I have used the RCBS 505 since I started loading nearly 50 years ago, and found it to be the best for reloading shotgun. I marked a spot on the bench where I set it up, and that made it easier to calibrate, as I was sure that spot was clear of my working area and would stay calibrated so long as I stayed away from bumping it. The one drawback about the 505 that I've found is that if any dust gets in the saddle for the beam arms, it will throw a false reading because the dust impedes the free movement of the beam arms. I discovered this when a buddy overcharged his 7mmRemington Magnum and had a nearly catastrophic failure, with a bulged chamber, broken bolt face, and the case stuck in the bulged chamber. When I went through his reloading procedures with him and compared his powder charges with my electronic scale, they were as much as 2.1 grains low and 3.5 grains high, with none right bang on. Close inspection of the scale revealed dust in the saddles blocking free swing of the beam arms, so a word of warning to all: If you load in a dusty area using a balance beam scale, either put it away in a dust free environment, or lift the beam out and clean the saddles and beam arms before you start loading.
I have also used the Hornady beam scale (not bad, but not as user friendly as the RCBs) and an electronic scale. The electronic scale was okay, but after comparing powder drops on both scales, I found that the longer the electronic scale stayed on, it seemed to wander in accuracy. When I moved into my new gun room, I found that the fluorescent overhead light affected the accuracy, as did having the window open for fresh air when a light breeze blew across the bench. If I hadn't been watching the empty pan when the breeze blew in, I never would have known that. As soon as the battery started losing charge the accuracy suffered, as well.
