How do you know how tight that is? You can guarantee that the bag will loosen up over the first few uses, so you will always have to add more sand after the first packing. You can be absolutely certain that the first time you put sand in a new bag you won't be able to get too much in.
If it is floppy, so that it flips from side to side and won't hold still, that is too loose. If it settles down to a firm base when you hammer the gun into place and doesn't want to move so that you can return the gun to battery, that's about right. If the bottom of the bag starts to bulge out, so the bag tries to become a sphere and wobbles on its bottom, and if you can't dent the surface of the bag with firm finger pressure, that's too much sand.
