ASG CZ 75 Update


Since doing the review on the ASG CZ 75, I have spent some more time with this replica. And I can now say with confidence that it does have hop-up, and that this does affect groupings and accuracy.

The hop-up takes the same form seen on 6mm replicas: A nub presses on a rubber or neoprene membrane and pushes this through a slot in the top of barrel. As the BB passes down the barrel, it hits the bump formed by this nub, which imparts backspin on the BB, affecting its flight and trajectory. This is the first time that I have seen functioning hop-up in a 4.5mm replica. Most people will tell you that there is no point in having hop-up on a replica firing steel BBs because these are too heavy for the backspin to substantially affect their flight. But this doesn’t appear to be the case…


You can just about see the hop-up nub in this picture – it’s the flat area on the right (top) of the inner barrel. This shows the nub adjusted for maximum effect.

Hop-up is adjusted by turning the slotted screw on the base of the hop-up housing on the inner barrel. If you turn this too far, it affects operation of the gun – with the hop-up turned all the way up (i.e. the screw turned all the way clockwise) the loading nozzle is locked in the inner barrel and the slide won’t retract. Easing it off a little allows the slide to retract, but leaves the blowback sluggish and the slide fails to lock back. Easing it off further frees everything up and restores normal operation.

But, how does it affect shooting? On a 6mm replica, hop-up is mainly used to adjust vertical placement of shots. At 6 yards, the effect is minimal, but at longer ranges it’s more noticeable. However, I have spent a fair amount of time playing with the hop-up on my 6mm replicas, and I have also found that adjustment tends to affect groupings at six yards – there is often a “sweet-spot” which provides the best and most consistent groupings, and this can only be found by experimentation and test. So, I wondered if the same would be true of hop-up on a 4.5mm replica?


Typical grouping at six yards before adjustment

The groupings I was getting with the ASG CZ 75 as supplied were reasonable, though with a fairly large lateral spread (around 2½” – 3″) and were inconsistent, sometimes the grouping would be much tighter. I tried adjusting the hop-up to see if it changed things. I started with the hop-up close to the middle of its range of adjustment, where the pistol cycled reliably. Hop-up is very sensitive, so it’s best to adjust by small increments (no more than ¼ – ½ turn of the adjustment screw each time). I did all test shooting from a rested position, to remove any errors arising from my technique.


Results were varied. Adjusting the hop-up did change the centre of groupings vertically, but only by a total of 1″ or so, and the lateral spread was still fairly large and inconsistent, varying from 1½” to 3″. Hmm, time for a re-think.

On my 4.5mm replicas, I tend to put a drop of silicone or Pellgun oil on the neck of each CO2 cartridge before piercing. This sprays a fine mist of oil on internal seals and helps to keep everything leak free and lubricated. I don’t do this on 6mm replicas, either gas or CO2 powered, because the oil also sprays on to the hop-up rubber and makes it less effective. I had used oil on most of the CO2 cartridges I had loaded in the CZ 75, and a visual check showed the inside of the inner barrel to be oily, which was almost certainly affecting the operation of the hop-up. Time to clean out the inner barrel and hop-up rubber.


I used a pull-through improvised from some gardening string and a small piece of cloth soaked in warm water with a little washing up liquid in it – this removes any oily residue inside the barrel and on the hop-up rubber. I did this several times and then finished with a clean, dry cloth to remove any dampness.

I loaded a fresh CO2 (without using oil!) and went back to shooting. And the results were much better. The lateral spread on groupings was reduced and much more consistent. By further adjustment of the hop-up I was able to get the CZ 75 to produce consistent groupings of under 2″ centred close to the point of aim. Result!


Typical results for ten shots, six yards, rested, after cleaning the barrel and hop-up rubber and adjusting the hop-up

So what does all this mean? First of all, hop-up does appear to work with steel 4.5mm BBs though the effect is less noticeable than on 6mm plastic BBs. However, if you use oil on your CO2 cartridges, that’s likely to affect the performance of the hop-up and may produce inconsistent groupings. If you have a 4.5mm replica with hop-up (and so far, the ASG CZ 75 is the only one I have come across) I’d suggest that you:

  • Stop putting oil on CO2 cartridges, and,
  • Clean the inner barrel and hop-up rubber to remove any oil, and,
  • Adjust the hop-up a little at a time until you find the sweet-spot for your gun.

And hopefully that will produce tighter, more consistent groupings closer to the point of aim.

Given that all this is contrary to the popular belief that hop-up doesn’t work on 4.5mm steel BBs, I’d be interested in comments from anyone else who has tried this. Are there any other 4.5mm replicas with hop-up? Does adjustment work?

And can we have hop-up on more 4.5mm replicas please? I had almost stopped buying steel BB shooting replicas because of the lack of hop-up adjustment. Pistols like the Tanfoglio Witness and the Cybergun GSG92 and Sig Sauer P226 X5 are fantastic replicas. But whether individual examples shoot above, below or on the point of aim seems to be purely a matter of luck and given that none have adjustable sights and you can’t try different weights of BB, there is nothing you can do about it other than fitting an optical sight. Being able to adjust the vertical centre of groupings on 4.5mm replicas would be a real step forward.

A short update video review and new shooting test is also available.

Target downloaded from:

Related pages:

ASG CZ 75 review

Tanfoglio Witness review

Cybergun GSG92 review

Cybergun Sig Sauer P226 X5 review

3 thoughts on “ASG CZ 75 Update

  1. Would it not be more sensible for the manufacturers to install fully adjustable (windage and elevation) sights on replica pistols so they can be sighted in properly? That way we could still use Pellgun oil and not risk potential seal leaks and/or damage. I would rather have that than fooling around with sensitive hop-up settings.

    • Adjustable sights are useful, but while they are fine on a replica of a target shooting pistol, I think they might look out of place on a replica of a combat handgun like the CZ 75. I realise that not everybody will want to play with hop-up adjustment, but at least it does give some scope for changing the point of impact vertically, something that’s sadly lacking on most 4.5mm replicas. And once it’s set, it should stay that way.

  2. Looks like KJWorks loves leaving airsoft hop up units on their converted 4.5mm pistol versions. I just purchased the Hatsan 250XT TAC BOSS which, of course, has nothing to do with the Hatsan company but is manufactured by KJWorks as a converted KJW MK-2 airsoft pistol. The 4.5mm 250XT also has that piece of rubber inside the barrel! The pistol even comes with a hex key for regulating hop-up! I agree with Pony Boy that that adding a hop-up unit to BB airguns is rather idiotic. Like Pony Boy rightly said if we want to use hop-up we cannot use Pellgunoil which is a must for keeping the seals alive! It’s not the problem of playing or not playing with hop-up adjustment. The piercing seal on my new Hatsan 250XT was so dry that it had to be soaked for about an hour with Pellgunoil before inserting the first CO2 cartridge. Just adding a drop of Pellgunoil on the very first CO2 cartridge didn’t stop CO2 leak. I must use Pellgunoil! No alternative here. Therefore, according to your experience hop-up is useless.

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