Re: MOA Rant
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Master_Blaster</div><div class="ubbcode-body"> Bases on angular discetion an MOA is minute of ANGEL. So if you cut a slice of pie that's a 45 degree angel the distance between cut lines with be say an inch at the center and 3 inches at the edge of the plate but the <span style="font-weight: bold">angel </span>is still the SAME. </div></div>
Well, if the <span style="font-weight: bold">angel</span> you're talking about is the one watching over you, then that just might be true. Likely even an <span style="font-weight: bold">angel </span>couldn't get your rifle to hold the same accuracy as measured by MOA out to just any distance though.
Seriously though, just because a rifle is accurate to within one minute of ANGLE at 100yds, doesn't mean it will retain that same level of accuracy out to any distance. This reality is why, for instance, military specifications call for a level of accuracy <span style="font-weight: bold">at a given distance</span>. For example, in the new SOCOM PSR rifle competition, the requirement is that a rifle be capable of 1MOA extreme vertical spread at 1500 meters for a 10 shot grouping.
Minute of angle is useful as a measurement of accuracy to shooters because it allows us to disregard measurement differences like metric/imperial and because it generally holds true within certain envelopes of performance (100 yds and 1000 yds are not the same envelope).
Now while wind, shooter, and atmospherics are non-rifle specific factors that you wish to take out of the discussion for the sake of argument, the other factors are not.
Obviously the pellet gun example (while funny) is flippant. I'm sure you are aware that a supersonic projectile can only be expected to remain accurate until roughly the point it enters the transonic stage which is generally going to be in the 850-1050yds range for .308. So I assume you are trying to argue that your theoretical 1MOA rifle at 100yds should hold that same accuracy out to the transonic threshold, not to infinity.
You might argue that BC and velocity are bullet/cartridge issues. You are right, but they are also internal ballistics issues.
A bullet is deformed and scored by rifling as it passes through the barrel, the level to which this happens and the consistency with which it happens relate to how the barrel was cut/buttoned/broached/hammered/etc. This is a factor that matters little at 100yds, but given 10 times that distance and time for air resistance and wind deflection to act upon the projectile, the amount of difference in how the bullet was impacted by its passage through the barrel becomes very important.
A bullet also has to make the jump from chamber to engaging the rifling. Certainly a big part of this is in the cartridge (seating depth), but how well a chamber is cut affects the consistency with which this happens even if seating depth and bullet concentricity are held to be perfectly uniform shot to shot.
A bullet is also affected by the crown of the barrel as it leaves. A well cut and maintained crown means that the bullet will leave the barrel consistently every time. A less great crown will impart inconsistent forces upon the bullet as gas leaves the barrel behind the bullet in a way that is not perfectly concentric and uniform. This effect don't mean much to a .308 at 100yds, but once you get out there, it can become quite pronounced.
The consistency with which the action locks up also matters, as does the consistency with which the brass contacts the bolt face.
Any harmonics on the barrel matter. This is why free floated barrels are generally preferred - less stuff touching the bbl, less randomness imparted on the bbl harmonics. It is also why devices exist that put weight on the front of the bbl or apply pressure or something else to cancel out those harmonics. In any case, as with anything consistency is key. A $2500 rifle with a bedded action and free floated barrel will exhibit more constant bbl harmonics than a factory tube dropped into a stock which touches the bbl in a slightly different way everytime you touch it or recoil shifts shit around.
Of course, optics mounting matters as well. How an optic mount is attached can exert non-uniform stresses upon the action or bbl which affect harmonics. This is why some people bed their bases and why some prefer two piece bases or, better yet, integral bases (although this, to a lesser extent, extends the issue out to the rings). A quality optic that is properly mounted will also not move as much. movement of the reticle within the scope or very slight shifting of parts that, after all, are attached not cast, forged, or milled as one, can mean a difference in POI at 100 yds that doesn't matter, but which can throw your adjustments out of spec at 1000 yds. Think of a very small cant for example, at zero, it doesn't really matter, but at 1000 yds and thirty some odd MOA of vertical travel later, you're off. Back down to 100yds, recoil of the next shot shifts it ever so slightly back to center, next one back up to 1K yds and slightly to the other side, you get the point, what matters little at 100yds, means ALOT at 1000.
Bottom line: quality of barrel, quality of action, quality of stock, quality of optics/mounting, and quality with which they were all put together, makes a huge difference at extended ranges.
That SOCOM PSR rifle may barely hold 1 MOA of vertical spread over 10 shots at 1500m, but I'm sure most of them fart 1/4 MOA 3 shot groups at 100 yds.
-Pete
