Diana 48 Question

[OKHunter1]

I am a bit confused and need help.  What do you make of a D48/.22 which groups noticeably better
with the action screw (in front of trigger) backed off a 1/4 turn from just snug. Not really tight at all,
actually loose enough to feel a slight hitch at the end of the cocking stroke as the action is seemingly
forced back into the stock. The gun is new and has never been opened up. It vibrates heavily when
the screws are tight, has no scope, and about 1200 pellets through her. Chrony numbers are good,
all other screws except the action screw have remained tight since the beginning, but again, it is a
vibrating mess with the action screw tight. The first thing I noticed with early shooting was that the 
gun settled and grouped better after this screw had vibrated loose. Since this is not supposed to be
good for accuracy, I treated the screw with blue locktite and snugged it up tight, assuming that the
gun was settling in, and overall this was the best course for improving accuracy. The POI shifted
high and to the left, and the groups opened up noticeably as she went back to heavy vibration, even
the vertical adjustment wheel screw on the sight would turn a couple notches with recoil. I fought 
with accuracy for a while until post locktite the action screw worked loose again from the heavy
vibration a 1/4 turn and now seems to be holding there. The gun has settled back down with that
hitch in the cocking action and has become noticeably more docile and deadly accurate. Quite odd.
Should I keep shooting with the action kind of loose in the stock? Could it be bad inletting or a
crooked screw housing?  Anyway I'm quite stumped and don't want to cause a bigger issue by not
addressing this one properly. Any ideas?

[DanD]

You might find this thread interesting.
http://www.gatewaytoairguns.org/GTA/index.php?topic=66631.0
Have fun!

[LAalex]

I would remove the stock from the action and check for hairline cracks.  It should vibrate less with the screw snug, not more. 

Scotty

[OKHunter1]

Dan your the man, great link, and ironic, as I have been reading bedding articles from all over the internet for the past 3 or 4 days, but have not seen this one.  This method sure gives me a new and much cheaper option to boot.  This problem has had me really stumped, but to be able to leave it the way it likes it, and floated in a soft bed of neoprene is great food for thought.  Love this site.

[MicErs]

Dan your the man, great link, and ironic, as I have been reading bedding articles from all over the internet for the past 3 or 4 days, but have not seen this one.  This method sure gives me a new and much cheaper option to boot.  This problem has had me really stumped, but to be able to leave it the way it likes it, and floated in a soft bed of neoprene is great food for thought.  Love this site.

Exactly.  I have not bedded anything since I bedded an RS2 with truck bed liner a couple of years ago but it definitely was an improvement.

[Springrrrr]

The spring guide on the 48 leaves a bit to be desired.  Compared to a Vortek or Maccari guide it is very loose.  The rear of the piston rides along the receiver (action) tube when the gun is shot.  The piston stem on my 48 at least was loose due to the fact it is simply crimped at the front of the piston.  The front screw goes into the trigger housing and pulls it snug to the receiver (action) when tightened, which then pulls the receiver against the stock at the rear.

Any one or more of these things could lead to excessive vibration, sometimes known a sympathetic vibration.  Each gun is slightly different and yours may be one that responds to spring and component vibration stronger than others.  By loosening the front screw like you did, you just may have disturbed the sympathetic vibration enough to smooth it out a bit.

You might check to see the receiver is being pulled against the stock at the trigger and not in some way messed up because of manufacturing tolerances is in order.

Mine:

It is hard to describe, but there is a very small gap at the front of the piston that I shimmed to tighten up the crimped area and make it so the stem is tight to the piston and no longer wiggles.

A Vortek kit with a fitted spring guide and a cover over the spring with a little heavy tar grease at the right points, pretty much killed spring vibration.

A fitted piston seal and a new breech seal was installed.

There is absolutely no vibration in my 48.  It has been toned down to about 825 FPS and even with all the above tweaks done to it, it still kicks considerably harder than the more refined TX200 and Walther LGU I have to compare it with.  It is much more hold sensitive too, but when I hold it right and repeatedly, it makes a fairly good showing.  Not quite as good as the other two, but better than many others I have seen.