Manufacture. Weight should not play a factor as the size remains the same. The DENSITY is what is changing. Now, this doesn't mean that bioval bbs are bad, but maybe they have not the same dimensions as the KA ones (for example 5.96 vs 5.93mm) which is not going well with your mags, or maybe the bio property is making them stick in the mag. Also, maybe you got a bad batch? Or maybe your mag is not good, but happens to like KA bbs. Who knows? You'd have to try a few different brands and a few different mags (I mean different models).
K3vX | Airsoft Gunsmith/Gun doctor in training.
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