Author Topic: Carbine QB78  (Read 5339 times)

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Carbine QB78
« on: January 23, 2008, 02:43:50 PM »
I'm thinking of converting my QB78 (XS in UK) into a carbine, how much can I take off without losing accuarcy. Also while the barrel is off, is worth getting it recrowned? I've done most mods to her but have done nothing with the barrel.
Any advice would be great!
Many thanks, Joe :)

Offline ribbonstone

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RE: Carbine QB78
« Reply #1 on: January 23, 2008, 03:03:38 PM »
Some things to consider.

Will shorten from the muzzle end. to do so from the breech end is a whole lot more work for no gain; the barrels are not choked so there isn't anything to lose from shortening from the muzzle end.  So you will recrown as you're losing that section of barrel.

Are some problems you need be aware of.
1. Many times, the bore is NOT dead center in the barrel once you cut back.  A small off center is just unsightly, and even those well off center CAN shoot well, but from previous posts, some can shoot pretty poorly once cut back. Have cut back two of mine (using a lathe to cut and recrown) and while there is a small amount of bore-off-centerness, accuracy hasn't changed.
2. The rifle will shoot a bit slower.  How much slower depends on several things, but generally a gun tuned for speed will lose a higher percentage of vel. than one running at lower speeds.  basically, it depends on how much co2 is being squirted down the bore with each shot...more gas needs the longer barrel more than the smaller squirts of gas from slower rifles.
3. Rifle will be a bit louder as well.  The gas leaving the muzzle will be at a slightly higher pressure, which gives it a bit sharper "crack".
Robert

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RE: Carbine QB78
« Reply #2 on: January 23, 2008, 06:11:53 PM »
Looking at it, I think I'm best off leaving her in her standard form. I know I need a better barrel band as I've yet to zero the scope in, shots are so way off I can't adjust the scope enough. Any ideas?
Thanks for taking the time to respond.
Joe :)

Offline ribbonstone

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RE: Carbine QB78
« Reply #3 on: January 24, 2008, 09:19:07 AM »
I'd take a look at the action screws (one under the barrel breech/loading port and the rear trigger screw), put new o-rings on the barrel shank, and be sure the set screw holding the barrel on is tight.  Would add that barrel band back as well.
Robert

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RE: Carbine QB78
« Reply #4 on: January 24, 2008, 09:56:56 AM »
I'll try that when my parts arrive from archer airguns, whats the best way to recrown a barrel and how hard is it to recrown a berrel?
Thanks, Joe

Offline ribbonstone

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RE: Carbine QB78
« Reply #5 on: January 24, 2008, 10:02:46 AM »
This one is short and to the point. Simple and to the point:
http://www.spiralsol.com/airgunmod/1001.htm

Isn't anything magical, what you want to do :
Is arrange for the pellet to be released by the barrel evenly all the way around.
Have the edge of the bore recessed slightly to keep it from everyday knocks,dings, damage.



Robert

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RE: Carbine QB78
« Reply #6 on: January 24, 2008, 10:05:26 AM »
Joe, this post on GTA a while back has some info as well:

http://www.gatewaytoairguns.com/airguns/forums/thread-view.asp?tid=5616&posts=6&highlight=recrown%20barrel&highlightmode=1#M40249

Hope that helps.

Bryan

PS- so you ride a supermoto?

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RE: Carbine QB78
« Reply #7 on: January 24, 2008, 01:08:54 PM »
Thanks for all the info lads, as soon as all the parts arrive and i've recrowned the barrel and it all works (fingers crossed) I'll post some pic's!
I pilot a KTM 660 Supercomp, full race exhaust, ported and polished inlet and outlet valves and race cam. My mistress!

Offline SDale

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RE: Carbine QB78
« Reply #8 on: January 25, 2008, 02:26:13 AM »
Sounds like it does alotta 1 wheel Salutes! I LIKE IT!

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RE: Carbine QB78
« Reply #9 on: January 25, 2008, 10:49:13 AM »
Does anyone have a metal barrel band that they no longer use/need?
I had a look at the barrel and it does need to be re crowned. Once I have purchased/begged/borrowed the tools needed I will attempt to re crown the barrel. The pellets I hope to use are an English make, called Logun Penertrators. Does anyone find their QB is pellet fussy at all?
I'm also thinking of attempting the breech mod but how much bigger do I go and what seal will then fit?
Joe

Offline ribbonstone

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RE: Carbine QB78
« Reply #10 on: January 25, 2008, 02:04:40 PM »
Don't have a spare metal band. there are some custom made metal ones, but the factory made metal ones are for the slightly thicker barrled versions.  Haven't found the plastic one to be too bad, except for the thin body screw used to attatch it.  I've lightly filed the notch (tiny round file)in the barrel that the screw has to pass through for a more normal sized screw and nut.  Just keep in mind, the barrel breech set screw stops the barrel's front to back movement, the band just adds some side-to-side support...so don't crank down on that barrel band screw too hard.

Know the Logun Penetrator, it was sold here (and may still be)...nice pellet, but have some us made ones that seem to work just as well.  Rifles haven't been too pellet-picky, shoot most good pellets well.  Are going to be some pellets that work fine in other rifles that you rifle will just refuse to shoot well...but that's true of every other airgun made.

You'd be flirting with the 12 foot pound limit with too many mods...might want to consider that.  What is fine today might be a little over the limit on a hot summer day...and that may be the day someone checks your rifle.
Robert

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RE: Carbine QB78
« Reply #11 on: January 25, 2008, 03:48:23 PM »
Hmmm I have thought about the mods may take it close to the legal limit, once my parts from Archer arrive and I find a barrel band, I'll get her chrono'd to ensure all is as it should be! Just worried about the accuarcy issue - just hope a recrown and better barrel band will cure it!

Offline Jaymo

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Re: Carbine QB78
« Reply #12 on: January 31, 2008, 02:00:05 PM »
I thought Archer had metal barrel bands for the QB78.
My AR2078/TF79T .22 likes the cheap Chinese Daisy wadcutters from Wal-Mart just fine. I pull the trigger, and they go where I aim.
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