How to free float?

Well I couldn' shoot too well at the 25 meter range despite all the caring I've given to the QB78. I thought maybe free floating the barrel would help - off goes the metal barrel band and what! The barrel lies now on the end cap of co2 tank. Why isn't the barrel staying straight, I've snugged the set screw so tight I fear something will break if I do it any further. Might it be the o-rings or something? And by the way what seems to be the correct size for barrel o-rings. I've use 9,25x1,78mm but think that maybe something bigger will fit. Sorry for my metric measurements, hope someone will understand

[ribbonstone]

..know it's a #012
Can do the conversion if you'd like, here are some more exact numbers:
http://www.efunda.com/designstandards/oring/oring_as568.cfm

Believe there might be a metric size that would work, but as I haven't tried it (and don't know the standard sizes for metrics), that's only a guess.

As for free floating, I've not had any accuracy improvement.  So long as the barrel is NOT hog-tied by the barrel band, accuracy seems as good with a band as without.   If you fit the barrel too tightly to the band, then found accurracy got worse (mostly vertical stringing).  On a guess, that gas tube tends to get cold...wants to contract...and if the barrel is too tight to the band, it gets stressed by the contraction/expansion.  

Band tight to the gas tube is good...I'll even epoxy the bottom 1/2 to the gas tube....but the barrel is a firm fit, but able to move in and out the band  if it needs to.

Believe to get any real benefit from free floating, one or two additional receiver screws would be in order....asking tht one screw to keep a barrel with 3 slipery o-rings in place is asking a bit too much.

So it's 0,364x0,070 inches and that is exactly 9,25x1,78mm. So I've been using just the right o-rings. That's not the problem then, and I'm using the RJ barrel band which let's the barrel float a little so I quess that's not the problem either. I've crowned the barrel and the crown is beautiful and uniform. I'm out of ideas how to approach this problem with accuracy because gun is useless if I can't hit the right spot time after a time.

[ribbonstone]

Dirty barrel.
   (OK..know you cleaned it...but sometimes they cough up bit of congealed grease (airgun snot) and it's always worth cleaning the barrel         again.)

Check for bolt probe/transfer port leaks.
   (Leak can vary from shot to shot...so the gas getting to the pellet varies.)

Mounts/rings

Scope (worth it to switch to any other scope you know is working right)

Change pellets (even if it USE to shoot them well).

So long as you've got the barrel off to change o-rings, make sure the two screws holding the action on are tight.

----
one of those odd things, but becasue of the way the barrel is fitted with o-rings and isn't a great fit to the receiver (breech), can get a gas leak to the back or front from between the barrel and receiver.  Bolt probe gets the blame for a backward leak (even if it's doing it's job perfectly).  So check both the breech end and the joint between barrel and receiver.

I've cleaned the barrel so clean that no dirt comes out. Today after the visit to a range I also honed the barrel with a little bit of smooth grinding compound and polishing compound to get any irregularities flatter and it seems even better now, also polished the crown inside the barrel so no catchy edges there. No leaks on seals, I've changed every one of them and with a small piece of paper on top of the possible leak I've checked all seals - so no leaks also. Scope is cheapo 'sniper' orsomething 3-7x20 -scope. I think it's not the scope either but you can't be sure. Have to buy new one just to be sure

The two action screws are tight as what and loctited on place. The only reasonable cause are the pellets. I've used H&N FTT and Gamo Pro Magnums. Gamos are all crap, even by looking in the tin you can see bend skirts and lots of lead flakes on the pellets. FTT's look good and worked on my last QB.

At the 50m range shot from solid bench position so no errors could have been done there and the pattern was about 10-12cm's so not even near a good result. Have to order a ton of new pellets and shoot like a madman to find the correct ones if there is such a thing.

[howie1a]

Lets go back to did the rifle shoot right before any work you did? if so it's possibleit could  be something that you have done maybe it's the RJ band it's a little bigger that the stock band maybe it is allowing the barrel to move to much. I don't know but in working on equiptment if  you do a repair and the equiptment works worse than before un do what ever you did and see if the problem go's away it's one of thethings  repairman do Since the qb78 is a non recoiling rifle I doubt the scope has been hurt by the rifle if it's not been on a springer.Also since the rifle is a non recoiling rifle I'm not sure that free floating the barrel is needed I understand the the free floating is to let the barrel move without any pressure on it from the stock and since we are not dealing with the pressures that gunpowder guns develope I'm not sure that free floating is a help.I can see it on a springer becouse of the spring movement is going on while the pellet is still in the barrel.I have a TF 78 which is a QB78 by compasso it shoots great the only thing I've done is recrown the barrel and choke the barrel and put a MB for a barrel weight and some stock work orther than that it's stock it's 177 cal it's one of the few rifles that I didn't do any tune ups on it was fine out of the box. howie .

[mikeiniowa]

Could be your barrel is too clean......shoot a tin of pellets through it to get it leaded up alittle, is the bore smooth? you can hone them and still have a rough spot, use a pull through with a really tight patch, pull slowly and feel how it goes through, should be smooth, is the bore straight?