Is this normal?

[redman]

When I was at the range yesterday, I tried some American Eagle in .308 Win. (BTFMJ 150 gr) for the first time. Between different types of ammunition, I usually have to make an elevation change; for example, when I shoot Federal's Gold Medal (BTHP, 168 gr), I dial in 6 mils, whereas Remington's UMC (FMJ 150 gr) get's 7 mils.

With the americal Eagle, however, I had to dial in 6 mils up, and 5 mils right to hit the bullseye... Does that make any sense? Won't different bullet types just affect the hight of the point of impact? How was the bullet pushed left?

Thanks for the insight.

 

[dan belisle]

Different bullet weights are different lengths, generally. They will affect (and be affected by) the barrel differently. As the barrel vibrates it actually moves in a small figure eight, so depending on where the barrel is in that cycle, the bullet will depart the barrel in a slightly different direction. - dan

 

[throback]

I have had that off center movement with different loads in .223. Not being any kind of expert it seems ODD. AE was involved here as well. If I recall it seems to shoot about 3 inches left and high from some 50gr handloads I was testing.

It'll be interestig to hear some of the more learned guys here clear this up and add to Dan's comment.

 

[Obtunded]

There is a tremendous amount of torque created when a bullet hits the rifling. Each bullet will create a different amount of torque and every load has different harmonics. That torque and its associated harmonic effects (that are the aggregate of pressure, twist rate and velocity) will change POI.

You will find that if you load develop your own ammo, ANY change to the loading (Charge weight, seating depth, ambient temperature etc.) will affect POI, not just vertically either.

 

[Grizzlypeg]

On my 223, if I shoot 45 grain or 62 grain bullets, they change only in impact elevation, but, if I shoot some 55gr fmj cartridges, they form a nice tight group impacting about 2" left. Switch back to the 45 grainer's, and its back in line. Supports the theory that the barrel is oscillating in some sort of circular pattern.

 

[redman]

First, let me add that my POI corrections are all off by a factor of 10... So, for the American Eagle, it's 0.6 mils up, 0.5 right (my scope adjusts with each click being 1cm @ 100m)

On my 223, if I shoot 45 grain or 62 grain bullets, they change only in impact elevation, but, if I shoot some 55gr fmj cartridges, they form a nice tight group impacting about 2" left. Switch back to the 45 grainer's, and its back in line. Supports the theory that the barrel is oscillating in some sort of circular pattern.

Exactly; I got a nice, tight group (for a box of 16$ ammo) when I dialed in the correction. When I went back to 168 gr BTHP, my windage correction returned to zero.

Thanks for letting me know I wasn't the only one! When I first observed it, I thought I might have knocked my scope off zero or something...