QB78D first impressions.

[pharoah007]

I picked up a QB78D today from Scopes and Ammo, thanks Eric.



I bought a seal kit and have also done a lot of research into tweaking this rifle, but I just HAD to try it out "stock" to see how she performs.

She looks real nice, and I had her zeroed in real quick. She was REAL accurate, I put a 3X9X32 scope on her. I have a approx 45 foot "range" in my basement and I could not miss.

Few things I noticed..

Is it normal the first time to have some gas escape (I do not know from where) when I backed off the pressure tube cap 1/4 turn to release the gas. The leak seemed to stop inside 15 seconds. The only seal I can see is on the bolt and it is dried out/cracked....so I have no reason not to think the others are the same (hence why I got a ring rebuild kit with it ;)


The action was REAL tight the first dozen or so shots....I REALLY had to push on it to cycle it back forward. This got better with time (put 50-60 rounds down range with first 2 CO2 Powerlets) and I assume will get even better when I disassemble and get a bit of lube in there (good idea to lube?)

If I had not done so much research on this rifle, I would be SUPER surprised, but I am glad to say I got exactly what I was hoping to get.

This is a kick ass rifle as is...I can now wait to tune her a bit :)

[SeanMP]

Nice buy!

Yes it's absolutely normal for a rifle to leak for a few seconds the first time it's charged up. I'll make an educated guess and say the seals have contracted and the first time they see gas it takes a second or two to pop out or in the case of the valve it actually forms to the seat under pressure.

However I've been inside dozens of brand new QB's and buying a seal kit is the right thing to do. When you strip it down do a really good job polishing out the breech in the area of the transfer port and set screw. Use a bonded rubber abrasive for your dremel to do this. Also polish the bolt and the bolt end of the breech but not to a mirror. If you polish it to mirror it wont hold lube properly.

Also stone the sharp corners in the barrel O ring grooves...lightly. Just break the edge.

And if your so inclined add two set screws just ahead of the breech port at 2:30 and 9:30. This will elliminate the infamous barrel wiggle.

Have fun with your project

[pharoah007]

Thanks for the info.

I will take emery/stone to all the sharp edges and burs. As for the bolt and bolt end of breach polishing...will fine steel woll be fine (000) ?

Also...I have no idea how to make set screws.....but I bet my tap die kit will come in handy ;-)

[Doc Sharptail]

Set screws can be had ready made. I'm pretty sure what's already on the QB's are 10X32 X 1/8". They are headless, and usually take an allen, or torx bit.

Just to add a little to what Sean has already posted:

Have a strong magnifying glass for this work. Pay attention to the inside of the bore, especially over the transfer port hole. This area is prone to burrs from machining. A little careful work to remove them with a small round needle file will work wonders for the bolt closing, and uniform accuracy. Give the crown a close inspection with magnifying glass as well. I use a gooseneck lamp with a 150 watt quartz tube in it for these inspections. It helps a lot to have good strong light.

I'd for sure replace the o-ring breech seal on the bolt. The tube end cap o-ring is also usually a weak point on these guns. That's the big white o-ring in the seal kit.

The 78-D is one of the best a/g bargains available. Good luck with yours.

Regards,

Doc Sharptail

[sholo]

Congrats on the new purchase!

While your in deburring/polishing mode, make sure you touch up the sides of the slots where the two cocking pins ride in. Mine were "cheese-grater" rough, and you could really feel it when cycling the bolt. A few light passes with a small file to get the worst off, then some 400, 600, and finally 1000 grit sandpaper wrapped around the shank of a drill bit or wooden dowel and it is now silky smooth

When you have it apart, you also might want to take valve apart and remove the filtering screen that's found inside. On my AR I only got about 80 shots through her when this screen decided to unravel and intertwine with the valve spring as well as trying push out through the valve inlet hole where piercing pin is, severely hindering performance . Not sure if this is a common occurrence, but something to keep in mind.

Have fun tweaking your newest acquisition , making an already great gun even better!


Todd

[pharoah007]

Thanks everyone.

I just MAY dive into this today.

As for my new O-rings. Should I use the supplied Industry brand CO2 airgun oil the comes in the maintenance kit....or should I use pellgunoil?

also when re-assembleing and I am lubing the breach/action. should I use one of the 2 above lubes? I also have spray silicone and CLP (CLP is used on firearms for cleaning, lub ann protection)

[pharoah007]

O, and 1 more question....is the hammer spring in the maintenance kit just a stock replacement spring....or is it a "upgrade" spring?

[rsterne]

AFAIK, there are no "upgrade" springs available for the QB.... When you consider that the stock spring is good for peak velocity at 1200 psi.... and will peak at 1800-2000 (more than the tube is recommended at) with simple shimming.... it doesn't make a lot of sense for it to be a common item.... I use QB hammers springs as an "upgrade" spring for a Disco when I want even more hammer strike than I can get with my "heavy" hammer spring....

Bob

[sholo]

Not sure what the chemical makeup is with the supplied oil, but as long as it isn't petroleum based it should be fine for the o-rings.
Personally, I use this stuff for my o-rings: http://scopesandammo.com/storefront/...nd-pumps-p-577

For lubing the bolt/breach (or any metal-metal contact for that matter) this is pretty good stuff. Just a little dab will do ya! http://scopesandammo.com/storefront/...ly-paste-p-261

Never heard of an "upgraded" QB hammer spring, so I would bet it is a stock replacement one.


Todd

EDIT: Bob beat me to it!

[AirGunEric]

petroleum based

I think you have that backwards- he has a Co2-powered gun, not HPA and almost all lubes excluding silicone are petroleum-based. The oil in the maintenance kit is fine for lubing anything on the QB if it's using Co2. As with anything, however, more is not always "better"- so no need to be lubing it up every 10th shot or such.