SMK XS78 CO2 to PCP conversion and Bullpup stock

[wazgilbert]

I’m restarting this project, that began as an excuse to get a 3D printer during Covid.
I had recently been given 2 near identical CO2 powered guns by my Dad, because he’d stopped using them for pesting in his back garden.
There was a Thumbhole stock ‘deluxe’ version and a bare vanilla version of the QB78 guns, marked as TH78D and XS78 by SMK.
He has used the deluxe one as his personal airgun for some years and when all the seals failed, a local gunshop basically sold him the idea to get the basic model and swap the stocks to keep the nice feel of his gun without having to do a strip down.
He then proceeded to take apart the action out of the deluxe and try to find any o-rings he could to rebuild it.
I received it in pieces with the working gun as a project. Of course I went on the web, found a spares kit, rebuilt the original gun and put the stocks back on the correct actions.
So now I had a ‘nice’ gun and a basic one to do whatever with…

 

[Peddy]

Looong forward to see how things turn out. The 78’s are a tinkerers paradise very simple to work on.

 

[wazgilbert]

Because this was done a couple of years ago, I realise that I didn't take many before pictures as I wasn't thinking of documenting the build at that time.
But as close as 'before' as I can get;

as you can see, Froggy's drop block was put on as soon as I had it in hand, and there's a 14.2mm slip on adapter to make it 1/2unf silencer compatible, but other than that, it was a regular XS78.

comparison to the TH78D that already had it's own slip on silencer fitted;

The mechanical parts are almost identical, just a shiny goldie lookin' bolt and trigger, the only real difference is the barrel ODs
XS78 14.2mm
TH78D 15mm
Both are .22 caliber.

 

[Rollo]

I agree with Peddy, very easy and satisfying to work on. Very simple and straightforward too. Did an XS as a winter project a few years ago. Mainly smoothing and polishing internals to get it working smoothly. A few upgrades and a decent power output ( below 12ftlbs before people jump in ), used it for targets and close range pest control - very effective.

 

[jamesleigh89]

The xs78 was my very first rifle... Thinking about it I think I might still have it in the loft. Seeing this post has just given me the urge to go dig it out to strip it down just to put it back together

 

[wazgilbert]

I’ve been a bit delayed waiting for some parts, the exhaust valve was leaking, the little seal is torn.

But meanwhile, this is what I was thinking of for the bullpup, how to get a trigger mech forward off the original and to operate it from there



The new trigger body was coming from a 3DP toy model someone shared for an AK47



Which I deleted a void that I modelled the xs78 mechanical parts (all in tinkercad, not even a real CAD program)


And then work out how to push the original trigger based on my rough trial

 

[jamesleigh89]

Mate, that looks awesome!! Are you keeping it CO2 powered or will you be converting to PCP?

 

[wazgilbert]

Mate, that looks awesome!! Are you keeping it CO2 powered or will you be converting to PCP?

I’ve got the drop block from froggys airgun parts and a regged paintball bottle set at CO2 ish pressure

 

[jamesleigh89]

Well good luck with the build pal. I'm looking forward to seeing the finished article. You will probably be the only person in the world with a bullpup xs78

 

[wazgilbert]

So far I had it as a CO2 powered one inside a plastic AK looking shape

Compared to the size of a TH78D

Covering the co2 end cap



And with a built in moderator to immitate the gas tube of the AK ( a bit )


Finished except I never made the stick on mag.

 

[wazgilbert]

The next step is to redesign the concept around the drop block and bottle


Apart from the leaky valve, which is on order, it’s functional. But I want to make it more of itself rather than an imitation of something else.

 

[jamesleigh89]

That's cool as f**k, and the size difference is almost by half. Maybe try your hand at a bit of wood working and try to make a wooden stock for it to sit in? Well done on the build tho. It looks awesome

 

[Berber]

I like this. I must not be tempted though, oh no.

 

[Richardbfes35]

I’m restarting this project, that began as an excuse to get a 3D printer during Covid.
I had recently been given 2 near identical CO2 powered guns by my Dad, because he’d stopped using them for pesting in his back garden.
There was a Thumbhole stock ‘deluxe’ version and a bare vanilla version of the QB78 guns, marked as TH78D and XS78 by SMK.
He has used the deluxe one as his personal airgun for some years and when all the seals failed, a local gunshop basically sold him the idea to get the basic model and swap the stocks to keep the nice feel of his gun without having to do a strip down.
He then proceeded to take apart the action out of the deluxe and try to find any o-rings he could to rebuild it.
I received it in pieces with the working gun as a project. Of course I went on the web, found a spares kit, rebuilt the original gun and put the stocks back on the correct actions.
So now I had a ‘nice’ gun and a basic one to do whatever with…

Hi I have the SMK TH78D deluxe in .177 I am just wandering where you got the dropblock for it and what is needed to convert it from co2. If you dont mind that is

 

[wazgilbert]

Froggy’s airguns is where I got it. Then a 13ci paintball bottle with regulator fitted ideally 850psi to be as similar to CO2 pressure. The one I got was claimed 850 but actually only about 500, so I dismantled it and rebuilt the Belleville washers stack to get a real 900.

 

[spaspeckerthedull]

Well good luck with the build pal. I'm looking forward to seeing the finished article. You will probably be the only person in the world with a bullpup xs78

ahem - mine from about 10 years ago inspired by the lelya 1, there are others. Currently bulk fill (the tube was made for me by porky yorky - RIP dave)

It's now in bits on the bench as i'm fitting a lane regged HW100 air tube to it

 

[wazgilbert]

I'm currently stalled on it, because I really want one of these kits;

SMK QB78 .22 Single Shot to 10 Shot Conversion Kit | Pneumatic Ballistics

www.pneumaticballistics.com

but it seems that every time I look at their web-site, it's out of stock, and if I call them it's "in two weeks; check the website", I was convinced they're out of business.

I've been playing around with a CP2 and Norris Sheldon's drop block instead.
bit of a creative block at the moment.

 

[wuggis72]

I'm currently stalled on it, because I really want one of these kits;

SMK QB78 .22 Single Shot to 10 Shot Conversion Kit | Pneumatic Ballistics

www.pneumaticballistics.com

View attachment 914040
but it seems that every time I look at their web-site, it's out of stock, and if I call them it's "in two weeks; check the website", I was convinced they're out of business.

I've been playing around with a CP2 and Norris Sheldon's drop block instead.
bit of a creative block at the moment.

Yeah I've looked at those a good few times. They're always out of stock. Have been for a few years now as far as I can tell. Unless some came in and I missed it.
I now believe they're still listed through lazy site management rather than any real intent to resume making/selling them.

 

[wazgilbert]

Yeah I've looked at those a good few times. They're always out of stock. Have been for a few years now as far as I can tell. Unless some came in and I missed it.
I now believe they're still listed through lazy site management rather than any real intent to resume making/selling them.

I enquired directly, was told 2-3 weeks, watch the website, and sure enough absolutely nothing in stock for 2 years (I called more than once, and he was quite short both times)
That rifle they designed I notice is also never available.