Well, let me start with the most important part of your question, in my mind. ANY range work must be done with a dead-solid rest. Period. You just simply cannot get a good idea of what is going on with your gun unless you are positive that your shots should be where you call them from an immobile rest. The difference in barrel length is something that is of some but not a great deal of concern, I feel. I have felt for the most part that when using factory ammo, that somewhere close to 50 fps. is lost for each inch of shorter barrel. I am sure others will differ with me, but I feel it is a close measuring stick. With that said, you can see how velocities can be affected with the same load in different barrel lenghts. It just then makes common sense that trajectories will change accordingly.
Get your gun shooting well for you at 100 yds., whether it is with some brand of factory ammo it likes best, or with a consistently-shooting handload. Move your target to 200 yds., and shoot again with a dead -on hold from your solid rest. Note the difference in drop. Do this again at 300 yds., and compare. This is the only true, proven way to find out how your own paticular rifle will perform at different ranges. Yes, there are calculation tables that will give you the theoretical drops, but until you shoot those ranges yourself with your gun and load, you will never know for sure.