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Massive point of impact shift, HELP

lksmith03
I have a 50 BMG conversion that I sighted in with XM33 Ball (American Eagle) I was at the range Sunday with some API rounds I had bought on the auction side and was shooting at a defective valve from work and couldn't hit it. I shot at a target board at 100 yds and the API rounds were hitting about 6 inches high and 6 inches to the left of where the Ball ammo was hitting. I expected to see an elevation shift due to different weight ammo, but am baffled by the substantial windage shift.
I put in a ball round and nailed the valve exactly where i intended (Actually .25" high but that could've been my fault)

Any Ideas?
I checked that the rings are tight and they are fine.
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Comments

10 comments

Date Votes
  • pip5255
    all ammo isn't created equal.
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  • allen griggs
    Different ammo, different poi.
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  • bpost
    What you witnessed is quite common for changing one type of ammo to another.

    FWIW my 375H&H shoots very accurately with 235 grain bullets, when loading 285 grain grand Slam bullets, also accurate, the impact at 100 yards is several inches different shooting from the same rest.
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  • Mobuck
    That's close enough to hit a truck at 1/2 mile. Considering the sighting system on an M2, you walk the hits in anyway. The ammo is developed with this use in mind.
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  • givette
    I believe the OP is addressing the left-right variance between ball and AP..not the (understandable) vertical variance.

    So, the question remains, as I don't have an adequate answer: What's the reason a round will curve, and another won't? Remember: Gun returned to Point Of Aim when the OP returned to ball ammo. Wind eliminated from the equation. Firearm eliminated, also as it was the same shooter/gun, using the same technique. Any thoughts?

    Thanks, Joe
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  • idahoducker
    I think it's the difference in vibration (harmonics?) with different loads, brands, etc.
    I had a similar experience recently while working up loads for my little M1 Carbine. At 13.5 grains of powder I get groups in the bull at 13 grains it's four inches low (expected) and two inches right. Everything about the two loads is identical except for the half grain of powder.
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  • 32 Magnum
    Check your crown for a possible very slight defect on right side of muzzle.
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  • allen griggs
    When the gun is fired, the barrel vibrates.
    A different brand of ammo will cause the barrel to vibrate differently, thus changing the point of impact.
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  • beantownshootah
    Well. . .this isn't that "Wanted" movie with Angelina Jolie! Bullets don't curve sideways in flight!

    I think a quick answer is barrel harmonics.

    A different load will cause the barrel to vibrate differently, and if the end of the barrel goes from a vibration "node" to the middle of a "wave" because of a different load, this can cause horizontal displacement of a shot.

    6 inches is still a fairly extreme deviation at 100 yards but if you consider that the effect may include the intrinsic inaccuracy of this particular loads, crosswind, and shooter variation too, it may not be quite as much as it seems.

    IE, if your ROUNDS aren't accurate, that ALONE could account for a 3" left-right spread at 100 yards. Did you group them? If so, what kind of group did you get at 100 yards?
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  • lksmith03
    quote:Originally posted by beantownshootah
    Well. . .this isn't that "Wanted" movie with Angelina Jolie! Bullets don't curve sideways in flight!

    I think a quick answer is barrel harmonics.

    A different load will cause the barrel to vibrate differently, and if the end of the barrel goes from a vibration "node" to the middle of a "wave" because of a different load, this can cause horizontal displacement of a shot.

    6 inches is still a fairly extreme deviation at 100 yards but if you consider that the effect may include the intrinsic inaccuracy of this particular loads, crosswind, and shooter variation too, it may not be quite as much as it seems.

    IE, if your ROUNDS aren't accurate, that ALONE could account for a 3" left-right spread at 100 yards. Did you group them? If so, what kind of group did you get at 100 yards?



    Actually they CAN curve, (like a golf ball) I've seen tracers do it from my 22, but not like the movie.

    I have never experienced any issue like that with going from one ammo to another, sure some shoot better than others but never thought that barrel harmonics would cause that much of a shift,especially since the barrel had .5inch of steel all the way around the bore (friggin barrel is 1.5in dia).
    the wind couldn't have played a part since the rifle was sighted in with no wind using the ball ammo and its POI was unchanged. Also the wind that day was from left to right so it couldn't caused a leftward shift.

    I haven't tested those particular rounds for groups since my shoulder had given out after a full day shooting the 50.
    I'll test that and give a report since a guy I work with is trying to get me some scrap steel from the weld shop
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