Author Topic: QB Quarter Bore  (Read 1827 times)

Offline Novagun

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QB Quarter Bore
« on: July 16, 2013, 05:58:04 PM »
This project started with a cheap Chinese Co2 powered rifle that was professionally upgraded in America before being sent to New Zealand . From that time on it started a new life altogether.

The professional modification included replacing the pellet probe on the bolt with a drill glued in. Note the drill flukes showing.

Then The owner added a home made silencer.

With luck the rifle would shoot groups like this but only with good quality JSB pellets.

Most of the time it shot like this  :-

If not much worse!

Following advice from airgun enthusiasts the owner went through many possible causes of the poor accuracy, from a badly formed barrel crown to pellet clipping in the silencer;which was happening but drilling it out didn't help.

It was eventually discovered that the bore in the barrel was larger than it should be and the pellets weren't engaging the rifling properly. A (.22) 5.50mm pellet (tin stamped 5.53) could be pushed through the bore with very little pressure.

The rifling marks look a lot deeper in the photos than they realy are. Apart from that both the probe/bolt seal and transfer port seal were both leaking a bit.

The decision was made to import a .25 inch bore choked BSA barrel blank England.

While awaiting delivery some preparatory work was started on the gun for its transformation. This consisted of opening up the front of the valve, getting the port in the valve to align with the hole in the tube and figuring out a higher flow option for the transfer port and discarding the O rings that the stock barrel was floated in.

Eventually parts the arrived from England.

The first attempt at modification proved that an angle grinder and a power drill are not a good substitute for a mill or lathe!
The valve piercing body didn't turn out well.

Another one was ordered from England for a second try (the key is a longer valve spring!)

The second try was much better although the angle grinder did slip once.

Some parts of the project were sent to a machinist for completion.

 A new .25 cal bolt probe was made  and the breach on the barrel was turned down to a very snug polished fit which required a soft face hammer to drive home in the breach block.

This is a far better means of attachment than the factory undersized breach floating on three 3 soft O rings and one retaining grub screw allowing it to move with the slightest knock. The new fit is so snug that it doesn't need any O rings to seal it. The feed tray section of the breach was made larger.

The bolt probe was turned down a bit thinner and  the tip shaped to fit the rear end of a JSB King pellet perfectly; further aiding bore to pellet alignment.
The next challenge was to get CO2 behind the pellet skirt to get them moving.
The bolt will have a tapered touch fit to a shoulder just inside the breach with an additional breach O ring just behind that.

The new barrel bore was cleaned of any manufacturing residue and a few pellets were pushed through the 1/4" bore to test the rifling and choke.

The factory bolt has a flow through the probe, but this rifle came with a tuners custom probe with a drill bit glued in for a flow past pin. You can see it in the realier photo.
 The new barrel was fitted and a chamber cut to fit JSB King pellets perfectly. A groove was machined in the bolt to add a breach O ring and a modification made to the transfer port sealing surfaces to fit a decent O ring.

A breach seal groove was cut and an O ring fitted.

The rifle was reassembled.



 It was tested with 5 different .25 calibre pellets at thirty metres. They gave differing results.   

JSB Exact Kings. 25.4gr
Benjamin Premiers 27.8gr
Predator Polymags 26.0gr
Daystate Rangemasters 36.2gr
H&N Barracudas 31.0gr[/size][/color]


Edit: Just found an old 10m test group for a comparison haha

The groups were all as good as expected with light pellets but the power available from a CO2 bottle certainly is the limiting factor for the rifle.  The JSB Kings pellets have a muzzle velocity of 596fps. That is adequate for a .25 calibre. The barrel rifling has a good twist rate but a little more velocity would reduce the size of groups and enable shooting well to fifty metres with heavy pellets.

The muzzle was threaded to accept a supressor and various samples tried to find one that helped the  accuracy of the rifle. Eventually a special light weight one was made.


And the best part is it really shoots well with this lightweight supressor one!

The rifle ended up a little shorter than the original. The new barrel was fifly millimetres longer but the different sized supressor made the difference.


The cost of the upgrade to this rifle amounted to as much as the initial purchase but the satisfaction and personal achievement in completing this undertaking overides any consideration of cost.

« Last Edit: August 15, 2013, 07:12:58 AM by Novagun »