¿Why does my airgun shoot heavier/lighter pellets lower/higher and/or to one side? ¿Shouldn't the difference in trajectory simply be up or down?
The thing is further complicated when a heavier pellet shoots higher than a lighter one.
¿What is happening here?
Well, several things could be happening, for example: I have found guns that shoot heavier pellets to higher POI's, even when the chrono tells us that the gun has a lower MV.
Sometimes that happens when the rearwards recoil of the gun is pronounced. And there is the clue.
By the time the pellet leaves the muzzle the gun is halfway through the rearwards recoil cycle and the muzzle is pointing a little higher up.
Because it seems that the pellets land higher, some shooters are convinced that a long first stage recoil cycle is better than a short snappy one.
They demand that their guns be tuned to "push into the shoulder" rather than give the shooter a fast rap. And they maintain that a snappy cycle is no good,
Reality is that most of the difference in this case lies within the shooter's head.
There is another effect that baffles shooters even more and that is when a heavier or lighter pellet prints not only with a vertical variation, that was expected, but also with a LATERAL variation.
Well, what is happening here is that the harmonics of the barrel make the barrel point to a different direction at every instant during the firing cycle.
Again, I've heard arguments that the harmonics can only be vertical oscillations because, after all, the barrel is held in place by the gun and the gun by the hands of the shooter, so ¿how can it move sideways?
Reality is that it can and it DOES. I am sure that if we could take a graphical reading of how the muzzle dances all over the place we would wonder at how can we even land a pellet or two in the target.
There have been in the past different "Muzzle Tamers", "HOTS" systems, "BOSS" systems and simple limb-savers borrowed from the archery world.
Most of them are complicated and require long shooting sessions to ascertain whether the small movement applied to a counterweight is beneficial or not to the harmony between barrel and pellet.
In the firearms world there is still another system that controls the location of the forward pressure bedding point common in many sporters, and does quite a creditable job with just one screw and one plastic barrel support inletted into the forearm of the stock. But there is nothing like that for the airgun world.
For one, most airguns have what would be otherwise called "Free-floating barrels" especially the spring-piston ones.
Some years ago, Diana came out with the T/H series and then the Pro-Compact series. In both these lines the rifles bear a very heavy muzzle weight that is sometimes beneficial, sometimes not so much.
Part of the Diana problems lie in that the fit between barrel and weight is somewhat loose and that the grub screws that attach the rather large weight are quite small. So, I have found some movement during the shot cycle of the weight, and there is no rifle that can be accurate with a large weight that moves differently each shot.
The challenge then was to design something that was simple, robust, would not come out of adjustment easily, but would still allow for the inevitable change of batch when the stash of "magic pellets" has been exhausted and a new batch needs to be acquired.
After a few disasters with the commercially available units of the past and the present, I settled on two carefully dimensioned muzzle weights. Much lighter than the OEM Diana ones; one that is appropriate for power levels of around 12 ft-lbs. and another that is suitable for full power rifles. The weights have a "hollow section" where thin ORings can be inserted and then the muzzle piece is tightened. To get out of adjustment you would have to loose an ORing or two and that is almost impossible without completely removing the muzzle weight itself.
Now, what is interesting here is not the resulting gadget, that will have a place in the products section, but what is interesting is how the process of tuning out the harmonics of a given barrel demonstrate that the barrel vibrates in all directions.
Let's see the target that shows the process:
For each group the muzzle piece was removed and one ORing inserted.
Starting on top left bullseye and using one of the holdover points, the group printed was 3.12 mm's.
Next on top right but still aiming low, the group opens to 3.81 mm's
Adding one more ORing and coming to the center bullseye, the group now opens to 4.77 mm's
Next is the bottom left bullseye where we add one more ORing and the group opens to 5.44 mm's
One more ORing (4 by now) and the group now closes to 3.43 mm's.
One more ORing (5) on the top left but at the center of the bullseye tells us that the goup opens again to 5.93 mm's
On 6 ORings the groups reach the 5.9 mm's
By 7 they close again to almost original size: 3.21 mm's
Now, the sizes of the group tell us that the harmonics are cyclical, that is you may find a few sweet spots in the middle but if you persevere you will find an overall minimum.
Granted that these groups were shot with the pellet that the gun had chosen as its favourite pellet, but still the differences are somewhat big.
Now, MORE important than the size of the groups is the "Major Direction" that direction where you find the most distant shots in a group.
In groups with 0, 1, and 2, ORings the spread is horizontal, while on 3, 4 and 5 are almost vertical, by 6 things go back to the horizontal; 7 is vertical again.
This rotation of the axes of the groups tells us that the barrel oscillates in all possible directions.
And that is something that is worth noting.
Keep well and shoot straight!