Author Topic: I ran out of steam before QB78 ran out of gas  (Read 7711 times)

Offline fisherdude

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I ran out of steam before QB78 ran out of gas
« on: June 03, 2010, 05:03:32 PM »
I purchased a .177 QB-78D a few months ago from Stephen Archer.  I am still not sure about what to do with it.  It does not have the accuracy I was hoping for so I tried a .22 barrell I found here in the classifieds.  Sometimes accuracy is good and sometimes it sucks. I have made no mods to the rifle besides a new seal.  I sort of developed a theory that accuracy diminished as the gas ran down and today I thought I would test that theory.  I got a target, my chrono, and two types of pellets and I was resolved to shoot it dry!  Just to see what it could do.  Temperature was 88 degrees F.  After about 80 rounds, I just put it up, it lasted longer than I did.  It was getting late and dinner was on the table.

What I learned was:  JSB exacts shoot better than CPHPs, I still want it to shoot better.  
So if anyone knows how to get these to shoot better, please let me know.  I am thinking about looking for a LW barrell.
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Offline ribbonstone

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RE: I ran out of steam before QB78 ran out of gas
« Reply #1 on: June 04, 2010, 11:52:20 AM »
Have a short list of non-drastic (can reverse them) things to try.

#1. Clean the barrel.  Probably did at least once, but they tend to cough up "air gun snot" now and again. Probably some hidden bit of the ear wax type grease they come shipped with from some hidden recess.
#2. Be sure everything is tight, esp. the fit of the barrel to the receiver and the receiver to the gas tube. If the polly port mod is just a little too high, the receiver plays see-saw on that high spot.
#3. Get some plastic ties; are going to use them as temp. barrel bands. Can bet by with duct-tape, but put a layer of Saran Wrap around the gun first to keep the sticky from peeling the stock finish. Can try one just ahead of the action to hold that in place, one where the rinky-dink barrel band is, or both.  Basically, if the barrel isn't stable, it won't shoot well. YES, it will look butt-ugly during these tests., but if you learn where it needs the extra support, are on the way to a good shooting QB.
#4. Adjust the trigger. When has a better trigger ever hurt accuracy?
#5. Change scopes.  Probably not the problem, but how would you know unless you swapped out for one you know is good?
#6. Try different pellets.  Most rifles will shoot several pellets well, but once in awhile will get a rifle that just has odd tastes (have one rifle that shoots H-points much better than anything else, but most will shoot JSB's very well).

These take a bit of work.
#7. Remove the barrel and check the breech end for how it fits into the receiver. If those o-rings are chewed, are likely to get a inconsistent gas leak that will ruin accuracy.  IF the barrel is loose-goose fit to the receiver, are likely to get poorer accuracy thn if it is tight (and he set screw tends to jam a ;loose barrel to the bottom, which opens up the passage on the top, which tends to allow a leak FORWARD at the joint of the reciever/barrel)..
#8. Push a few pellets down the barrel with a are feeling for loose spots, rough areas, or constrictions.  Not a hell of a lot you can do about them, but we tend to check for them anyway.
#9. Check out past posts on recrowning and fire lapping.  
#10. Check out the BREECh end of that barrel.  If there is a burr or rough spot around the transfer port's passage though the barrel, it will snag pellets and cut them  as they are chambered.


Offline patton123

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RE: I ran out of steam before QB78 ran out of gas
« Reply #2 on: June 04, 2010, 07:22:34 PM »
I know what you mean about the terrible accuracy of the 78......your not alone. It's the biggest complaint w/ the rifle. I tried a bipod on the barrel and accuracy improved greatly. I found the barrel moves when shot, so if you steady the barrel accuracy does improve.

If you like CO2, look into the RWS 850. It's FUN and accurate.


Offline Stephen Archer

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RE: I ran out of steam before QB78 ran out of gas
« Reply #3 on: June 05, 2010, 07:28:25 AM »
Sorry to hear of your trials with the QB78 Deluxe!

Looking at the test targets, it's not clear to me how big the groups are and for how many shots (apologies if I'm missing something obvious here). Were they shot outdoors and was there any wind? I'd just like to understand quantitatively what accuracy you're obtaining and then relate that to factory specs for these guns. But if the JSB group is for 40 shots at 30 yards and the group size is a couple of inches (ish) vertically, I personally would be very pleased with that performance at my level of shooting skills - particularly given the muzzle velocity changes you experienced.

One thing for sure is that the muzzle velocity chart you have is very different from my experience. Your gun starts at about 570fps and falls to a consistent 500 fps after about 25 shots. Just that muzzle velocity fall would obviously lead to some vertical stringing. How quickly did you fire the gun? It may be that the muzzle velocity dropped initially as the gun cooled through being fired until it reached a balance with the ambient temperature and speed of firing, so the muzzle velocity stabilised.

I find that the first few shots are normally a little slow and that the gun then shoots fastest for the next 10 - 20 shots, accuracy is generally consistent. The muzzle velocity usually dies fairly abruptly after about 50 shots and accuracy falls apart at that point with vertical stringing. I'm normally shooting indoors at 60 - 65 degrees F and your results are at a much higher temperature, so this may cause a difference.

As always, Robert has many good suggestions for practical improvements to these air rifles. I too would look to the barrel/breech fit, and also the fit of the front band and the crown of the barrels involved for possible improvements.

Please email me with your order number and I'll check our testing records if this was a "Gold Service" gun.

Stephen Archer
Archer Airguns Inc.

Offline fisherdude

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Re: I ran out of steam before QB78 ran out of gas
« Reply #4 on: June 05, 2010, 11:36:20 AM »
The target was shot outdoors.  It wasn't particularly windy, I live in a forested neighborhood where there are a lot of trees that block a lot of the wind anyway. I installed 2 fresh CO2 cylinders and popped it 3 times before my first "for record" shot.   The targets were 3/4 inch red stickers.  I alternately fired CPHPs and JSB Exacts.  I had previously been shooting indoors (air conditioned) where I was experiencing a precipitious drop in velocity after about 30 shots.  But at 88 - 89 degrees F, it didn't displaty the precipitous drop, after 25 rounds it it just settled in at about 500 fps. Like the energizer bunny, it kept going and going.   Altogether I fired 80 rounds - 45 CHPs and 35 JSB  Exacts. My plan was to wait 30 secinds between shots but some intervals were longer.  I have the gold service target that came with the rifle, but I bought it in 177 and I found a .22 QB79 barrell that I put on if for this test.  

I am an experienced competition shooter (black powder) and since I read in this forum about others getting all shots to touch at this distance (30 yards) with thier PCP and FT rifles-- that is my end goal.  I have tried 4 brands of 22 pellets thus far and the JSBs are the only ones that are showing much promise on getting there.  I think the bigest obstacle is variations in CO2 pressure - I don't know how to even this out.
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Offline spysir

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Re: I ran out of steam before QB78 ran out of gas
« Reply #5 on: June 08, 2010, 09:16:58 AM »
Listen to Robert, thinking you are using the barrel I sold you, it will group so one or several of the things Robert mentioned is coming into play. You did use new o-rings right? Next add a second barrel lock down screw, then???????  .  Maybe try five shot groups w/JSB 15.9 with a one minute interval between shots and please keep us posted, they are good rigs once you get them sorted.