Gamo cf20 mod version any information.

[Shed tuner]

Hi i think your half right.... I pulled the old o rings and had a look at testing with some generic ones that fit and i have noticed a scratch where the o ring sits and down the side of the chamber which has turned out to be a crack in the transfer cylinder, to finish off the outer transfer cylinder seal has cracked too.

Time for more parts from chambers and hope this is where the air is going.

I am amazed that the part that stops the piston is plastic, makes sense to never dry fire these!

that'll leak

[Phil Russell]

Maybe it won't help but a few years ago I needed a new trigger unit for a CF20 and I called BSA direct ... they sold me a new trigger unit cheaper than I could have bought one elswhere. I also got a new end sight unit/underlever catch unit.
Cheers, Phil

[rovercoupe]

Just for giggles i super glued the crack and tried a rubber o ring instead of the plastic one and made no difference, whould have thought it would have gone up a bit in power.
The only thing that worries me is getting the plastic seal onto the transfer cylinder without breaking it now!

[rovercoupe]

Well I am stumped, new parts are in and it is still low on power, i now own a stack of these and they are all around the 7ft lbs mark. So it looks like they are detuned but is there anything I can do to get the power back? Going to try and get a standard piston tube and see if thats different. I doubt they would change the gun in any other way than a smaller spring and possibly changing the piston in some way.

[adie jaques]

Change the seals in the breech area and make sure you have a good seal, get rid of that awful piston weight/guide and fit a nylon top hat and maybe a 20-25 gm brass weight, make sure the cylinder bore is true and the seal is no more than a moderate single finger push, all the way down, with the loading port closed, try it open and feel the compression or any leaks, when pressing hard on it with the aid of a dowel or similar.
Another problem could be an undersized transfer port, the best size would be around 3-3.2mm, you would need a suitable sized reamer and long arbour to achieve this. I would favour this is the problem, on reflection. Nothing too do with power, the std trigger is in bad need of tuning and can be very good.

[rovercoupe]

Thanks for that, all these have not come with any weight in them so once the lathe is up and running then it could be a good first project to knock up a few bits! Will do a full leak test first though to see if it sheds any light on it.

[Shed tuner]

with it stripped, try pushing a pellet through the barrel, see if it's excessively tight anywhere (e.g. that stupid smoothbore muzzle section)

[rovercoupe]

Got bored of this and had to have a break but back on it now, the seal was rubbish round the transfer block so replaced with a o ring and got a good seal. Also chopped the stupid end off and tried with new spring, old chopped spring and with a bit of weight in the piston end.

No change in power all over the place but no more than 6ftlbs! This is a long shot but i am not using a proper chrono but a app so to rule it out I am going to bite the bullet and buy a proper chrono and see if the twang is causing strange results on the app. Only reason is i got a 6ftlbs difference by swapping pellets on another rifle which is really odd!

[gwyn]

Got bored of this and had to have a break but back on it now, the seal was rubbish round the transfer block so replaced with a o ring and got a good seal. Also chopped the stupid end off and tried with new spring, old chopped spring and with a bit of weight in the piston end.

No change in power all over the place but no more than 6ftlbs! This is a long shot but i am not using a proper chrono but a app so to rule it out I am am going to bite the bullet and buy a proper chrono and see if the twang is causing strange results on the app. Only reason is i got a 6ftlbs difference by swapping pellets on another rifle which is really odd!

Hi Rover, MY EX mod CF20 went t**s up last year, it had previously been chrono'd at 8.5ftlb and was very accurate,not being in to pulling guns apart i took it for repair,turned out it had a cracked main spring,so new spring and seals fitted and came back chrono'd at 9.5ft lb.
Well the other week it went t**s up again!!suspect the same thing again,still shoots but erratic with some pellets appearing to hit a card sideways but almost on the aim point?
I cant be bothered to have it repaired again (out of the warranty by a few months)so have brought a new 440.
I see you are in Leeds, i am in Shipley,its yours for free if you fancy a trip over? message me if interested.its even got a Welsh Willy blade fitted!!
Don.

[rovercoupe]

Right, so i have the gun off Don and its nice, really nice but it is way low and wrong so its been apart today and the spring is bent and compressed and the breech seals were not great so replaced them and noticed at the start of the barrel its corroded away and there was a ring of lead round it. Really hard to get a pic but it looks like its had it. Stuck the new spring in it and it was horrible so put the cut down spring in it with a bit of weight and its output is 4.6 flbs max. It sounds right so stuck a offcut of railway sleeper (softwood) downrange and shot it and max penetration was 18mm, min was 14mm this was at 6m. Gave it a shot with the .22 and got 8mm and the chrono said that was doing 10.2 ftlbs. Now i know this gun has had it in terms of accuracy but with a cut spring it feels like its putting out decent power.

Think its time to bite the bullet and buy a proper chrono.

[rovercoupe]

Pic of barrel, all the others have rifeling at the start and this one is all corroded at the start.

[Shed tuner]

you know the barrels come out right ?

Under one of the forestock screws, there is a grubscrew (allen head); back it out. Then heat up a bit to break the loctite, and unscrew the barrel.

(obviously remove all plastic bits first).

[rovercoupe]

I do now!

[gwyn]

Hello Jon,interesting findings! i thought the spring was going to be the culprit.
The barrel picture is interesting,i imagine the lead my have come from the tail ends of pellets, chopped off as they were launched into the barrel from the breech loader.
Perhaps remove a 1/2" off the tail of the barrel?
Strangest, is that until that spring went it was still an accurate shooter!
I look forward to forthcoming episodes!
Don.

[rovercoupe]

Well its been a interesting week, new chrono arrived and the fully resealed gun with chopped spring was way up on power too close for my liking so a few pennys out of the end and got it down to a nice 10.5ftlbs, smooth as silk with a delrin spring guide and virtually no preload and super easy to cock.

For reference the weak point seems to be the transfer port plastic seal which once replaced with a o ring will send the power upto the moon! Every single one has leaked out of the many i now own. Wether this was deliberate to keep the mod spec ones low i do not know.

A big thank you to all who have helped me through this saga I now have some guns to rebulid and bring back to there former glory.