Victoria B.C.: Modifying the KWC .177 Replica Mauser 712 for Bolt Hold Back (More in description)

This KWC CO2 powered air gun is a very good replica of the real Mauser 712, which was a 1930s full selective fire “machine pistol.”
Out of the box it operates in function and handling almost like the original real thing. Given the price, I was very happy with it as it was. However, my other automatic KWC BB replica handguns hold the bolt open after the magazine is empty, which is one of their best features.
So...
I set about staring at the parts and worked out how to make it hold back and stop firing after the magazine was empty. This is especially important in a gun with selectable fully automatic fire. The only way to know the magazine was empty was to count the shots, or guess, or feel, when it was empty - especially tricky in full auto. This distraction resulted in taking a lot away from the experience and aura and inevitably led to wasted CO2.
My mod does not hold the bolt all the way fully back, nor does it have a catch to keep it there once the spent magazine is removed. But importantly, it STOPS firing and holds open.
My mod also opened a little space in the magazine so I now load 20 BBs (the real gun’s magazine held 20 cartridges). I can get two complete magazine loads out of one CO2 cartridge. This means 40 shots in semi-auto or full-auto without worrying about when the magazine runs out or wasting CO2.
I must note that I can usually get 5-10 more shots out of the CO2 in a third loading. When I try this it usually does not hold the bolt back after the last BB simply because there is no longer enough CO2 to push the bolt back far enough, which is typical of blowback BB or air soft guns.
I’m good with small tools and measurements and am quite experienced at tinkering with mechanical things. If you are tempted to try this make sure you are reasonably skilled as the warranty will be voided.
I’ve continued to test and shoot this gun numerous times since finishing the video and it works well every time!
Cheers

[@jacquesn6019]

Thanks for video.
Very useful. You are a smart guy.
I don't understand why the manufacturer doesn't make this modifications who reflect the reality of the M712.

[@tamserku]

Hi Terry, I have been looking for a solution to last shot hold open on the Broomhandle and found your mod very interesting. Just want to know how the mod is working out after using it for a while now? Any issues at all? Thanks.