Beginner's build (long)

[ribbonstone]

Decided to answer some questions I was too busy to do before. We get all  â€œinto” modding our new QB’s and don’t stop to test after each step.  This time decided to do some testing between steps to answer some of the new-guy’s questions.

Plan was to get velocity readings:
1. Box stock
2. Transfer port to poly-port
3. Bolt hole enlargement/bolt probe.
4. Shim mainspring
5. Valve.gas-path smoothing
6. Bulk fill


So far, have got 3 of the 6 mods chronographed.

Box stock: about 510fps
Poly port adds about 35fps (up to 545)
Bolt probe adds about 20fps. (up to 565)

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Can skip this long part if you’re results-only orientated. It’s the how’s and why’s.

Yard/garage sales...can go to 100 and never find an airgun, but show up when you’re nearly broke and there is sue to be one.

Found a QB 78 .22, looks to be about 5 years old or so....probalby shot once and put away and forgotten.  Looked better than many I’ve just taken out of the box, so after a bit of discussion and “dickering”, took it home for $32 (he started at $40 and I started at $25 so we kind of met in the middle).  Had to borrow $7 from a buddy until later.

At home, it leaked....but the leak was from the end cap, and there was a spare one at hand (from converting QB’s to bulk or tankers).  A couple of drops of pellgunoil and she held gas.


BOX STOCK (using Superdomes for all tests):
Shooting 14.5gr. @ 508fps.

Took her apart and was bone dry inside.   Cleaned the barrel (filthy), cleaned the hammer , back half of gas tube,and inside the receiver. Lightly lubed.  Replace the transfer port seal (this one wasn’t too mangled) with a poly-port. Had a spare muzzle brake, so removed the iron sights and attached that.  One scope was unattached, so this rifle got a little cheap Daisy 4X32.

{Odd thing. Never had one before with a color case hardened hammer.  Though it was dry grease, but once cleaned, it really is color case hardened.}

Shooting 14.5gr. @ 546fps.

Took it apart again with intentions of drilling out the bolt probe (which has worked just as well as a bolt probe for me).  BUT, the dammed tip of the probe snapped off.  That bolt was hardened to brittleness.  This is my 6th QB, and none of the others were brittle-hard, but be aware that they can be.

Annealed the bolt (heated it and let it cool slowly), drilled it, made the bolt probe.  In this case, had to make one that allowed for the o-ring, so it’s of 3 diameters (rear shaft to fit the hole in the original bolt...middle band the same diameter as the original probe, and front shaft to push the pellet and let the gas in.

Can't see the shaft that's soldered into the bolt, but the middle (fat) section and probe are visable.


Once the probe was fitted (soldered to the bolt) and polished, added a new bolt seal and tested again.

Shooting 14.5gr. @ 568fps.

It’s cool, windy, and raining today...about 75 degrees, thanks to tropical storm Fay, so vel. would be a bit higher in normal summer temps.

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When i take it apart to do the other mods, will snap a picture of the color-hardend hammer.  Intention is to do the bulk-fill mod and see if that gains the “little bit” I believe it does.

Then shim the hammer a bit to see what that gains.

Then taking the valve apart and tossing the screen/filter.

Then taking the valve apart and smoothing the gas flow/enlarging the gas path.

[ribbonstone]

Left off with the rifle shooting 565fps but we hadn't touched the valve or the mainspring yet.  To get to 565, and smoothed a few rough spots inside the rear of the gas tube, cleaned and lubed the hammer-bolt.and maintspring, and added a poly port.

I'll leave it to this site for a description of how to take everything apart.
http://www.gatewaytoairguns.com/library/Charlie’s%20QB-78%20Tune.htm

This is a standard valve:


Just above the silver tube on the left are three circles.  A fiber filter, metal screen, and a solid washer.  OF these we DO NOT put the fiber fileter or metal screen back in.  they do filter the gas, but slow the gas flow to the valve.



Tested with just that modification:
14.5gr. @ 588fps (increase of 12fps)


Took the valve apart one more time.  The first order of business is to make the solid washer a bit more open to gas flow.  Can be neat or sloppy, makes little difference (and you can tell, I took the sloppy path).  With a simple Dremel cut-off wheel, made slots around the outer edge.

Can grind a bit of a radius on the piercing pin while you're at it.  Proably does little for gas flow, but it does make the pin better at self-centering.


Next up would be to radius the valve stem head a bit. Can get drasiatic here, or just a smooth curve.   I've done them ddrastic on a lathe, but for this one did a small curve using a drill to hold the valve and a file.  Be careful not to bugger up the seal and DO NOT LET IT OVER HEAT!!!



Tested with  the piercing pin and radiused valve stem head,:
14.5gr. @ 601fps (increase of 13 fps)


The last simple "trick" was to try bulk fill. Get the same results from running on a remote line.



14.5gr. @ 613fps (increase of 12fps)
Any UK shooters...we just broke the 12 foot pound limit...expect the airgun police to be knocing on your door.

Which all totaled is an increase of 101fps from the way the rifle shot out-the-box..and we haven't touched the hammer spring (yet).

Not a giant fan of wacking the valve stem harder...but a little shim seems to harm nothing.  Get too thcik a shim and the hammer becomes very hard to cock.  But from past builds, would expect an easy 20-30fps and still retain reasonable cocking effort.

We'll see.
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You'll notice that I used some valve parts from other (spare) valves for the pictures.  Sharp eyes will also notice a change in the barrel band screw.  Like many people, I broke the horrible little screw.  So i marked it's spot, gooved the barrel to allow a thicker screw to pass, and substituted a common 4-40 screw and nut.

[jonny1o1]

on a QB79 gun can the piercing pin be left out?

how would i go about tapering the valve stem, do a little at a time?

20 seconds filing with a drill

1 minute cool time
 
and repeat?

can the valve spring be lightened?

[ribbonstone]

IF you figure out a different way for the valve return spring to work, could leave out the piercing pin. There are ways but remeber that that return spring can change the vel. of the rifle . Hammer hits the valve...that spring is part of the resistance of the valve to opening. Too lite and a lot of gas gets wasted (valve open longer than it needs to be).

On the valve seal head, I'd chuck it in a drill, clamp the drill to a table with a c-clamp or two (or in a vice), spin it and use both hands to guide a small (6") fine cut flat file.  I'll stop every minute or so, touching the brass with a finger (stop the drill!)...want to keep it cool enough to be able to touch. Keep a soaking wet rag handy to help cool it off if needed.

[ribbonstone]

Want to keep this simple, using hand tools most people will have (or be able to borrow).  Certainly not the ultimate in mods., just stuff the average guy would do in a day. Are some pretty drastic valve jobs that can be done, and do offer more speed.....are hammer lightening jobs that can give longer shot counts and less noise....all sorts of stuff for the QB has been worked out.

Back to shaping the valve stem:

The valve seal used seems to soften at about 200-220degrees (if you boil it, tends to get "mushy") so I'd keep the temp. down to "hot to the finger tip, but not a burning hot" when grinding.  If you can't touch it and keep your fingertip there, then it needs to cool down.

Bascially, just want to remove the square edge...leaving a flat in the middle to contact the piercing stem...but radius the edges well for a smoother gas flow.

[Splash]

Very nice write up Rib. Thanks for taking the time to, check what each mod does for performance.

[ribbonstone]

Left off with the rifle making just over 12 foot pounds of energy bulked, under 12 foot pounds on 12gr.s.

The last thing to adjust is the mainspring tension.  By adding washers under the mainspring, are basically making the spring pre-tensioned and stiffer.  There is a limit to how much you can pre-tension as sooner or later the spring will bind up before the sear catches and the gun will be impossible to cock.  Better to stop while the bolt still travels a bit past the point where the trigger sets.

For this one, used two thin washers.  Still easy to cock and if desired would guess twice the thickness would fit in this rifle.



As promised, a picture of the hammer. IT ISN"T SUPOSE TO LOOK LIKE THIS...all of the others are the silver gray of hardened, but not color hardened steel. At first I though it was discolored from dry drease...but I cleaned it.  Can see on the back 1/2 where I spun it in a lathe and dulled a steel lathe bit trying it for hardness...belive me, it’s color case hardened and a pretty good job of it.  If it is stained, then it's stained INTO the metal, not on top of it (which is a pretty good description of color case hardening)

Results are as follows:

With 2 shims:
Running on 12gr.: 14.5gr. @623fps for 12.5 foot pounds.
Running on bulk fill: 14.5gr. @ 637fps for 13 foot pounds.


I’ve decided to stop here.  this is basically what a stock Benjamin 392 gets with 8 pumps.  Even on 12gr., will still have  about 500fps/8 foot pounds of energy left at 40yards. that’s enough for what I’ll use this rifle for.

Did grow a better (old) scope. Not much call for a straight 4X of this size, but I had one and figured this would be a good rifle to put it on.

And did test for accuracy at 20 yards.  Little gun doesn’t do bad at all, but seems to prefer some pellets that NONE of my other rifles will shoot well, and turns it’s nose up at other pellets that shoot great in most of my other guns.

Who cares so long as it shoots something well?

[jonny1o1]

so after i tapered the valve stem and left the peircing pin, my 79 shoots alot faster

would HPA add a couple more FPS?


i dont have a chrony,

the pellet penetrates almost 1/2 in in 5/8 spruce plywood with CR points
 anyone have a chronied rifle that shoots like this?

[ribbonstone]

850PSI HPA tank would add a little bit, probably not enough to notice unless you chronographed it, but would shoot nearly the same speed in the middle of winter (co2 will be giving much lower speeds in cool weather...way-way lower in cold).  850PSi gained from 1% to 4% vel. depe3nding on the pellet....call it going from 600fps to about 615fps.

Cold go to a higher regulator pressure (like 1200PSI) for an increase in vel. but how much of an increase would be a guess (depends on the state of the gun's tuning...is a limit to how much gas can pass through a rifle no matter what the pressure).....my guess would be about 8-10% (from 700fps to about 750-770fps).

[Jaymo]

Honestly, that looks like rust, not color case. Metal oxides are hard and abrasive, and would dull a bit. That's ok. Browning is just a controlled rust, and yours looks like it has a nice smooth surface rust(browning). A little patchy but not rough. Nothing wrong with that.
Or is that just how it looks in the pic? I thought about case hardening all of my CO2 and pneumatic hammers with kasenit or maybe hot blacking them with a hot salt bath of distilled water, lye, and saltpeter.

[ribbonstone]

Could be...won't come off with anything I've tired on it, including an attempt to lathe it.   Have lathed other QB hammers, and while they have a thin  surface hardening layer, they will lathe.  Inside of the tube isn't rusted, but the hammer might have self-browned.

Next time I take it apart, will try some stronger chemicals as abrasives aren't getting it done....but the "blochy" patches of blue shwoing though make me thing that even if it's rusted some, there's a thicker carbon/hardening layer than normal.

[Jaymo]

You could always take some 0000 steel wool soaked in oil to it. If it's smooth, and not rough, I'm not sure I would try to remove it Just my opinion.