Reset scope for different pellets?

[GarthThomas]

I usually shoot CPHPs, I find they are adequate for my use but I picked up some JSBs and find they are very good,  actually better.
 My experience has been that the 18.1 hit exactly where my scope is set for the Crosmans but the 15.9s and the 14.35s don't. The groups are all excellent but it looks like I will have to reset my scope for each type of pellet. Is this normal and if not does it indicate a traceable problem that I could rectify like another re crown or a slightly bent barrel or my scope being out of level.

[Nikoman]

Garth,
that is normal. Different pellet types will have different POI.

[AmBraCol]

Garth, what is odd is you getting the same point of impact for both the Crosman and JSB's - which are very different pellets. What you're seeing is one or more of the following.  1.  Different weights give different velocities so at different distances you'll have a higher or lower point of impact for different pellets.  2.  Different barrel vibrations.  As you fire a gun the barrel vibrates. This will change with different pellets. As the pellet exits the barrel it will exit at a different point of the vibration pattern resulting in a high, low or horizontally dispersed shot. 3.  Different pellets will spend different amounts of time in the barrel.  This means that they will exit the barrel at a different point of the recoil cycle.

So there's nothing wrong with your gun.  You can either adjust your scope for each pellet, or you can learn to use "kentucky windage" to hold the crosshairs over or under or off on an angle from the target.  This isn't as hard to learn as you might suspect, it depends on getting to know your rifle and its trajectory with the different pellets.  I once had an old Daisy 880 that had the sights way off.  But I could hit anything I could see by holding off from the target.  If I recall correctly I had to hold low and to the right as it was hitting high and left.  At different distances I'd have to hold different amounts, but since I spent so much time with that rifle I could hit anything I could see within range.  Mil dot scopes make the process easier.

[GarthThomas]

Thanks, how about if at 30yds I'm right on and at 12yds their hitting about 1/4"-3/8" left but still in a tight group. What could cause that.

[AmBraCol]

Thanks, how about if at 30yds I'm right on and at 12yds their hitting about 1/4"-3/8" left but still in a tight group. What could cause that.

Usually that would mean you've got a cross wind.  If you're that far off to the side up close then you should be even further off further out.  BUT if you've got a cross wind blowing it could possibly push your pellets into the target - or, if it's the other direction, it could blow them even further off!  Pellets, especially light .17 caliber ones, are easily blown about by a slight wind.

[GarthThomas]

It's a .22 and I have found it with different pellets that if I zero the scope for close or far even though I'm getting good groups they will usually be off the vertical depending on the range, and with no cross wind. 

[AmBraCol]

It's a .22 and I have found it with different pellets that if I zero the scope for close or far even though I'm getting good groups they will usually be off the vertical depending on the range, and with no cross wind.

Different pellets, different trajectories. Vertical differences are normal.  As soon as the pellet leaves the barrel, gravity acts upon it.  What you don't see when adjusting your scope is that the line of sight is straight but the barrel is subtly inclined up or down in relation to the line of sight.  Your pellet flies in an arc, not a straight line.  If your barrel is on a horizontal plane then the pellet actually starts to fall the instant it leaves the barrel, due to the action of gravity upon it, but since you're using a scope your barrel will actually be pointed slightly up so that the line of flight of the pellet will intersect the line of sight.  You will actually have two ranges at which the pellet intersects the line of sight, those ranges depend on many factors such as the height of the scope above the bore, the initial velocity of the pellet and the shape of the pellet.  So perhaps you'll be "dead on" at 15 yards but at 20 you'll actually be hitting above your point of aim and at 10 you'll be hitting below your point of aim and perhaps at 30 or 35 you'll be "dead on" again but hitting below your point of aim at ranges past 35.  (actual ranges will depend on the factors mentioned above).  Now let's say you pick another pellet.  This one puts you dead on at 15 yards also but because it's faster, and more aerodynamic, it actually flies better and doesn't slow down as fast as the first one so you're "dead on" again at 40 but high at 20, 25, 30 and 35 yards.  If you plot the actual height of the impact over the point of aim you'll then be able to draw a facsimile of your pellet rifle's trajectory WITH THAT PELLET.  Ironically, sometimes you'll get a better trajectory with a pellet that starts out slower but has more mass so it carries better than one that starts out fast.  Plus there's the factor of "going transsonic" that plays hob with all kinds of things as well.

Horizontal dispersion, however, is usually caused by external factors of wind.

[GarthThomas]

I must have had too much Barley juice last night, I meant horizontal movement. 

[breakfastchef]

Thanks, how about if at 30yds I'm right on and at 12yds their hitting about 1/4"-3/8" left but still in a tight group. What could cause that.

Internal alignment issue in a scope can certainly cause this. I've had scopes that would shift POI when I changed the AO.

[GarthThomas]

I never thought of the AO affecting that, I leave the power at 9x for everything. I am going to try to level the scope though and see if that makes a difference, if not then I will have to upgrade the scope quality.