Shoei MKb42(H) Firing version

Absolute superb blowback action (look how his arms shake while shooting).

In the past modelguns did what they want to do; nowadays it seems, that they start to do what the user wants.

Cerwyn, keep on with the good work!

Jati

Thank you for your comments Jati 


All credit for this vastly improved firing performance as well as cartridge durability, must go to 8ace. The man is a Genius   


The standard Shoei MKb42 proved disappointing in its firing performance, but there's no denying that it is a truly superb replica of this particular rifle, as are all of Shoei's products.

Shoei Cartridges are brass, heavy at 25 grammes and require 2 caps to generate enough blowback.




The model's bolt and carrier assembly is heavy, big and needs to travel a long distance so strong Blowback power is essential.
Shoei achieved this power using a cartridge design so well sealed internally that no smoke or sound can escape. Two caps generate a lot of power which pushes the bolt and carrier back, but causes damage to the cartridge rim and point where both halves screw together very quickly.
One problem I found with the cartridges supplied was that the Piston was a tight fit in the case's bore. So tight that after firing, the piston would be jammed firmly in the cartridge's nose. The majority of pistons would need drifting out which shouldn't happen! The bore measured 7mm in diameter so Shoei's Tomio suggested reaming them out to 7.1mm. That made slight improvements, some pistons would push out now but many would still jam.
The only way I found to stop them jamming was to reduce travel in the cartridge by adding 0 rings on top of the piston. Please see:




Even this wasn't 100% successful. Pistons were breaking and cases flaring at the joint where both halves screw together.  

Rim damage was so bad it rendered the cartridge useless and unusable in very short time.

Thanks to Japanese legislation, bolts are made of Zinc which also wears away quickly. Hencve the need for a replacement, steel bolt face which can just be seen in tyhe photos at the end of this article.


8ace's cartridge is Aluminium, the MkI version only weighed 8 grammes. I've yet to weigh the improved MkII version but it's an efficient, durable cartridge that only needs 1 cap to cycle the gun effectively and reliably.


I will provide more detailed information later showing internal mods to Bolt and Carrier, Magazines, Cartridges etc. as well as more videos 

Custom made MkII Cartridge for Shoei MKb42


In cartridge firing trim the Bolt and Carrier have to be fixed together rather than have the original floating bolt. The mating surfaces of Shoei's parts were not 100% flat and level so both surfaces were corrected by machining. The fixing screw threaded into the back of the bolt, through the carrier but isn't really strong enough. A larger diameter stud was inserted into the bolt and secured to the carrier by a Nyloc nut and washer.
After several firing sessions the carrier's mating surface split, as can be seen in the photo below. A sleeve was made to fit, giving a stronger assembly.


Split Carrier




 




 
Thanks to 8ace, now this glorious model performs as I'd want it to, and as it should do