Here's a little bit of a puzzler for you.
I went out the other day to set up a new set of scopes and mounts and to try changing over from 130g ammunition to 150g ammunition in my 270 rifle. The rifle is a Parker Hale Safari and the ammunition used was PPU (SP). The scopes (An unbranded set but guaranteed Fog Proof, Shock Proof and Recoil Proof) and the Scope Mounts (A brand new set of Nikko Stirling Platinum Mounts) were fitted to the rifle and were checked for tightness both before and again after the zeroing session. I am positive that they hadn't moved.
I checked the zeroing with the 130g ammunition while sat at a bench at (Aproximately) 100 yards and found the rifle's zeroing to be pretty much spot on. I then set out a new target (Exactly the same range and shooting position) and "double checked the zeroing" with 1 round of PPU 130g (SP) ammunition. The resulting shot showed that the rifle and scope was spot on for this ammunition. I then loaded the rifle with 4 rounds of PPU 150g (SP) ammunition and carefully shot them at the same target. The first round went 4cm low, which is something that I expected it to with a slightly heavier bullet head. However the next shot went marginally lower and left again from the previous shot. When the third and fourth shots were fired they did exactly the same, each shot going exactly the same amount further both low and left than the previous rounds, giving a very regular diagonal "laddering" effect.
I have my own thoughts as to what has happened but before I say what they are I would welcome your thoughts and opinions on this "unusual situation".
A photo of the resulting target is attached for you to form your opinions from. To put things into perspective the red "bull" is exactly 40mm dia.