A while back, the wife got me a Gamo Whisper when I told her I wanted a pellet rifle.

Seemed to do well with open sights, and after a few days, I decided to put the included scope/mount on it.

When I did that, I found I couldn't sight it in.  Started at 25 yards and noticed that it would make good groups for 3-4 shots, then it would start hitting several inches low and left, high and right, etc.  But, each time the zero moved, it would group there for a few shots, then move again.

I swapped out the factory scope with another 3x9 scope that I previously had on another gun and worked fine, even if it was a little cheap.

Same problem.  Groups nicely for 3-4 shots, then starts grouping somewhere else several inches away.

I gave up, pulled the mount off, and shot it for a while with the open sights.  Seemed to work much better that way, except the open sights are terrible, and my eyes aren't much better.

Started looking and really didn't like the two-piece mount that the Gamo comes with.  So, I decided to order a one piece mount that clamps directly to the dovetail.  The new mount also has a stop that I adjusted to keep the mount from moving.

Tightened the heck out of everything and took it to the range.  Started shooting and working my shots into the bullseye.  Got it almost there, then the groups jumped way left and low again.  

Walked it over again, and just about when I got it there, it jumped way right.  Did this two more times and finally gave up.  

What gives?  I keep reading the reviews and lots of people seem to like theirs and many don't have these problems. Anybody have this problem with their Gamo? Wondering if there is something else I should try, or should I just give up and get something else?

From what I under stand - air rifles are very very hard on scopes......

Red

Don't give up just yet.

The rig isn't one that I have ever used, but I looked it up and see it is a rather strong model.

Two things to keep in mind.

Break in: The rig may need more time to settle and break in. The seal and lube are probably still factory fresh and a typical spring gun takes most, if not all, of a tin of pellets to break in.
If your rig is still bouncing funny due to lube and break in, you will get flyers and POI shifts.

Spring gun recoil: Springers are very hard on recoil issues. Scopes can be broken, mounts can slip, stocks can come loose. Be patient at first. Keep and eye open for things that are external that you can observe. If those scopes are not "air gun rated", consider trying one that is. Air gun rated scopes do not have to be very expensive. There are several brands that make entry level fixed or variable low power air gun scopes. It sounds like you understand the mounts need to be pinned or prevent slippage, that is important and usually something you can spot once you start accumulating cycles.

Try and watch those two issues while you keep working on technique. Spring guns are very hold sensitive and not easy to bench rest. 25 yards is probably one or two beyond the top of the ballistic arc for a typical scope line. Don't go much farther out till you can hold under an inch at that distance. Once your POI stops drifting, then work your way 5 yards at a time to about 40 yards. This should take at least a tin or two over several (many) sessions.

If your stock, mounts, or scope, is to blame, it will not settle out and you should be able to investigate to find it, however you will need to be patient at first unless you are positive that the rig is done with break in and that your technique is not to blame.

If at all possible, visit with a nearby air gun specialty shop or a field target club and let an expert give it a run. That will end the question in short order and put your mind at ease.

Thanks for your help. Here are my thoughts.



I read more about it last night and I think this could be contributing to the problem.  The groups seemed to shift less this time than they did last time I shot it.  Of course, I changed mounts in the intervening time, but I think this probably had something to do with it.  I shot it a decent amount without a scope before I got my new mount, which undoubtedly contributed to the break in.

Something else I need to do is clean it, as I have read that cleaning can have an effect on accuracy.  I haven't done that yet, and have no tools to do so. Is there a way I can drag a patch through the bore without a .177 cleaning rod? Maybe get a string through there and tie a patch to it, like a poor mans boresnake?  I'm sure Hoppes is a no-go...right?  What should I use?



I tried both standard and airgun rated scopes with similar results, so I think probably this is the least of the issues so far.  But, I'll keep an eye on it.



I think this is the big issue.  Remember when I said it was getting better with open sights?  I wasn't as worried about my hold when shooting with open sights and probably relaxed a bit. It did much better without the scope. Of course, my expectations for precision were lower, too.



I just pulled the scope out of the mount and torqued the scope stop down.  Now that I think about it, I forgot to do that when I put the new scope mount on.  I think that might make a difference. Everything seemed good and tight, so I'm not sure anything moved much, but I'll keep checking on it.



I live in WV.  There are no air gun specialty shops here.  

To clean an airgun, we use a tool that looks like a piece of shrink tubing with a strong piece of line looped inside. The loop sticks out of one end and there is handle on the other.

The shrink tube gives enough rigidity and flexibility to shove the whole thing down the bbl from the muzzle and out the loading chamber. Once there, the loop is opened by sliding the shrink tube down a little and enough for a patch to be placed inside the loop. The shrink tube is then used to choke down on that loop to secure the patch. The patch is saturated with cleaner/lubricant and pulled through the bbl. Some strokes with a dry patch will tell you if there is leading.

It takes several shots to settle the bbl again, but every bbl and solvent takes a little experimentation to determine how sensitive a rig is to fouling and cleaning.

If I get home for a minute I can take a photo of the ones I bought from Mac-1 and the cleaner I use. I looked at AOA and Pyramid and was a little more than surprised I didn't find what I would like, but AOA did show their own cleaner/degreaser. I have not asked what is in there.

You really don't want to over do it with air rifle cleaning and it is easy to do more harm than good. I would suggest trying to learn to use a pull patch to prevent bbl damage and to learn to detect fouling.

Not sure where you are in WV or how often you travel near Field Target Clubs, but it would be worth the stop if it comes up. You will learn very fast when you meet other experts.

I improvised

I took a thin piece of vinyl coated wire, thinking that the vinyl coating would protect the barrel.

Made a loop at one end, threaded a patch through the loop, and pulled it through from the breach end while lightly oiled.

It was filthy!  I did that a couple times with different patches until they came out somewhat clean, then a whole bunch more times with a dry patch to get all of the oil out.

Looked down the barrel.  Didn't see any evidence of leading.  Looked nice and clean.

If it had been a super expensive precision gun, I wouldn't have done this, but I figured what the heck.  I'm sure the vinyl protected the bore well enough and I took the extra step to bend the end of the wire away from the direction of travel through the bore. Seemed to work great, except that it was a monster to get it to go through that stupid suppressor on the end of the barrel.

After your post, I got to thinking.  I've had a couple of guns that had to be cleaned regularly during the first few hundred rounds or they would be all over the place.  I almost sent my Ruger SR22 back for this very issue...it was shooting all over the place, but when I'd clean it, it would settle down and shoot nicely for a while, then it would need to be cleaned again to restore accuracy (seems counterintuitive, but that's how it worked).  Now, you can run it all day long and the accuracy never suffers. I guess there's no reason that a pellet gun wouldn't be the same way.

Anyway, I'll be interested to see if this has any effect. Like you said, it may take a few shots to get it to settle down, but I'm hoping for good results.  

Thanks for your suggestions.