Accuracy Issues

[DeeDubya]

I see quite a few posts in other threads concerning accuracy. There is a subsonic/supersonic discussion up in the 97B forum that made me think of some other factors relating to accuracy. One of the most misunderstood anomalies in handgun shooting is this one:
 
I was shooting 115gr bullets in my 75B with good accuracy but wanted to try a heavier bullet for better penetration and functional reliability. However, when I switched to 147gr bullets my group moved up the target by nearly 6 inches. It still groups well but now shoots high. 
 
Why would a heavier bullet shoot higher than the lighter bullet? 
 
Handguns recoil in a slightly different way than rifles. The relatively short gun has a tendancy to pivot on the axis of your wrist (barrel flip) thereby causing the barrel to tilt upward instead of just pushing straight back. The fact is that a heavier bullet causes a tiny bit more flip than the lighter bullet, causing the heavier bullet to print higher on the target.
This effect can be compounded by a difference in velocity. A lighter bullet will normally have a higher velocity than the heavier bullet. This means that the heavier bullet stays in the barrel a nanosecond longer than the lighter bullet therefore it's trajectory will be ever so slightly higher as the barrel is tilted higher. 

[b5.5dan]

Very informative!
So could you say that, as a general rule, if you use heavier ammo you will always shoot higher?

I think that the thing that makes this most interesting to me is that I'm at a point where I am still trying to "master (yeah, right  ) my pistol, but I switch ammo a lot. It never occurred to me that my vastly different performance from day-to-day could be blamed, I mean, could be partly due to my switching ammo.

Thanks for the info!

[ZG47]

The S.M.L.E. No. 1 Mk III, standard British battle rifle of WW1 and WWII Battle of France and standard Australian/Indian rifle for both World Wars, does the same.
 
The Mk VI (215gr roundnose) ammunition always shoots higher than the c. 1912 Mk VII (174gr specially constructed spitzer with profile of 195 gr conventional spitzer) ammunition.
 
The No. 4 rifle behaves differently, however, because it has a different receiver and a stiffer barrel that is basically a Long Tom (L.E. Mk 1) barrel shortened to SMLE length, although you cannot swap them around due to different breech arrangements. The use of a heavier barrel may have been inspired by the Australian approval of the Long Tom profile for target shooting.
 
NB The Australians adopted the heavy barrel version as their standard sniper rifle in WWII and kept them in service until the purchase of Parker Hale Model 82 rifles.

[DeeDubya]

"Very informative!
So could you say that, as a general rule, if you use heavier ammo you will always shoot higher? "
My experience with handguns, yes heavier bullets will nearly always print higher at moderate ranges. Exceptions would be at longer ranges (I.E. metallic silhoutte) where velocity slows and gravity takes control.
This can be evident with rifles too, although it would be at close range.

[DeeDubya]

Here's another little accuracy issue that can mess with your head.
 
I occasionally shoot prarie dogs with a buddy of mine. Last round I took my 243 which I had sighted in precisely at 200 yards. I quickly learned that I could hit them at 175-225 yards but missed every time at 100-150 yards. Then again I was dead on at 50-75 yards. 
 
I prefer to take head shots. Here's what was happening. 
 
 When sighting in at 200 yards with most rifles the bullet will have to travel up through the line-of-sight in order to fall back to zero at 200 yards. So the bullet may pass through the line-of-sight at 75 yards, be 2" over the line-of-sight at 100 yards and finally back down to zero at 200 yards. So... my gun was zeroed at 75 yards, 2" high at 100 yards and zeroed again at 200 yards. In taking head shots I was shooting over the little critters at 100 yards and for all they knew, there was a thunder storm nearby. 

 
 

[ZG47]

Provided that you keep the pressures the same, heavy bullets shooting high may be generally true. There are always unpredictable rifes, such as those with military barrels that have been slimmed down by gunsmiths.
 
The craziest one I read about was a sporterised 6.5 swedish mauser on which the barrel had been turned down so much that you couldn't touch the barrel after two shots had been fired. I would need to reread the article but I doubt that the rifle was accurate for more than 2 or 3 shots.

[DeeDubya]

Many times older rifles can be rejuvinated back to accuracy by simply cutting an inch or two off the end of the barrel and recrowning it. The last few inches of the barrel are most important because "this is what the bullet last touches before it is airborn". The tip of the bore collects more dirt and dust than farther in. Many ranchers would rest their rifle on the truck seat with the barrel down on the floorboard. It seems to be a safe practice but this is when dust gets into the barrel and acts like sandpaper when a bullet passes over it, thereby honing out the last few inches. It's a fairly inexpensive process if you're using a scope, but the price could go up if your're retaining the front sight.