The items you are counting must be uniform in weight. No counting scale will ever function with any high degree of accuracy if there is a significant variation in the weight of the items. The larger the initial sample (30 pieces versus 5 pieces) the better results the counting scale can provide since it averages out any variations in the piece weights for accuracy.
Also, please don't buy a parts counter with too large of a maximum capacity. A large portion of customers buy a parts counting scale with more maximum capacity than they typically will be needing. The reason is usually along the lines of needing to weigh small items and a few larger items. If the majority of the time you're counting no more than ten pounds of parts at a time, then a 10 lb. capacity counting scale is a great choice for you. If you need larger capacity options, you can choose buy a couple of different capacities, or buy a scale with accumulation function, or buy a model with remote scale capability like the GSE 675.