Author Topic: Modifications QB 79.  (Read 3399 times)

Offline Novagun

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Modifications QB 79.
« on: July 13, 2013, 08:59:04 AM »
This project was undertaken by an air gun enthusiast with some experience and limited workshop facilities but confidence in his abilities. His aim was to improve the standard Chinese made rifle by improving the finish to parts and to convert it from CO2 power to high pressure air. In other words converting it to a pre-charged pneumatic rifle.  He stripped it down to find it wasn't too bad for a stock one, other than a course feeling hammer that was catching in the tube and the felt filter from the valve being shredded through the valve:

He polished all contact faces of the hammer and bolt assemblies so they slide freely in there respective tubes:

And spent a fair while shaping the valve stem to  increase the gas flow and to hold the valve spring

He removed the piercing pin arrangement which was not needed and opened up the case of the valve a lot for air flow and set up a seat for the spring. It would have been better finished if he had a lathe and mill to do that part.

Still a bit more work to do on this part.

The end of the bolt was machined off and a 1.5mm pin probe added, as well as opening out the brass valve body a fraction - again to aid flow.

The trigger had a long heavy pull with a lot of play but was modified to be light and crisp 

The QB variant guns have a simple trigger unit that can be adjusted to suit the shooter.

A lighter spring fitted.
The sears were cleaned up using an oil stone, keeping a close eye on angles!

Polishing was done working down through grades of wet and dry sandpaper finishing with 2000 grit.

The  polishing was finished off with a light burnish of Molybdinum disulphide grease and washers were added to take up side play

The trigger was then adjusted to suit the shooter.

The rear mounting hole was slotted so the trigger unit could be mounted a millimetre or two further back on the main tube, to move the sear rearward and allow a little bit more hammer throw.

Both tubes were cleaned polished so that both the bolt and hammer slide very smoothly in their respective tubes.
The machining marks were removed from the tube slots and  the barrel port opened up a little using a Dremel grinder.

Got the gas block opened up the same as mine to provide a few CCs of regulated air volume

The regulator was examined with the the expectation of using a belleville washer stack in it to increase the pressure but it was found to be an older low pressure set up with a coil spring and shim. It could be converted to use washers.

The probe pin was modified.

It was not straight but an attempt to straighten it in the lathe as much as possible was made but it is made of hardened steel. Although not perfect it does push the pellets straight. 

Next came reassembling the valve with a new EPDM oring and some decent O ring lubrication then fitting it to the tube.

Hammer assembly lubed lightly and reinstalled

Transfer port seal

Then clean off excess lubrication and put the breech back on. The operation of the action is now very smooth.

The High pressure air regulator was shimmed to give 1000 pounds per square inch.

« Last Edit: July 29, 2013, 07:51:14 PM by Novagun »