Well, i believe in barrel vibration harmonics. More so on some rifles than others. A short bull barrel, is not going to be affected nearly as bad as a 26" standard profile "snake whip" magnum rifle barrel. And as for something like a 30-30 with a barrel band somewhere in the middle and almost at the end of the barrel, i dont have a clue what harmonics would do with them. I do know that the amount of rounds in a tube magazine will affect the p.o.i. I have checked this for myself, and found it to be true. At least with my marlin 30-30. Which is the rifle i am talking about. I used to load it with reloader 7. I switched to imr 3031, and had a heck of a time. Finally found a load that would work, but is 400 fps slower. Well, i bought another bottle of reloader 7, but for grins, i tried a couple of different loads with reloader 15, and a couple with varget. I did not spend a lot of time trying to develop loads with the last two, they just didnt work as good (with the little bit of loading i did with either of them) the reloader 7 has been the best i have used so far, and that is what i will probably stick with. I have a couple of other rifles that i need to get a really good load worked up for. So this question seemed like it was begging to be asked. As i really do not want to work up 300 loads to find out which is best. That is horribly time consuming, and expensive. I am going to see if any of my buddies has a copy of "pet loads" , and see if i can borrow it. And just fine tune one of those loads. If nobody has a copy, i will check our librbary, and last resort, i will just have to buy one. But i would still like to know "why". I do get what you guys are saying about the pressure curves being different. It is to bad there is not a mathematical equasion (that did not need a phd in triganometry and calculous) to perform that would tell you what powder, at what velocity would be the best bet for a particular barrel length and size. If einstein was a balistician, we would have that formula! Lol!