The situation, 2 shooters, 2 rifles, a T3 and Prohunter in 243. Both with 5x30x50 Z6i BT newly installed on top. Our local supplier had run out of our usual reloading bullet supplies so rather than messing about we bought cheap ammo to zero on and try the BT, before working up a new load with the only bullets our local supplier had in stock. All should be simple, as the swarovski has the PPU ammo in the programme so the BT should be bang on. However we knew the PPU flew slower than the published figures due to the shorter barrels in our rifles, however we had the speed from the chrono from a few weeks before, 2810 - 2830 fps.
We zeroed both rifles at 100 yards, and achieved 10p sized groups, in both rifles, so happy. I ran the swaro app on my phone and we needed 6 clicks up at 200 yards, all very easy. so we did this in both rifles and did a test at 200 meters, expecting a group at about bull, but double size, so say 2 inch ish? Cold and windy, 10 - 15mph so a bit of drift would not worry us, and it would be checked again before stalking, we just wanted to get the new toys almost there and ready.
The problem, the group was open and 5 inches low, despite both rifles having 6 clicks added to the BT as directed by the app. Everything tested and correct.The app was adjusted for the slower expected speed, but to achieve this crap group and drop it would have had to drop from 2820 ish to 2300 ish FPS, in 2 rifles in a week. Typically the chrono we had with us had a flat battery, but it was only there as part of the zeroing box.
The question. Why in 2 rifles was there an unexpected 5 inch drop in POI and a far larger grouping over an additional 100 yards, from the expected one. The only possible change is batch of ammo, but as factory bought i would not expect this to loose 500fps?
We are out tomorrow again, with fresh batteries in the chrono, but the wisdom of SD would be appreciated.