Okay, here is what my brain is telling me in response to both of your posts. To get consistency at range, you THEORETICALLY (if I have my theory all figured out right) need, or would benefit from, a high ballistic coefficient. Your ballistic coefficient, if I remember correctly, is derived from the mass of your projectile divided by surface area of your projectile, and an oddly shaped pellet (as opposed to a relatively streamlined spire point bullet) may factor into this differently than normal.
Therefore if I were building my own benchrest pellet gun, I would look for something with a good heavy pellet relative to its bore size.
Now, to compensate for drop and wind deflection, you need a relatively high speed projectile. However, you do not want to attempt to break the speed of sound, because that makes bad issues for some reason that I never fully understood. However, what I do understand about supersonic projectiles is that when they go back under the speed of sound they are affected in a bad way. So, if you can keep them supersonic fine; but if you are going to be back under the speed of sound by the time your pellet reaches the target, it would have been better both for your accuracy and efficient use of the energy imparted to the projectile if you had just stayed under the speed of sound to begin with. To my understanding, the ultra-high velocity pellet guns are still only going 1200 fps or maybe just a little bit more at the muzzle. If the speed of sound is 1100 fps, which depending upon your altitude and temperature should be a pretty close estimate, even a really fast .177 pellet will be subsonic by the time it reaches its target, probably even by about 25 yards (just a guess on that distance), but definitely by 100 yards.
So, back to my hypothetical benchrest pellet gun, I want something with a heavy pellet and probably running about 1000 fps max with whatever pellet I am using. It doesn't matter if the manufacturer says that it shoots 1600 fps with a 4.5 grain .177 alloy pellet, if you put a really heavy pellet in it to get its velocity down to about 1000 fps, you will be much better off at range because of the massive difference in the ballistic coefficient. This is just mentioning the BC, not even the whole problem of that pesky sound barrier!
This is all after you find a gun that performs well, and have figured out what kind of pellet it likes. With firearms, they have pretty much figured out what works and what doesn't, but at the present time, it would seem that long-range pellet gun shooting is more of an art learned by trial and error than a science. PCP is probably your best bet, yes even for fifty yards. I would be looking for something with a high energy output in .177 and very heavy pellets to go with it.
That being said... I don't have any PCP guns. Yet. I am just too cheap to get a PCP when it costs more than a .22 LR! Forget a .22 LR, they are more expensive than many sporterized or beat up Mausers! And... as my name implies, I like Mausers. So... being short of a good PCP gun, I have used my cheaper break-barrels to good success out to about fifty yards. I have used them to some success out to and a little beyond 100 yards. Obviously not as good as you are looking for, and really not good enough for anything but "fun shooting", but at least it is something. I have never used a gas piston, but from what I read about them, I'm not going to get one anytime soon. But if you don't already have a coil-spring gun, it might be worth it for you.
Looking at the above group, it would appear that your gun just doesn't "like" those pellets. I don't know why your shots were high. As an aside, none of my guns (break barrel and multi-pump pneumatics) like GAMO brand pellets.
Just as a disclaimer, I am in no way an expert in the field of air rifles (or any other field for that matter). Neither am I a physicist in any sense of the word. I just try to understand "why". Sometimes it helps, and sometimes it doesn't.
Wow, that ended up being a long post...
