I've noticed this strange phenomenon with my .177 Diana 24 (T-05). At 15 yards, my POI will change consistently up and down every 7 to 10 shots (yo-yo-ing)roughly half an inch: a consistent string of shots at one POI, then a sudden shift up for several shots, then a shift back down to the original POI, back up to the higher POI, on and on.

My data aren't here with me, but my last string tested 60+ with these generally repeatable results.

Assuming it's not the shooter (me, seated FT leaning against a wall for support) the pellet (CPLs), or the scope (I've tried 4 different ones with the same results; iron sights don't give me accurate enough results with my eyesight to make a good comparison), what could cause this phenomenon?

Hard to believe barrel lock-up issues could be so consistently in repeatable strings, but I can't think what else it might be? I honestly don't recall that this rifle ever did this when new, but it seems to have been doing this for a while now (with my thinking it was the scopes having been damaged from being on more powerful springers).

out to be the scope in my case. After changing the scope, my 24 returned to the "pellet stacker" status. I'm assuming you have checked for loose stock screws, mount and etc. Also, this problem can be caused by the scope being set near it's vertical limit. If that is the case, shim the scope, bend the barrel, or get an adjustable mount. Otherwise, try a different scope and see if it eliminates the problem.

Jonathon,
I have the identical problem with my TO1 '68 Mod 24, with both .177 and .22 barrels.
It beats me, I have changed every thing including the lock-up stud. I haven't changed the stock.
I've tried different holds and bench rests. I know I'm shooting straight 'cause the strings are dead vertical. It should be at least cloverleafing if not one holing. Its driven me to distraction and I am on the verge of giving up and going back to CO2 for garden plinking.
Let me know if you find the cause. Over a chrono it is very very consistent.
???????

definitely don't give up on springers, or even Diana's, or even break-barrels for that matter. When they work, they are a treat.

However, I will add that my Diana 24 is the only Diana I still have, and the only break barrel I still have - fixed barrels remove one possible variable in the accuracy puzzle.

I've put five different scopes on it, so either all my scopes are bad (except they work fine on my other rifles), or there's some other cause.

Yup its not the scopes, I've tried several that on other spring guns I've got great groups from.
My stock had a crack but that has been repaired super solidly. However I'm not prepared to source a new stock until I know it can one hole!

eliminated the stock screws and mounts being loose, it almost has to be the barrel lock-up, or possibly a broken spring. There is some horozontial adjustment that can be made in the lock-up (probably won't help the vertical problem much), but if the lock-up is worn or damaged, then it's time to check with a "real" air-gunsmith. Hopefully, he will be able to make the appropriate fix. My model 24 is extremely acurate, even better than my R-7. I know that is heritical to some, but it is the truth, the whole truth, and nothing but the truth.

problem witht he seal.

Hey Jonathon and Billy Bob
When I bought my 24 about 18 months ago it was stringing,
I have:
Replaced the lock up stud,
The breach seal and shim,
The mainspring,
The main seal,
The piston,
The trigger unit,
The end cap,
Tried .177 and .22 barrels,
Tried 3 scopes and mounts.
Over a chrono at the local shop its super consistent 596 fps 5.4ft/lbs with Gecos
Here it is and its darn well beaten me.

http://picasaweb.google.com/Crabbet/Original24TO1/photo#5045945888631614658