Posted: 7/22/2017 10:18:23 AM EDT
| In my .308 I normally load 168gr bullets and that is what my rifle is sighted for at 100 yards. When I shoot some cheap 150gr or 180gr bullets just to foul the barrel, my shots aren't even on the target backer. I would say I'm off by about 12-18" high and 12-18" left. Is this just because of the different bullet profile and weight or is there something else going on? I would normally see an impact shift in my .223 or 7mm rem mag, but only maybe 2-3" at 100 yards. This is very frustrating when I am testing several loads with different bullets. |
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Inside 300 yards, barrel "harmonics" determine the PoI.
Different bullets, being pushed by different powders, cause the barrel to vibrate differently, and long with this, the bullets exit at different points in time so the barrel may be pointing instantaneously somewhere other than where you think it is pointed. All sorts of things influence this vibration: rate of pressure rise, length of barrel, the way the barrel is threaded onto the action, and the way the action is torqued into the stock. If you have a 36" long barrel (play along) 0.010" of barrel vibration can displace the PoI by more than 1". So, overall, yes, what you are seeing is one reason many of us end up shooting only 1 load in any given gun. |
| When i switched loads from a 140 AMAX to a 140 Eld the charge stayed the same and the speed was pretty similar. Â the AMAX was .2 mils left and .2 high at 100 yards. Â I only shoot one load out of my rifles, but a ballistic calculator can account for any offset if you do want to use multiple loads in one rifle. |
Completely normal. You should be working with 1 bullet at a time, tinkering with powder charge, seating depth, primer selection, etc... and then setting that load aside when you're happy or just plain finished with it. When you pick up a new bullet, you're developing a new load.
And 
Those groups both use 168's in a .308. Top are Speer, Bottom are Hornady. These groups were fired back to back on the same target paper at the same POA. The groups were over 6" apart vertically and about 2" horizontally. Powder charges were nearly identical, everything else was identical. MV's were within a few FPS average. You can get very large departures from your prior POI/POA intersection with a tiny change in load spec. |
| I experienced similar experience today at the range shooting my 308 RPR. Sighted my rifle at 200yrd with Fedral 168 gr. 1MOA no problem. Switched it to PPU 175gr and my holes were ending up on the bottom left diamond while I aimed at the center of a sight in paper target. That's a huge deviation! To make sure my scope did not get loose, I followed up with Federal 168gr and 1moa result. |
| I've recently been testing several different 95gr bullets in a 243. Three types of bullets, same weights, same case, same charge, same seating depth. When switching from one bullet type to the next, the first round is a flyer almost every time. Example, I shoot 5 rounds of Berger bullets, then 5 rounds of Sierra bullets; that first Sierra bullet is a flyer. Groups overall are in the same POI, but that first round is out. My theory is it is taking 1-2 bullets to reach a new equilibrium. Each bullet down the tube is depositing copper, but also picking a bit up. This will be different for one bullet tyoe than it is another. Ad this to what has been previously said about barrel harmonics and you can see how your POI can change just by switching bullet type, let alone bullet weights and charges. |
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Different lots of the same bullet (Sierra 224 90 gr SMK) have shown differing points of impact, even with same base to ogive measurements.
I got lucky and switched lots during the sighting stage at 600 in an informal F Class match. I was gtg and looked at my ammo box and realized I had hit the end of the last box. I had the rare good sense to try another round. 0.75 MOA shift low and to the left  Glad I had marked the shift in my ammo box.
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