You've not really given enough information on your Diana 34. Do you know what trigger system it has? For a period of time during production of the T05 series guns Diana used Chinese barrels with twelve lands and grooves. Some of these were quite accurate, others were not. If you've not done so, clean the barrel. I use a patch dampened with Ballistol or Hoppe's Number Nine for any new or used gun I purchase. The trigger could possibly use adjustment or even disassembly and polishing. With a bit of practice half inch center to center five shot groups should be attainable. In short I'd get another Diana 34 before I looked at anything Gamo, but then I have a half dozen different iterations of the Diana 34 models.
Like Roadworthy suggested, clean the barrel. Check all your stock screws for tightness. I'm going to guess at only 6-8 years old, the trigger is a T06. It will say which on top of the receiver. Whether T05 or T06, you'll find plenty of info in the German Gate for tweaking either or just post a thread there and ask. If you haven't seen that yet, click up on the right where it says Select Gate and in the menu, click on European Gates, then on that page, click on the German Gate. Might also check for any play in the barrel: side to side or up and down as it will cause fliers if the barrel isn't locking up right for you. Next and probably biggest question is what pellets are you using? Might be as simple as finding a pellet your rifle/barrel likes better that what you're currently using, so you may want to test a few different pellets and weights to find one your barrel really shoots well. Hopefully that covers a few of the bases for things to look for and try.
Like Roadworthy said, The artillery hold, with some spring piston/gas ram rifles you'll want a loose hold. Let the rifle rest on an open palm, don't grasp the forestock. Let the butt of the stock just rest into the shoulder, don't pull it in. Don't grasp the rifle with trigger hand, just enough pressure to keep in position with relaxed thumb. Any hold/pressure differences between shots will affect POI. Just moving forehand up or down the stock one or two inches will open groups.
HistoryI bought a used RWS Diana 34 (.177) about 6 or 8 years ago for some pest control around the property.I never found the rifle to be too accurate. I'd get fliers all the time.After some time there was a distinct "crunch" when charging the rifle so I ordered a Vortek rebuild kit of sorts. A new seal, new spring etc. So the crunch went away.I had hoped that after the rebuild my accuracy issues would go away but that didn't happen. At 25 yards I'm probably around a 2" group. Fliers all the time. I even thought my scope was broken or losing zero so I bought another one. Still no luck.My capabilitiesI used to shoot pistols competitively (not for accuracy) but I did have plenty of trigger time, and had a solid grasp of fundamentals and follow-through. At one time I used to shoot 2-3k rounds per month. With a 9mm Glock I could hit whatever I wanted. No flinching, no yanking. I can manage pretty decent groups with a 5.56 at 100 yards with non-match ammo. So I don't consider my trigger bad with regards to actual firearms...but perhaps it is bad with airguns. As far as I understand there is a difference. The 34 recoils quite a bit. My shootingMost of my pest control is opening up a window on the 2nd story of my house and letting my rifle sit on the sill while I take shots. I take most of my shots within 20 yards but have taken some as far out as 50. I would like something quiet as I do have neighbors and this is a semi-residential area. Chipmunks are ruining my driveway and now I am at the point of needing to spend 35k on repaving it. My questionsI would like an accurate rifle that I can depend on. I'm not sure the Diana 34 is it...though everything thing I read says it should be. I have messed with the trigger to try and lighten it. It never changed much. It's not the best. Mediocre. Heavy and mushy without a clean break...but I wasn't expecting a 1911 feel. Is there something I'm missing to shooting this rifle?My cousin just bought a Gamo Swarm Maxxim Gen 2 in .177 and I feel that it is more accurate than the 34...but still leaves room for improvement. I'd like to be confident in hitting chipmunks out to 30 yards.If I didn't want to break the bank what would you do? New rifle? If so which one? I was looking at the Gamo Maxxim in .22 or the Hatsan Speefire Vortex in .22 (Both are $200)Given my type of shooting (randomly popping open a window and taking a few shots in between meetings), should I even stay with a break action rifle? PCP (no experience here)? Springer vs Nitro piston? I'm all over the place and would appreciate a bit of direction from those who know more than I do.
OK great info. Thanks everyone. It makes sense now that the recoil happens before the pellet exits and that being a big difference firearms and airguns.Most of my shooting I don't use a support hand. I rest the rifle on a window sill or a small sandbag.It seems like the general consensus is to go light on the hold and let the gun recoil as it will. I'll try that.With all this being said... Should I even go through this effort if fundamentally the recoil will disturb the shot? Should I just switch to a different platform that doesn't do that. (PCP?)Do nitro guns recoil less and thus will be less prone to this?