Thank you Fiftydriver (Kirby)! That's starting to make sense. I hope I didn't offend anyone with my toilet example above but I had not yet read your post. I'm just trying to understand... That's what I'm looking for. Great explanation.
I always used to think people were saying that their measured groups at longer distances were smaller than their measured groups at closer distances, which is why I was quick to exclaim BS.
But I've come to learn that what they were probably saying all along (or perhaps I was misunderstanding all along) was that the moa calculated from the measured groups at longer distances was smaller than the moa calculated from the groupings measured at closer distances. light bulb
As Fiftydriver said, bullet groupings don't necessarily create the outline of a perfect three-dimensional cone over distance. If after the bullets stabilize in flight, they spread/disperse outward at a lesser rate per distance traveled than they initially spread/dispersed while they were in unstable flight, then one can understand how it's possible to print a lesser moa group at the longer distance than at shorter range. The actual measurement of the groups will always be greater at longer distances, however the moa calculated from the measured groups can be smaller at long range than from groups at closer ranges. Imagine that the coning shape of the group expands at a lesser rate after the bullet flight stablizes than it expanded prior to bullet flight stablization.
Now is everyone thoroughly confused? :confused: Or not. ;)
There are others out there with much more scientific knowledge then I have on the subject. What I have is practical real world experience and this is what I have learned from shooting very long bullets to high velocity in fast twist barrels. Here are a few things I have learned:
The bullet is not unstable when it leaves the barrel, BUT, what is happening is that as the bullet is in the barrel, it is forced to rotate around the axis of the bore. When the bullet exits the bore, the bullet then will try to spin around its center of gravit. This is seldom exactly the same point as the axial center of the bore. AS such, there is some "wobble" if you will in the bullet.
This happens until the bullet settles down or "goes to sleep" and spins true around its center of gravity. Think of it like a childs toy top. When you first release it it will have some wobble and it will also move around a bit on the ground, then it stabilizes and spins much more even and true. Same thing happens with the bullet.
here are some general things I have found to be true:
1. The longer the bullet, the more range it takes for the bullet to fight off the effects of the rifling and spin true around its own center of gravity.
2. The faster the twist, often the more you will see this.
3. The higher the velocity, often the more you will see this.
If you take a conventional bullet, it also does this but the relatively low weight of the bullet and its relatively short length do not show this as far down range
For example, a 50 BMG will often take 250 to 300 yards for the bullet to completely go to sleep and spin true. My 7mm AM for instance will be very similiar to this shooting a 200 gr ULD RBBT as will the 338s based on the 408 CT case.
In most cases, with conventional chamberings, this range is more like 100 to 150 yards. In most varmint chamberings with low bullet weight, its generally well under 100 fps.
So how do you know?????
Well, the best way is simply to measure groups at various ranges. If the rifle is accurate and the load is a good consistant load, its relatively easy to do. For example, many of my personal heavy rifles will shoot 1/2 moa at 100 yards. At 300 yards, the group size will often be 1/3 moa. At 500 yards, its not uncommon to see 1/4 moa. That this tells me is that once you get past 300 yards or so, the bullets are generally shooting nearly the same or smaller moa group levels out to very long range.
It is not uncommon at all to see this type of rifle shooting very long, very heavy bullets at high velocity out of a fast twist barrel to see a load that shoots 3/4 moa at 100 yards that can shoot 1/2 moa or even less at 500 yards.
Most believe that once a bullet is sent on its path in flight, the groups it forms will be conal in shape. That is generally not the case with a heavy, long bullet.
Now this is really a simplistic way to look at it but it happens time and again. It has nothing to do with stability really, its just the bullet trying to fight off the forces impossed by the rifling and fly true and spin around its own center of gravity.
Again, I am sure many will read this and say the terminology is wrong and I am sure it is but I believe the ideas are sound from what I have seen testing many rifles with this type of bullet.
With conventional weight and velocity bullets, it will be much harder to see if possible at all.
Very well said!
I don't think I could have written that in those easily understood terms!
Well done Kirby! MagnumManiac