20yrd groups awesome but 30 yrd groups are poor

[Dhc8guru]

I haven't really run into this problem with any of my rifles. At these relatively short distances, usually the difference between 20 and 30 yrds is minor.
Now with my Hammerli 850 (.177) shooting at 20 yrds, I am seeing about 0.400 groups with JSB Exact 8.43gr and CPHP's are about the same. Then at 30 yds they open up to 1.25" at best with the JSB's and even worst is CPHP go to over 2". I have tried about 10 other various pellets and most were worse.

Right now I am chalking this up to a pellet issue. At first I thought maybe the pellet velocity could be an issue, but certainly 700fps is plenty for 30 yards.
I am a bit stumped on why I am seeing the groups open up so wide in just 10 yards. ​​ ​

 

[30cal]

Have you actually measured the velocity? Could be a bad barrel. Pellets tumbling maybe? Co2 losses power at cold temps. Its winter could be to cold out? Is this a new problem? Or a new gun? Have you cleaned the barrel if its a new gun? Bad scope or loose mounts. Just some thoughts. Good luck.

 

[greg]

Looks to me like pellets destabilize after a certain distance. Do you have a scope cam to attach and try see pellets in flight? 
[edit: clarification]

 

[jps2486]

I suspect your pellets are spiraling. That could be caused by shooting too light a pellet at too high a velocity. Best keep the speed below 875 fps.

 

[30cal]

The Hammerli 850 is a CO2 gun. If its stock it won't be shooting to fast.

 

[MarkUK]

Same table, same rest, same hold?
If you moved out to a different site to do the 30 yarders, you may have introduced something like parallax or side winds into the equation.

 

[Dhc8guru]

Lets see...its a new gun,. Well actually a warranty replacement to another 850 that failed. I haven't measured the velocity yet as Its been very overcast for weeks. I can say that it is hitting very hard. I have a metal swinging flip up target that is made from 5mm plate. Its a firearm target. Anyways, it has no problem at 20 yrds flipping the target up with 8.4 grain pellets. Temps have been in the high 60's to low 70's , wind has been under 5mph, I am just shooting from a seated position and resting on my knee. Fairly stable but not like its locked in a rifle rest. I suspected tumbling pellets. but the holes in the targets are clean. Only when I tried some H&N sniper magnum 16 gr pellets did the holes finally ripped like the pellet was hitting at an angle. 
If the barrel was bad, I think it would have poor accuracy at any distance? They are Lother Walther barrels and I haven't read of any barrel issues on the gun. ​Certainly can't be ruled out. I am going to re-clean the barrel, the first one was just a quick swab and do a close inspection.
The patterns even though crappy, they are consistent. Crossman HP's are a vertical string to the left of center. The JSB's are round and in an arc pattern.
I ​​ will see how the new pellets shoot when they get in. Unfortunately the weather turned cold and won't warm up to the upper 60's till Saturday.

Thanks for the help so far.

 

[30cal]

The first thing I would try is a stable rest on a calm day. Its the only way to know for sure.

 

[Dhc8guru]

I think your right. My hold was steady evident from the 20 yard results. The more I think about it, I think the wind was a bigger factor then I considered. That extra 10 yards spans a pretty open area of my yard and with the pellet slowing down, the wind has more time to toss the pellet off course.

 

[chasdicapua]

The small errors in the consistency of our hold and trigger pull get exponentially magnified the farther out we go. I would do what .30 cal says and get the gun as stable as you can on a steady table. This will tell you if it's the gun or not. I have to say, every single time I've thought to myself, "something is wrong with this gun or this scope", the more I looked into it and checked it out, the more I realized it was me!

 

[30cal]

So true!