When I was at Camp Perry last summer, the military were definitely shooting Black Hills blue box. I use it in competition as well, the 77Gr Moly Coated version for 200 and 300 yard ranges. I roll my own for the 600 yard course, using a different, moly-coated bullet.
I am sure that you all know the twist rate is dictated not by the weight of the bullet but by its length; long skinny bullets require a higher twist rate than shorter fat ones. Furthermore, the long skinny bullets have a better BC than the short fat ones. Compare the BC for a 40 grain Nosler bullet to the BC of the 75 Gr Hornady AMAX. Of course, heavier bullets are usually longer than lighter bullets so using weight as the indicator for twist rate usually works well.
However, that is not always the case. Compare the 75 gr AMAX to the 77 gr SMK. You will immediately see the AMAX is longer and with a sharper ogive than the SMK, even though it weighs less. So, while your rifle may shoot the 77 gr SMK properly, it may not be able to stabilize the 75 AMAX.
Further, in order to be stable, a bullet needs to retain a certain velocity and rotational speed. For instance, my 75 AMAX loads reach 600 yards in .84 seconds out of my 1:8 twist 20 inch barrel. It leaves the muzzle at 248,400 RPM and 2760FPS (according to my chronograph.) There is a certain threshold in rotation that has to be reached for the bullet to be stabilized. For this one, I believe the figure is 175,000 RPM.
I will not bore you with a discussion on coeeficient of drag and spin decay, but suffice it to say that the rotation and velocity decrease rapidly and at long ranges if your bullet is not supersonic and ois no longer spinning quickly, it may become "erratic". This will be more marked in shorter barrel as the MV is less than the one from longer barrels.
Further more, the air density increases when it is cold, (we all know that airplanes toke off quicker when it's cold outside,) and so the drag on the bullet increases. This is exacerbated by the fact the powder in the bullet might be affected by the temperature and you actually experience quite a drop in muzzle velocity with the corresponding loss of stability downrange.
All this is to say that you should shoot more. :-)
P.S. If you are in any way interested in long range accuracy, get 20 inches of barrel or more and get a 1:8 or faster twist rate. You can then play with the longer (heavier) bullets knowing your rifle can handle them.
If, on the other hand, you are only interested in shooting hordes of zombies coming for your iPod, then I suspect you are fine with your 1:16 or smaller barrel.
The above does not, of course, apply to the military and LE contingent among us.