Huben Huben GK1 .22: A Beginner's Journey to a DIY Mini Carbine

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End Receiver Picatinny Mount

And now we hit the CRUX of converting a GK1 into a mini-carbine. Figuring out the best way to mount a stock.

The general idea is to mount an adapter somewhere on the pistol that has a picatinny rail. Then you can purchase an off the shelf picatinny mount stock. (Example)--or make your own!



The 2 common places to mount an adapter would be:
 
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Option #1

The receiver end of the pistol.

Stock mount receiver end.jpg


The stock would mount straight back, probably with a drop to get best facial clearance with the scope.





Close up of the mounting location:

End receiver view.jpg
 
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Navig said:
I decided to go with the receiver end for the simple reason that it has 2 built in mounting points. The bottom of the stock has only one, which would bring in the problem of how to stop an adapter from rotating. Might come back to revisit that one later.
Click to expand...
Just use MoistOnes rear Picatinny adapter. It looks and mounts perfectly. Almost looks like a factory supplied precision fit adapter.
Also very solid print density.
The only possible improvement, IMHO, would be an exact replica made out of anodized aluminum.
Have fun.
Huben K1~06.jpg
 
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Yah, I have linked to his product page in this thread previously. I am doing this DIY style (AND posting every stop along the way), because I'm finding it fun! Also, in a few more posts, I'm about to diverge from the typical receiver block mods out there, for the very reason you pointed out in your comment.

But first, measurements!

To plan out my adapter block, I first started taking important measurements:

End plate mounting distance.jpg


Bottom of block to center of bolts = 25mm
Distance between center of bolts = 20mm, or 10mm from horizontal midpoint
Flat portion or receiver plate, width = 17mm
Left and right angled portion (“wings”) of receiver plate, length = 6mm
(Not pictured) clearance from bottom of plate to just below where the iron sights sit = 29mm



And some angle measurements:


End plate mounting angles.jpg


Receiver plate horizontal angle (left and right wings) = 30°
Receiver plate vertical angle with wood stock = 128°
 
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Navig said:
Next order of business: Let’s make sure it works out of the box.


** Edit--the general experience is whenever possible, load pellets before adding pressure (so skip down to the loading pellets section). Doing so allows you to operate the magazine release lever under significantly less tension.

I pressurize with an Air Venturi Rovair, my backup is a Air Venturi refurb carbon fiber tank (my local scuba shop refills me at $100 for 10 fills), and a Hill Mk4 hand pump. For how-to’s regarding filling, see the How-To section of the forums.

View attachment 438778

Using a high pressure airgun compressor is fairly easy. Set the pressure on the dial. Connect the included fill probe to the quick release foster fitting on the end of the compressor hose. Insert the fill probe/hose into the GK1. Turn the compressor power on. Select 120VAC. Push the green Go button. Close the bleed valve knob. Filling to 4500psi/310 bar takes about 7 minutes. The compressor shuts off automatically. Open the bleed valve and release the hose pressure. Remove the fill-probe. Good to go.
Click to expand...
How long can you allow pressure to remain in the gun before it dissipates or does this happen at all from your experience.
 
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Strangely enough, my GK1 keeps loosing air nearly daily, and in direct proportion to tins of pellets!

I think a standard pcp should not lose any appreciable air in the span of many months. People with more experience might be able to give more info. About 2 years ago, I found my Marauder SAM creeping down maybe 250psi in the span of a month, which prompted a leak check. Found a leak at the fill port, Benjamin sent me a replacement.
 
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Designing the adapter block

In Sketchup, I found the easiest way to design a block is to tackle it in layers or sections.

The first portion I designed was the interface with the back end of the pistol:

First I designed a cross sectional plane that fits against the beveled shape of the rear of the pistol. I am going to leave out the picatinny mount for now--it is easy to design and add after.

I added 2mm of width to each side of the block, making the block wider than the pistol. This was to accommodate a full intact circumference around the bolt heads, which would result in a stronger block, rather than having no material on the far edges of the bolt holes.

Cross section.jpg
 
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Once I was happy with the cross sectional surface, I used the Push tool to push the surface into a 3d block, 29mm tall (the height that fits under the iron sight).

Block design2.jpg


I added a second layer, making the block thicker by 4mm, to the end depth I planned for. This block addition can be taller by 3mm because it lies beyond the end of the iron sight.
 
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Then, as I mentioned previously, I added the bolt holes using 2 different sized cylinders, one for the through hole of the M5 bolts, and a second larger diameter cylinder (but more shallow) for the countersinking.

Basically, you create the cylinder, position it where you want (based on the measurements picture above), and use the Solid Tools→Subtract to subtract the cylinder from the block. One cylinder for the thru hole (r=2.8mm) and a second larger one for the countersink (r=5mm, depth of 5mm).

Block design3.jpg



Here is the beauty and power of these sort of solid tools. If you flip the block around, you can see that the program automatically cuts the 3D hole right through the beveled interface surface:


Block design5.jpg
 
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Navig said:
I then printed it up with prototype settings (super thin walls to speed up print time). With 0.8mm thick walls 10% gyroid infill, it took about 45 minutes of print time.

View attachment 447970



And mounted up:

View attachment 447971


View attachment 447972


I liked the fit, thought I was ready to add the picatinny rails and finalize the block....
Click to expand...
This build thread is interesting, HOWEVER, MoistOne has already perfected this design for picatanny attachments two months ago. It's available right now and It fits perfectly. Here is a pic of MoistOnes.
1000012878.jpg
 
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scottblair said:
This build thread is interesting, HOWEVER, MoistOne has already perfected this design for picatanny attachments two months ago. It's available right now and It fits perfectly. Here is a pic of MoistOnes.
View attachment 448010
Click to expand...
I picked up one of these a few weeks back and it fits very well and seems sturdy. I never know how a 3d print will hold up to stress, and this could be a very stressed hinge point, so we'll see how it hold sup over time.

For those looking at ordering an A3 Tactical stock option, make sure you order the right one. Somehow I managed to order the version with a 2-bolt connection rather than the pic rail connector. A3 has been great to deal with though on swapping out just the hinge - so a + to them there.
 
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Yes, I continue to be aware that Andrew has a great product out there, as I replied previously (scroll up). I even continue to link to his online shop.


My next post, though, (still prepping it), I came across a problem that took me in a different direction. Various posters have brought legit questions about the strength of 3D printed plastics. Interestingly for me, tho, it was not because there is a lot of hinged pressure on it from the attached stock. If my picatinny turrets start to fade, I can just print another--if you have a 3d printer, this all about 50 cents in plastic and a few hours of print time. I ran into some more concerning structural issues...
 
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