I have a RWS M-350 MAGNUM .177 caliber "PREMIUM COMBO", from Firearms and Supplies in Hancock, MI (www.airgunsUSA.com), which includes the Bushnell Banner 4-12x 40mm (Special Airgun Scope) and the EZ-Plus One-Piece Scope Mount with integral scope stop. This is a one piece mount which has a pin for preventing the mount from moving in relation to the dovetailed base. The scope mount also has sticky tape that lines the clamps. It seems like a solid mount to me.
The scope, a Bushnell Banner 40MM, 4-12X, #71-4214 The package can be seen at http://www.airgunsusa.com/desc/rws16155pc.html

I assembled this package and sighted the rife in. I purchased a tin of 500 Beeman Field Target Specials. This setup has been good at providing me with quarter-sized patterns from 30 yards.
I don't know exactly when this started, but now, after 10 to thirty rounds (I'm estimating), the pellets are dropping five or six inches below what it was sighted in at. I can resight in the rifle and after a while it does the same thing. It typically does not wander right or left; it just drops. I cannot see that the scope or mount is moving...
The seller thinks it is the scope, but he says it is a very dependable scope. I have shot about 700 pellets through this rifle and in all other respects it is a great rifle combo.
What do you think is going on?

You say that, each time you rezero the scope, the point of impact falls five or six inches after a couple dozen shots? Does it consistently fall to the same level each sighting session? Does the scope have a lockable adjustment? (Some scopes have to be unlocked in order to run through the clicks, then locked back down.) Are you shooting the same way each time? That is, not zeroing standing and then shooting benched, or vice versa.

It does sound like the scope, although the model looks like it ought to handle the recoil of the 350.

To be honest, I am guessing about the number of rounds because, when it seems good, with half a dozen accurate rounds into a target, I go hunt with it. I have probably never shot more than twenty rounds into a target at any one time. I am careful with the rifle and scope and essentially never touch the scope; particularly when cocking. I haven't banged it into anyting and haven't dropped it.

I consistently use only two pellets, the Beeman Field Target Specials (round nose match pellets), @ 8.9 grains or the Predator @ 8 grains. These shoot fairly closely to each other. There is perhaps a 1/2 inch drop on the heavier Beemans @ 30 yards.

Yes, the deterioration is consistent within an inch or so. It always has dropped four to six inches and if it goes off to the left or right it is an inch or less.
The scope has click adjustments but no locking features. I have never noted the setting number and whether it has changed from that. Is that possible? It's not that easy to turn.

I "bench" it on my hot tub cover, so I essentially stand and lean over on my elbows and rest the front stock in my hand. This is how I have done it the last three or four times. I can do a one inch square pattern, or better, this way at 30-35 yards.

I haven't found anything definitive on the internet that seems to address my problem. That's why I'm here. I wanted to see if I could find someone who has had a similar experience before I send the scope back to Bushnell (only to find that there is nothing wrong with it).

It would seem to me that if this has happened several times you ought to be running out of adjustment - unless the elevation knob is turning back somehow.

It has about 7-1/2 full turns available, and I had four left (to go up). I recall it requiring one or more turns to bring it back in. This jives with their calibration clicks, where one click = 1/4" @ 100 yards. There are 60 clicks per revolution. Meaning that one revolution = 5" @ 33 yards.
I had at least four revolutions left, so if it started out somewhere near centered when I mounted it (and did not require me to screw it down a bunch when I first mounted the scope), then it suggests that the settings are changing! I know I have re-adjusted the scope upwards at least four times! This would have required, on average, a full turn each time, and YES, I should have been running out of adjustment. Yet, I still had more than half of it left.
Perhaps I should call Bushnell...

far from it. it seems though that it would be unlikely that something is happening inside the gun. it would take a pretty big drop in velocity to create that much drop. it would be unlikely that it would show after ten shots then be ok after re-zero then repeat itself. eventually the pellet would not leave the gun. so, i think we can eliminate a velocity drop. this gun is a break berrel. is it at all possible that your locking mechanism is loose and is causing barrel to droop after several shots? that seems the only mechanical thing that could do what you describe. a bad piston seal COULD do it maybe but probably not. if its a bad seal, as the gun warms up maybe it starts to blow by. the gun cools down while you re-zero. you shoot and it warms up and blows by. its a stretch i think. iron sights would create the same pattern if it was
a seal. it might not if its the barrel droop thing because the sights are on the barrel.

after eliminating those possibilities it has to be the scope, or mounts.

again, i am not expert. just trying some logic.

http://www.bushnell.com/general/riflescopes_banner_71-4124.cfm is the scope I have.
They talk about a 60 inch adjustment at 100 yards.
A click is 1/4" @ 100 yards, which = 240 clicks and there are 60 clicks per revolution. This would be four revolutions, whereas I can get it to turn 7-1/2. Same for left and right. This is strange...
I'm going to try the iron sights and report back. Bushnell said to send it in but I don't think the girl I talked to knew anything at all.

Start out in your normal, consistent fashion and zero the gun. Take a bit of whiteout or something equally removable and mark your turret setting. Then do ten or twenty shots in a row and see if it's dropped POI. If so, look to see if the turret has rotated. If it's the scope, I doubt you would be able to continually adjust it up without A) running out of clicks and B) failing in the same direction every time. Usually, scopes tend to wander all over the place when they go bad.

The iron sights should give you an indication of whether or not the gun, itself, is not holding POI.

Just be sure to shoulder/hold it the exact same way each time to rule out anything you may be doing.

I mounted the iron sights and started calibrating. I started at about 65 feet and then moved back to exactly 100 feet. I had to grind some off the notched, removable insert because I could not get the rear sights low enough. I put forty to fifty rounds through the rifle.
Finally I was able to get an evenly distributed tennis ball sized spread of five shots at 100 feet, with one bullseye. Seems pretty good to me... Is it decent accuracy for me/gun? I was standing and resting my elbows on the top of my gas grill.
Now I just need to go back and shoot about twenty rounds through it, at 65 feet, and see if they start to drop. I'm kinda an old fart and my trifocals give me fits with the iron sights. I liked the scope... Is there any workaround for using iron sights when either the target or the sights end up being blurry?

Being similarly afflicted with presbyopia (that's what they call it when you get old and loose flexibility in your lens hence can't focus close any more) I've found two things that work for me:

I can use an old set of reading glasses from when I first started using them. They correct hardly at all, but bring things into focus at about three or four feet - I can shoot pistol with open sights that way - I'm told that what you do is focus on the front sight and let the rear sight and target sort of take care of themselves - this works reasonably well. The other solution: try a red dot. I've taken to that for pistol FT and found that it works remarkably well.

Then back to 65 feet? At was distance was the drop occurring? Might be easier to spot if you're at 65 feet with the iron sights.

Your point is why. It will be easier to see the target. I have not had a sunny day in a while and it's hard to really see the target lines (just black marker lines) at 100 feet. 65 feet is somewhat arbitrary, but I have a rock outcropping that keeps pellets on my property and the hot tub cover is 60-65 feet away. I'd say 60 to 100 feet is my usual squirrel shot distance. I went out this morning in flat light and couldn't really make out my iron sights while trying to aim at a gray squirrel. Missed twice.

I'm not sure what you mean by "at what distance was the drop occurring". When my scope (or whatever), gets noticeably out of whack it was usually 4-5 inches drop at 65 feet.
I know there is a rise in the trajectory of a bullet before it starts to fall and I do expect it to shoot higher as I move in. This is more data I am interested in acquiring.

Regarding the sights, I've never heard of "red dot sights" before and am overwhelmed with the options and wonder whether the scope versions would even stand up to the recoil of my air rifle. They mention them being appropriate for shotguns and crossbows. Just as appropriate for high velocity air rifles? Also the prices of these sights go from $9 to $400. Again, baffled. It's been a very long time since I've been to a gun show or store.

I've never heard of "red dot sights" before and am overwhelmed with the options (on the internet), and wonder whether the scope versions would even stand up to the recoil of my air rifle. They mention them being appropriate for shotguns and crossbows. Just as appropriate for high velocity air rifles?
Also the prices of these sights go from $9 to $400. Again, baffled. It's been a very long time since I've been to a gun show or store.
Is this what you are suggesting (a replacement for the front sight)? http://www.opticsplanet.net/picture-1-ncstar-red-dot-sight-plastic-red-dot-weaver-base-dp.html

You might try an inexpensive red dot. IMHO - you don't really gain that much with a more expensive one. I have a Tasco which cost me $35 at Sports Authority. I've tried it on my RWS 52 just for a few shots - normally it's on my Crosman Custom pistol (CO2). The red dot is cool for us with older eyes - everything focuses as at long distance - you just have to concentrate on getting the dot over what you're trying to hit (assuming you have it zeroed). There can be collimataion error with the cheaper units, but I've found that as long as you keep the dot more or less in the middle of the field of view it really isn't significant. If you like what it does for you then look at further options - A1 optics makes a unit with a 3 MOA dot (cheaper ones are 5 MOA) and a 2x adapter for about $50 - I don't have one yet, but probably will before long.

You need to do some definitive testing. Stop changing distances. Its tough to diagnose this kind of thing when the range keeps changing.  Sight the rifle at 20 yards, keep shooting at that range, see how many shots until POI changes. Do the same thing with iron sights, see if the POI changes. You can't really tell whats going on inside the gun without a chrony. Velocity testing will tell you if the problem is the rifle or the scope. Theres a few things that could be happening here, no way to tell   without real test results.

1. the breach seal, or spring

2. the mount, it is slipping

3. the scope, BYE, BYE scope

but Dave is right, mark the mount in the rail with Nail Paint. I use my wife's WHITE nail bottle to mark the end of the mount and shoot, shoot and shoot if it is sliding I will know 3 rows of 10 pellets with a waiting time of 30 seconds between pellets.

Next, inspect the seal (breach) and see if it is locked down with a tissue paper on top of the breach seal assembly if it lifts then you have a bad breach seal, meaning the tissue paper PUFFING up

then go open sights and check if you are 1/2 or 3/8 at 20 yards everytime you did this test.

warren

PS: Bet you, it is the mount, rubber breach or the scope. if it is the spring YOU WILL need a chrony

and remember "it's 30% the gun and 70% the shooter"

That is what I meant about 100 feet or 65 feet. As mentioned, if it's dropping you should see it at twenty yards. Maybe not four/five inches but will still be there. And don't forget to set your adjustable objective to the right yardage.