get down to 1 MOA.

Yes, with third-party adapters and stocks. With a carbon stock the total weight stays at about 1.3 kg. With a Picatinny adapter you can fit most stocks on the market. Once you fit a stock you will want a red dot too, because the open sights sit low for your face. These are not Huben products.