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Posted: 10/22/2002 1:02:05 PM EDT
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I've got a strange problem that I'm hoping someone can shed some light on why it might be happening. I've got a rifle that has 2 very different points of impact. The rifle is sighted in for Win 64gr Sp, about 2" high of point of aim at 100 yards. M855 ammo is 3" right, and the same general elevation. Yet my Bushmaster 16" heavy shoots the same SPs and M855 with virtually no difference (maybe 1" higher for M855). The rifle in question has an old Colt SP-1 lower receiver, and a Colt upper with Colt 16" M4 barrel. I have noticed this problem with both a Bushmaster 11.5+5.5" short barrel, and a Colt M4 16" barrel on the same upper receiver. I'm inclined to think that there is a problem with either the upper or lower, but have no idea as to how to proceed to track it down. |
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Think barrel harmonics. When a round is fired, the barrel has movement(whip). As the bullet moves down the barrel, the muzzle is moving, or whipping. Since the muzzle is moving(limited stoke), the POI is determined by what position the muzzle is when the bullet leaves the barrel muzzle. A stiffer, shorter barrel will have less muzzle movement than a longer barrel, but will still have movement. This is why different ammo shots to different points of impact. Also, when you tailor a handload to a rifle, what you are doing is making the bullet leave the barrel at the end of its whip/stroke(farthest out-reach). This end stroke/harmonics, will place the bullet at the same POI due to the longer duration that the barrel will spend at a given point(barrel running out of whip movement, and moving back 180'ish), verses having the bullet leaving the barrel some where in the center of a stroke. The standard deviation of FPS of a round will cause the bullet to leave the barrel in different points of the long stroke if you try to time the bullet exit to anywhere else than end of stroke. Also, the barrel whip is not up and down, nor side to side, but in a elongated circular motion. This is why your different types of ammo are ending up at different points down range. But if you don't believe me, then it's just Voodo magic, and the rifle needs to be burnt as a Witch!!!!!!!!!! Hope this helps. |
There is a solution!!!!! Buy huge amount of the same ammo and sight all the rifles in using the same ammo. This way, you don't have to worry about the change in POI. 
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