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Author Topic: QB works  (Read 85 times)

Offline Mintie

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QB works
« on: August 08, 2018, 10:29:52 AM »
Picked Seppo's QB up for some work a few weeks ago, I hadn't been inside one of these before so was quite interested in giving it a go. This particular QB was one of Rev's old "builds" but it was all standard apart from a bolt handle and a green paint job.

I pulled it apart and cleaned years work of gunk and dirt out of it and replaced the drop block seals to test it in its standard form.

Geco 6.9gr - 555 fps for 4.75 fpe
Crosman Premier heavy 10.6gr - 512 fps for 6.1 fpe
JSB Monster 13.4gr - 484 fps for 6.9 fpe
H&N Rabbit Magnum 15.7gr - 445 fps for 6.9 fpe

Pretty underwhelming really considering its a full sized rifle with a 20" barrel and most Crosman 2240s are doing that out of the box on pistol barrels. I did notice that even with very heavy pellets it still seemed to be wasting some co2 with a cloud out of the muzzle with each shot, thought about that for a while as it didn't add up with the low power results and came to the conclusion that the flow of co2 through the valve and up the transfer port must be restricting it and slowing the shot cycle down.

I opened the rifle back up and had a go at modifying the valve a bit to increase flow and added some extra hst with some washers. This gained about an extra 60fps in speed but I still wasn't happy so I dialed a friend, Razier gave me some more ideas and after a bit of reading I did most of the work mentioned here - https://www.airrifle.co.za/threads/38719-Modding-a-QB78-QB79-the-way-I-do-it

So now a total list of mods include:
Oring as TP
Barrel and breech TP bored out slightly
TP in valve angled for better flow
Valve spring replaced with a lighter spring
New valve spring guide
Valve body milled out to allow for much better flow
Re shaped valve stem end to improve flow
Valve piercing pin, washer, collet and filter all removed
Probe end cut off, drilled and a piece of drill bit inserted to create a pin probe
Hammer polished
Extra HST washers (which I might remove yet one by one to fine tune)
Drop block hollowed out to increase plenum space
Orings all replaced with Polyurethane ones

I forgot to get pics of the final valve work but will probably have it apart again today to check it all out and take pics, It pretty much looks like the valve in the link but the spring is longer, the spring guide is a cut down smooth bolt and the stem is reshaped a bit.

I was happier with the final results but as mentioned above I will look to fine tune with the HST washers to make sure its not wasting gas.

Geco 6.9gr - 725 fps for 8.1 fpe (170 fps and 3.4 fpe increase)
Crosman Premier heavy 10.6gr - 660 fps for 10.3 fpe (150 fps and 4.2 fpe increase)
JSB Monster 13.4gr - 662 fps for 13 fpe (178 fps and 6.1 fpe increase)
H&N Rabbit Magnum 15.7gr - 584 fps for 11.9 fpe (139 fps and 5 fpe increase)

I think 13 fpe is probably about the max you can expect from 177 on c02. The poms seem to have pretty good results from shooting rabbits etc with sub 12 out to 40m or more so I think this is a pretty good result. This rifle would get some good power with a .22 or .25 barrel but the only ones of those I have spare at the moment are too short and wouldn't do it justice.







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Offline Akzle

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Re: QB works
« Reply #1 on: August 08, 2018, 04:00:06 PM »
If memory serves right, you can hog off most of the "valve body" leaving the top and bottom, with just 5mm straps joining them... can't remember the numbers increase in volume it gave.

If you over-bore the ASA adapter you'll bleed gas - it's really obvious after you've done it once... don't go deeper than the tank O-ring!

Can't remember what else. There was a quarterbore on here some time ago, and I think wingman has tweaked a few.

Nice guns for the price, shame they're unobtanium now! Where did you find this one to buy? Or is it still this 'Seppo' character's? and did he get it off Rog?

Offline Mintie

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Re: QB works
« Reply #2 on: August 08, 2018, 04:24:34 PM »
Seppo used to be on here a fair bit years ago but has been into powder burners more lately. Hoping that rebuilding this for him might re kindle his airgun love! It's pretty rare to see a QB up for sale these days and when they do come up its often around $500!

Raizers Quarter Bore thread is great reading and an impressive rifle in the flesh but I wanted to stick with c02 on this one to see what sort of power could be produced from it.
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Offline septictank

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Re: QB works
« Reply #3 on: August 08, 2018, 08:30:02 PM »
It most certainly has me excited again. The mynas in the backyard are going to hat you though @Mintie

Offline Novagun

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Re: QB works
« Reply #4 on: August 09, 2018, 11:36:59 AM »
Raizer hasn't shown up here for ages. That mod he did to the qb was excellent stuff.
I was looking at a QB 78 but to import one from the USA is too expensive and the Gamo and Umarex guns don't excite me so that leaves the Crosman ratcatcher.  Maybe, but by the time you do the mods to it you or I have a gun that is pretty ordinary.

The one of Sepos looks good even with the green stock.
« Last Edit: August 09, 2018, 11:38:37 AM by Novagun »