Tuning The same but not!

[Mycapt65]

Today the wind was hawking all day. Shot a few of my 20s and 22 HWs. It finally laid down enough to break out my 177 HW95. I started left to right with the blue groups. Started off pretty good with JSB 8.44 Exacts. Then the groups fell apart and started vertical stringing. Frustrated with the gun I didn't even finish the the last group of five. For ships and giggles I decided to try 4.53 sized JSB 8.44 Exacts. Wow what a difference. Started to get dark and had to get ready for company so I stopped after the two five shot orange groups. Might as well go out on a high note.

I'm at a loss to explain what happened no other changes were made in between groups and they were shot a minute apart. Go figure?

Distance 25 yards
Dime sized stickers.

 

[Windmill01]

I see a chronograph in the picture with your rifle. Did you at all have a chance to use it when the stringing commenced.
Gary

 

[Mycapt65]

I see a chronograph in the picture with your rifle. Did you at all have a chance to use it when the stringing commenced.
Gary

No. Unfortunately not. But the rifle usually looses about 10-15 fps in about that many shots. The average POI sank when it fell apart. A minute later the POI was back on target with the larger pellet. The regular 4.52 Exacts were sloppy loose and 4.53s were noticeably tighter.

I think as the gun warmed up the harmonics changed and started tossing the looser pellets. The rifle's accuracy and POI was immediately spot on with good fitting pellets. I believe the additional back pressure affected the piston bounce and harmonics.

 

[Windmill01]

Your hypothesis sounds good especially if the poi is spot on with the tighter fitting pellets. I’m really no expert but I have experienced a similar phenomena where I could see the difference in poi but I could hear the piston slam into the end of the compression chamber when using loose fitting pellets. Turns out I needed a new piston seal.

Gary

 

[Mycapt65]

Your hypothesis sounds good especially if the poi is spot on with the tighter fitting pellets. I’m really no expert but I have experienced a similar phenomena where I could see the difference in poi but I could hear the piston slam into the end of the compression chamber when using loose fitting pellets. Turns out I needed a new piston seal.

Gary

IIRC the ES was high with the looser pellets and the gun has noticeably more jump than similar models with larger calibers. To some degree this is normal but maybe I should do more testing and check my records. I'm pretty sure that seal was very recently replaced.

I was having drastic power loss several shots into a session during an extreme heat wave. I attributed it to drag from excessive thermal expansion of the full face seal. I wonder if the seal couldn't recover it's shape now that it's cooler out.
I'll chrono the gun with a few different pellets.