The FX Impact is a truly iconic airgun. In fact, it is such a good airgun that it was the first one to convince me that bullpups weren’t just a gimmick but serious pieces of kit with genuine benefits over conventionally designed rifles.
Once the Impact won me over, I went on to own several sub-12 and FAC models. I have used them all from the original offering through to the MKII, M3 – and still have a couple that I can’t bring myself to part with.
Most recently the FX Impact M4 Compact has joined my lineup, and it has been a winner from the get-go. This sub-12 bullpup has excelled at everything from farmyard pest control to squirrel culling and sniping rabbits from the support of a bipod, earning itself a regular slot in my hunting arsenal over the past year.
My Impact M4 has featured in several hunting videos on the Airgun Extra YouTube channel but it has taken me a while to get round to sharing a detailed review. Here goes…
The FX Impact M4 Compact retails for around £1,775. That is quite an outlay but this is about as good as bullpup airguns get and it has serious pedigree – remember, the Impact was the first bullpup to really win me over.
This little PCP really does live up to its name and is extremely compact. It weighs 2.8kg unscoped and is just 640mm long. Those proportions make it easy to carry and it also handles very well in the confines of a hide or inside farm buildings – hence my fondness for using it when staking out squirrel bait stations and for rat shooting inside barns, stock sheds and grain stores. Despite being very small, it is very solidly constructed and has stood up to plenty of rough and tumble out in the field.
Below: The FX Impact Compact is a small bullpup airgun that's well suited to pest control
You can tell at a glance that the Impact M4 has been built for functionality. That said, it isn’t a plain looking gun and certainly has a style of its own; a style of clean design and reassuring simplicity. The bottle serves as the forend for your lead had and behind that is an underside Picatinny rail for accessory attachment. There are two additional accessory rails – one on each side.
I really like the AR-type pistol grip, which in this case is the FX UG1 design. Its hard rubber finish is very grippy and that is enhanced with stippling. It feels great in the palm – it is very comfortably sculpted and has a good swell. It’s nice and steep and that combines with its ergonomic contours to deliver very good trigger attack.
The cheek-support is fairly basic, but its curved profile makes for a comfortable contact point. There is a slight righthand bias to it, though I think it should still be fine for lefthanders. Right at the very back you have a height-adjustable butt pad. Adjusting it is a quick and simple operation – just slacken off the knob and move it up or down to wherever gives you the best fit.
Below top: The UG1 grip is comfortable and very practical
Below bottom: Adjusting the height of the Impact M4's butt is a quick and easy job - the multi-shot magazine can be seen beneath the cheek support
The FX Impact M4 Compact has a 500mm barrel with the Superior STX liner. The M4 platform gives you a lot of adaptability with a host of different barrel lengths and liners. A chunky shroud houses the front section of the barrel. This shroud provides a bit of sound suppression, but I’ve been taking advantage of the half-inch UNF thread to fit a moderator which has made it whisper-quiet for stealthy hunting.
Scope attachment is via a 30MOA Picatinny rail which gives you about 200mm of clamping space. The M4 has a very strong top rail for improved rigidity, and it incorporates a dovetail rail on which the scope rail sits. Slacken off the fastening screws and you can actually slide the Picatinny rail forwards and backwards for added adjustment to ensure perfect eye relief with any optic.
Below top: The barrel shroud on the Impact M4 has a UNF thread for silencer attachment
Below bottom: The Picatinny scope rail can be adjusted forwards and backwards
The Impact M4’s magazine has a very generous capacity. Mine is .22 holds 28 pellets, the .177 holds 34 and even the .30 holds 20. It is a tried and tested magazine that is easy to load but it has been uprated to ensure that it still runs smoothly when loaded with heavy slugs. The magazine is driven by a sidelever action that is well-positioned just above the grip. It features a nice big drop-down handle and can be reversed for lefthanders. As I’ve already said, I have been using this gun for almost a year and the cocking and loading system has never let me down. It is swift and dependable and its positioning soon makes for intuitive operation, even when hunting in total darkness.
The Impact M4 has a brilliant trigger, which is completely free from the sponginess you often encounter on inferior bullpup airguns. The match-type blade is adjustable for reach, height and angle. The two-stage mechanism also features FX’s Quick Set Trigger system for easy adjustment between either a very light or more positive setup. I must admit that I haven’t had to touch the trigger setup on mine as it was perfect straight from the box. It has a short first stage that comes to an unmissable stop before a light and very crisp second stage break.
As for the safety catch, it is a switch-type setup that is very quiet in use and positioned in just the right place to operate either with your thumb or trigger finger. It is in the safe position when it’s pointing upwards and you just flick it up so the arrow is pointing forward when you’re ready to shoot.
Below top: An oversized dropdown handle makes for positive operation of the sidelever
Below bottom: The FX Impact M4 has an excellent trigger - the main air gauge can be seen just in front of it
The Compact model of the Impact M4 family is fitted with a 300cc carbon wrap bottle. Maximum full pressure is 250 bar and from that you can expect around 300 shots at sub-12 power. Onboard pressure is displayed on the Wika gauge by the neck of the bottle. When it’s time to refill, just pop off the cap from the inlet beneath that gauge and then snap on the supplied Foster connector.
My .22 calibre sub-12 Impact M4t is running at around 11.5ft/lb with 16-grain JTS Dead Center pellets. It is doing it very consistently, with variation typically staying within 4 fps over a ten-shot string. That consistency can be attributed to an AMP regulator which is certainly doing its job. The pressure for that reg is shown on the Wika gauge in front of the magazine slot.
Being sub-12, this model features the smaller plenum, but FAC-rated Impact M4s, which can churn out well over 100ft/lb, have the 75cc Power Plenum. You get a lot of tunability on the Impact M4 through easy regulator and valve adjustment and the FX Quicktune system. Obviously, you don’t have quite as much control on sub-12 models, but you still get a simplified version of the excellent Quicktune system.
Below top: The regulator gauge is positioned on the underside of the butt section, just in front of the magazine
Below bottom: Sub-12 models of the FX Impact M4 feature a simplified version of the acclaimed Quick Tune system
I’ve been very impressed with the Impact M4’s performance on the range from the outset. It is a very accurate little airgun, which is why I’ve been using it so much for hunting, and doesn’t seem to be too pellet fussy as long as you pair it with quality domed ammo. Running on those 16-grain Dead Centres, it is virtually pellet on pellet at 30 metres and still producing ragged single-hole groups at 40 metres. That’s shooting from the support of the bench and, in windless conditions, it is still very accurate at 50 metres.
To sum up, the FX Impact M4 Compact has given me extremely good service over the past year. Even with a silencer fitted, it is extremely compact, but it still has a good shot capacity and there’s no questioning its potential for precision shooting. It isn’t cheap but the price includes a lined hard case, and I genuinely believe it is one of the best bullpup airguns out there. Whether targeting rabbits, rats or grey squirrels, it has been doing a brilliant job for me.
MAKE: FX Airguns, Sweden
MODEL: Impact M4 Compact
PRICE: £1,775
LENGTH: 640mm (25.2in)
WEIGHT: 2.8kg (6.2lb)
BARREL LENGTH (Compact model: 500mm (19.7in)
MAX FILL PRESSURE: 250 bar
Article first published March 10th 2026
