You may remember the shifting POI problem with my twin-tube 909 [www.airgunrendezvous.com]
Well, it went off to a friend of mine for a power tune and a little bit of work on the POI problem, which included removing the plastic fillet between the top tube and the barrel.The tune was quite conservative, involving mainly opening up the airways.

On the weekend I fired a few shots through it. Here's a 7 shot string over the chrony, using 212.5 grain Barnes Raptors: 626 648 677 685 677 665 649 Average 661 ES 59

This represents an increase in peak fps of 12.5% (609 fps pre-modding) and power increase of 26% (Now 220 peak fpe vs 175). Not too bad, with 7 utilisable shots!

However, my accuracy testing showed that the old bogey still remains.

In the picture above you will see that after I adjusted the elevation, the last 5 shots fell quite nicely, with a bit of vertical stringing on the last 2. This was off a 3000 psi fill. I then refilled to 3000 psi and shot the string on the left of the target. The first shot was spot on, then #'s 2 and 3 fell low, before #4 returned to where it should be. I continued off the same fill on the next target(below):

#'s 5 and 6 fell nicely, and #7 fell away, but that was not all that surprising at the end of the fill.
The shots off the third fill (2800 psi) strung vertically, but that was mostly down to me adjusting my hold.

So I'm still at a loss...Gary's suggestion re the tubes flexing may still be the issue, though the two 3000 psi fills did not behave quite the same (excepting that it would have been more helpful if I hadn't had to sight in on that first string). What I didn't do, and will this coming weekend if the predicted storm doesn't arrive, is chrony all of the target strings, to ensure that the nice even string I chronied prior to shooting the targets is repeated throughout the target shots.
A work in progress...

Cheers

Neil

PS Afterwards,I felt a little disgruntled, and so did one of my dogs...

Thats sucks Neil
I hate it when something like that happens. I can't rest until its sorted. It takes all the fun out of it until its fixed.....

Well, for me at least.........
Hope you get it sorted soon

Hi Neil,

Thanks for the post. That's great!

Doggie looks alittle "bent" there. haha.

Here's what I'd humbly suggest. Recording the Chrono data for a bunch of strings takes alot of slugs and gets you lost. I'd proceed thusly:

1). Prepare a bunch of identical targets. Looks like you have that. But ... cut them the same exterior size card stock, so you can stack them and have them register the same later.

2). Set up so you can absolutely duplicate the bench hold each time. No cant. No difference in where the gun is bagged. No difference in your grip. etc.

3). Now. Set up three cards (targets). Fill the gun to 2,600 psi. Shoot one target. Fill back to 2,600 psi again. Shoot the second target. Fill back to 2,600 psi again. Shoot the third target. Retrieve the targets. They "should" over lap, and all the holes register.

4). Now. Do the same thing for 2,800 psi. Fill to 2,800 - shoot one target, etc., repeat. Retrieve the cards- stack and examine. All the holes should register when overlaid.

5). Now. Do the same thing for 3,000 psi. Fill - shoot - overlap - examine. All holes should register.

6). Now ... you've only burned up 9 slugs, and a bit of time. You have all the information you need to determine if the rifle is consistent. All you need to tell if the slug/barrel are a good match. We're gonna have to assume that you were perfect. That's why you take time to set up a bench situation that can be easily, and absolutely duplicated 9 times in a row (even with the interruptions of 9 fill cycles).

7). The three cards from each of the three shots at each individual fill pressure should overlap well. You've removed the variable of fill pressure. Each was shot at the same PSI. You've removed the factor of the tubes moving over a given decending pressure cycle - each series was shot at the same psi.; thus the same tube postion can be assumed. You removed yourself from the varibles, with good bench technique. If the series do not overlap well - then the hammer/valve cycle is not consistent. Or, the barrel/slug are not a match.

8). You can now examine the relationship between the holes of the three cycles. You should be able to see the shift in the point of impact ... each slug would make when shooting a group.

And, again ... you've done all this with just 9 slugs!

Gary

Neil,

Sorry to hear the gun still has issues, at least the dog's still faithful! Thanks for the post though.

I think Gary has a good plan for you, especially considering what you paid for shipping those slugs!

Paul - Thanks, looks like I have a way forward now from Gary....

Gary, I love that idea! 9 slugs is a big saving too, both in money and time! I'll be onto it as soon as weather permits and will report back. Thanks

Jerry - You're right, I don't like "wasting" those gold slugs but as they are my larger game hunting slug I need to do my testing with them.

Cheers

Neil