If you have read my review of the Umarex Buck Mark URX, you’ll know that I was quite impressed with it – for a budget airgun, it isn’t a bad shooter. However, for me at least, the shooting experience was not as good as it might be because of a heavy trigger-pull. The point of this article is to find out if this can be easily be improved. Caution is required here – as far as I am aware, it isn’t possible to buy spares for this replica, so if you mess things up, there is probably nothing you can do other than buy another. So, it’s sensible to go step-by-step, doing only a little at a time.
Disclaimer. Airguns can be dangerous. Taking them to bits and especially messing with things like trigger assemblies can make them shoot differently, unpredictably or even (if you are particularly unlucky) not at all. So, don’t do this modification unless you are confident about your air-gunsmithing skills. In fact, don’t even think about it… And if you do go ahead anyway and mess it up, don’t come complaining to me!
It’s also worth mentioning that you probably shouldn’t test fire this replica without loading a pellet. On most spring-powered air pistols, the effort required to force a pellet down the rifled barrel creates a cushion of air inside the piston chamber. If you fire without a pellet in the barrel, the piston slams forward with much greater force and this can damage the seal.
OK, with that stuff out of the way, let’s look at disassembly. Which fortunately is pretty straightforward and requires only a decent cross-head screwdriver, a 3mm hex key and something to drift out small pins. Before you start, make sure the pistol is not cocked!
Here is our starting point. Six shots, six metres, freestanding, Umarex Mosquito 4.3gr pellets. This is fairly typical of the groups that I’m seeing with this pistol before modification. I have shot smaller groups, but it always feels like I’m fighting the trigger pull. It isn’t terrible, but I am certain this replica is capable of better.
Let’s start disassembly. First, remove the two cross-head screws, arrowed, to release the grips.
Take them off and you’ll be left with this plastic frame.
Next, remove the two 3mm hex-headed screws that retain the accessory rail (the longer rear one also joins the upper and lower body).
Finally, remove the two cross-head screws, one either side just ahead of the trigger guard (arrowed).
And that’s it. Now gently lever the upper body from the frame.
Now we can see the trigger mechanism and the sear (arrowed).
When the gun is cocked, the sear engages with a slot in the front of the piston (arrowed).
When you pull the trigger, the sear moves down and allows the piston to move forward, propelled by the main spring. All that is making the trigger heavy is friction between the sear and the piston.
One thing that becomes immediately obvious with the gun disassembled is just how heavy the trigger spring is – my improvised gauge suggests that it alone accounts for almost three pounds of the pull weight. That’s a fair chunk of the total, so perhaps cutting a coil off the spring may make things better?
Before I do that, it’s time to look at the sear itself and think about what I can do to make it better? I’m going to do this step-by-step to try to find what, if anything, makes a notable difference.
The obvious answer is to decrease friction by simply lubricating the sear and the front of the piston housing. I did this using a PTFE grease with Teflon, which is also safe for use on replicas because it doesn’t degrade seals.
I reassembled the gun and tried shooting, and the result was; no discernible difference in the trigger pull. I wasn’t entirely surprised – on disassembling the pistol, it has obviously been well lubricated in the factory.
For the next steps, it’s easier if you remove the sear completely. This is fairly simple. Drift out the two pins (arrowed) from the right – both pins have knurled ends, and must be removed and reinstalled from the left side of the frame.
Then, the complete trigger and safety mechanism can be lifted out of the top of the frame.
This also gives access to the trigger spring, which is below the end of the sear and mounts on to a plastic pin moulded into the frame.
Next, remove the cross-head screw, arrowed, and remove the safety. This gives access to the single pin that retains the sear – again it must be drifted from the right and removed on the left.
With the sear removed, polishing is much easier. The face you need to polish is arrowed.
The next option is to try hand-polishing the sear. I don’t want to remove much material here, only to make the sear slide more easily as the trigger is pulled and hand-polishing removes very little metal. I polished the sear using Autosol-sovol metal polish until it looked nice and shiny.
Then I reassembled, and the result was, no difference. Hmm…
Next, I cut a single coil off the base of the trigger spring. You have to be careful here – the trigger spring keeps the sear pressed against the piston when the gun is cocked. Remove too much tension from the spring and the gun may not cock properly. Below you can see before (above) and after shots of the trigger spring. Removing only one coil does make the trigger-pull lighter and it does not affect function. You could remove more coils to make it lighter still, but there will come a point where the spring won’t have enough force to lock the sear into the piston.
With everything reassembled, the uncocked trigger feels noticeably lighter but, when I try shooting, there is no obvious difference and certainly very little improvement.
OK, time for the final and most drastic step – using a Dremel attachment in a power drill to polish the sear more aggressively. This is where you can all too easily turn a functioning replica into a wall-hanging, so you really must be patient, do a little at a time and then reassemble and test.
I do some initial polishing, reassemble, and I can’t really feel much difference. So, I try again and remove a little more. You can see the result below – I have actually removed very little material and only on the upper part of the face of the sear.
This time, when I reassemble, the trigger does feel different. Before, the trigger felt like an on/off switch – you pull, there is no response and you continue to pull harder until, suddenly, it releases. Now, you pull and you can actually feel the sear moving until it reaches the release point. That’s better, and it does feel lighter. And, just as it did out of the box, it continues to lighten-up more as I shoot.
Six shots, six metres, freestanding, Umarex Mosquito 4.3gr pellets after the final round of grinding. It took around twenty shots or so before the trigger lightened up enough to make this sort of group fairly easy to achieve.
Finally, there is an appreciable difference! I could go on to remove more material in an effort to make the trigger lighter still, but there is a risk that I’ll get to the point where the gun may not cock properly or may even fire when the safety is released. At the moment, it’s better than it was and everything works as it should, so I’m going to stop there.
Was it worth the effort? I think so. This is not a difficult replica to disassemble and reassemble and the difference in trigger-pull is noticeable and this directly translates to tighter groups. If you are fairly confident in your ability to work on airguns and patient enough to go step-by-step, I recommend this as a distinct improvement over the original.