We can safely assume it's the lack of space to add a focuser to increase charge particle compression. I'm pretty sure even the zephyr units couldn't perform shield piercing attacks. At best they had a crapload of stopping power and shield attenuating properties. If the BT units did ever shoot through the shield we can also assume its because the continuous fire caused the shielding to be unstable even for miniscule moments. But all in all, the BT units don't have piercing ability at all.
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