I wouldn't recommend using the .12g BBs, IMO they're garbage. Some BBs may have a seam around them which will chew up your hop up rubber as they pass through, not to mention the hop up is fixed and set for .20g BBs.
You're lucky to get a good KJ out of the batch... I've heard many horror stories of KJ guns breaking right out of the box and some need a whole lot of work just to get shooting decently. KJ QC is really hit or miss!
If you're looking for a cheap upgrade, open up the hammer spring housing and drop a BB in there, and put the spring back in... INSTANT HAMMER SPRING UPGRADE.... ghetto styles, increased FPS at the cost of one BB!
I do it for all my guns.
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