Air Gun Home :: View topic - Question for rsterne or any other QB78 Gurus

radar · Silver Status Member

If I need to boost output modestly, say from 500 fps, up to about 550 or 600 fps MV on a stock QB78 in .22 cal., (lets just assume 14.3 grain pellets and 2 CO2 Powerlets at room temperature)...can I do this with a small hammer spring preload without losing too much efficiency in shot count, or do I need to get the transfer port seal upgrade kit too (or instead of preloading)?

Thanks!

SMP · Silver Status Member

You can get that increase from just changing that seal.

The best way is to machine (drill press) a tiny little flat on the breach and on the valve and then use a slice of 1/4 nylon tubing for the transfer port. Make the slice just long enough that there is a few thou of compression when you tighten the breach screw.

Ignore the custom ports and look at the flat machined in the breach. you can use a 1/4" end mill or even a 1/4 drill bit ground flat and sharpened

As well you can open the port passage on the bolt (perpendicular passage) It is very restricted.

These mods will make use of the power thats already there and will affect your shot count the least.

rsterne · Moderator

^X2....

Leave the hammer spring alone (affects shot count a lot) and just let the gun breath a bit better.... A 1/4" diameter flat on the top of the valve and the bottom of the breech and a short piece of poly tubing and drill out the bottom and front of the bolt to 5/32".... I gained about 100 fps that way....

Bob

radar · Silver Status Member

OK. I see, and thanks for that.

Two questions:

1. Is this all downstream of the valve, or upstream and therefore under pressure at all times?
(I need to know if I'm trying to seal permanently against up to 900+ psi gas, or just enough to stop gas from significantly bypassing the breech/chamber during the hammer blow so I can prepare myself for the work.)

2. Is this a good deal that will do as I need instead of hand re-working it?
http://www.archerairguns.com/QB78-Seal-Kit-p/aaqb78sealkit.htm
...or this one?
http://www.archerairguns.com/QB78-XP-Tune-Kit-p/xptunekit.htm

I don't need or want a lot of power at the expense of shot count because I don't need a CO2 powered version of my Discovery. Air is cheaper than CO2, and I'd be perfectly happy with my old Daisy 22SG's (540-ish fps) performance out of this gun. I just don't want, like, a 4" drop from me to the tree, and being a .22...you know.

The second kit claims 13 fpe with the .22, and that would be OK, but that's the very highest desirable power level for what I am using it for.

.

SMP · Silver Status Member

1) It is down stream of the valve so your not trying to seal full pressure full time

2) the first kit is not worth the effort. The second kit does have the improved bolt in it but as Bob and I said you can achieve 99% of the gains by replacing the transfer port seal and drilling the orifice in the bolt.

The rework is very very easy and it's worth doing even if you buy that kit. The important bits are the two tiny white seals...those are the transfer ports. They seat and seal sooooo much better against a flat surface. And your not chasing them around trying to make sure they stay put on the round tube.

Here is a picture of that kit(Eric not Archer) with one of the transfer ports turned sideways so you can see that it really is 1/4" white poly tubing

rsterne · Moderator

Either of those kits would be fine.... Don't forget, that 13 FPE claim is at 75*F.... The more power you want, the fewer shots you will get....

Bob

radar · Silver Status Member

OK, thanks guys.
Ya, at 75 degrees--that's like only late spring on through early September here. I think I will go ahead and get the second kit with the improved bolt and the seals already cut, and then do the milling as you showed.
Then I should be happy with the power at real Iowa temperatures.
(You know our January temperature is 5 degrees lower than Maine?)

Thanks again!

rsterne · Moderator

IF I understood the video and the Archer ads properly, they are using a "rubber" seal and they don't mention milling the valve or breech.... I am assuming that the tubing they are using is flexible enough to conform to the curved surfaces without milling them flat....

Bob

radar · Silver Status Member

Their breech seal appears to be nylon or poly to me, as it seems to be a section of whitish tubing(?)

I'll know more when I get everything in my hands and can examine it. I have ordered the kit above, and will decide whether or not to mill the sealing surfaces when I see what I'm dealing with, but I'm leaning toward milling it flat. I may not even use their breech seal, but I like the bolt design.

SMP · Silver Status Member

rsterne · Moderator

Although I didn't try an extended probe bolt in a stock gun.... it was worth 30 fps in a 900 fps QB compared to a flow through bolt that was drilled out to 5/32" plus.... I have no doubt that the performance gain of Archer's bolt over a stock one would be significant.... I think their "XP" kit of bolt and seals is a good idea, and I wouldn't hesitate to use it.... All you are doing is improving the flow downstream of the valve.... While it will have some impact on the CO2 consumption, it should have little effect on the efficiency (FPE/CO2 used).... I don't think for what you want I would open up the valve front, in fact I question that on CO2 at all, as you would tend to get (more) liquid in the valve, which is wasteful....

Archer have an HDD which they claim will recoup most of the shots lost from their XP kit.... I've never used one, but you might want to check into it....

Bob

radar · Silver Status Member

Sean, if I mill it...if I decide it could use it after having a good look...then I will for sure cut my own seal to get that "bearing crush" thing going on.

Bob, I was going to ask you guys about that debouncer. Some guys on the GTA recommended it to another QB78 owner in an old thread. I almost wish I'd tried that before the tune kit, but I didn't see it until later. I will stick with the tune kit plan, but am interested in how the debouncer works, beacuse I may be able to mod something later on to get the same effect. If it just adds hammer mass, no problemo. If it adds mechanical dampening in the form of drag/friction on a new (added) part, I need to do some thinking on it.
Anybody know the principle?

radar · Silver Status Member

Nevermind. I found out.
It increases "total energy" available--that's shot count only and not muzzle energy--from what I interpreted in reading the posting by Stephen Archer himself.
Also, it helps keep temperature fluxuations down by minimizing unproductive gas usage. It relies on the friction of the hammer pin slots, and says if those have been polished during a tune-up, then it won't work so well. It also works best with the shorter springs and lighter hammers of the factory guns that happen to have them (randomly, apparently, as it is Chinese made Laughing

).

I may have to make me one of these things. Pictures for sure, if I do. Mr. Green

SMP · Silver Status Member

Pictures would be great. I never have been able to figure out that goofy little gadget.

Alstone · Moderator & Site Supporter

The de bouncer is slightly thicker at the ends of the U shape, in other words the legs of the U are tapered towards the end, and when the hammer tries to bounce back it jam’s in the thick part of the U which expands in the slot and stops and more movement.

Personally people I know who have tried them have been disappointed.

Al