My QB79 .25

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   Cross posting from another forum, but here is my QB79 project

03/04/2013

 Raizer:340354 wrote:

I brought a Mike Melick "power tuned" .22 QB79 back in August last year which I was rather excited about at the time lol

Was doing 15+fpe out of the box

Before doing to much shooting I had the barrel threaded and added a Porky style silencer

And when I was really lucky I would get groups like this

But only with JSB pellets

And most of the time they were more like this 

Also had some 10m groups that were over 12" ctc at 15m with superdomes!

After working my way through multiple possible causes with advice from members of this and other forums I soon found that the stock barrel was bigger than .22 and pellets pretty much weren't engaging the rifling, the only reason I can see that the JSB's were doing ok for the most part is there very soft skirts.

The marking looks a lot deeper in the photos than in person!
Also the probe/bolt seal and transfer port seal were both leaking a bit.

I decided I didn't want to spend $60+ on getting another stock barrel sent to me only to risk getting another dud so after talking to an excellent machinist on other forum we decided .25 was in this guns future!
So I now have a BSA blank winging its way from John knibbs   

While waiting for that to get here I've opened up the "piercing body" of my valve (badly), lined the valves port up with the hole in the tube, found a 5mm id tube to use for the transfer port and replaced much of the stock screws for strength.

Aiming for 40fpe still on co2 at this point, next step will be a 17ci hpa tank shimmed to 1100psi.

04/04/2013

 Raizer wrote:

As said, my first attempt at opening out the front of the valve didn't go too well 

Jonballs is checking if he has one for me, so hopefully my 2nd attempt will go a lot smoother!

Trying to figure out how to do my transfer port and still have the valve seal to the tube

04/04/2013

 Raizer:340538 wrote:

Take 2 should look more like this

The spring is currently still held in with the factory washer, Mike Melick ground it into a triangle when he did the original tune, he also cut a seat for the spring into the valve stem

09/04/2013

 Raizer:341873 wrote:

Customs Declaration
1x rubber oring
1x grease
1x steel tube for machining

declared value: £15 

 ;D

So my barrel arrived this arvo, haven't taken it out of the bag yet, but just looking through the plastic the blueing and rifling looks really nice!
Kinda disappointed it doesn't have the BSA logo stamped in it haha 

(the "oring" is a new breech seal for my Hatsan)

11/04/2013

 Raizer:341873 wrote:

Take 2 went a lot smoother, I had one slip with the grinder but thankfully didn't manage to destroy the piece!

Brought a longer spring for the valve too.

11/04/2013

 Raizer:341911 wrote:

I just got an update on the machine work

Here's a quick progress report for ya.. I have turned up a new .25 cal bolt probe and turned the barrels breach down to a very snug polished fit which requires a soft face hammer to send home.. 

This will be a far better means of attachment than the factory undersized breach floating on those 3 soft O rings and one retaining grub screw allowing it to slop all over the show with the slightest knock. The new fit is so snug that it wont in fact require any O rings to seal it. I will put it in the mill tomorrow and open up the feed tray section of the breach.

I will turn the bolt probe down a bit thinner and shape the tip of the probe to fit the rear end of a JSB King perfectly aiding bore to pellet alignment further. 
The Kings are a perfect candidate pellet for this project as they are light, short and tend to out shoot any other pellet in all .25 cal rifles. The challenge you now face is blasting enough CO2/air up its skirt to get them moving..
The bolt will have a tapered touch fit to a shoulder just inside the breach like a Theoben Rapid with an additional breach O ring just behind that. 

I gave the bore a good scrub to rid it of the factory crud and have pushed a few pellets through the new 1/4" bore and there is plenty of bite in this bad boy.. the choke is nice and tight and the pellets exit showing good strong rifling grooves in the head and skirt. 

12/04/2013

 Raizer:342140 wrote:

Latest update 

 [color=teal][size=10pt]Well Raizer, Your Qb79 is officially a .25cal! The barrel is fitted and I have cut the sweetest JSB chamber in it sized perfectly for the Kings.
I also shaped the bolt probe tip to suit the internal skirt angle... tomorrow I will machine a groove in the bolt to add a breach O ring and a mod to the transfer port sealing surfaces to fit a decent O ring. 

That will only leave the crown and 1/2x20tpi muzzle thread to complete before hopefully some testing on paper tomorrow..[/color]

Edited by Raizer - 14 Apr 2013 at 3:58pm