[Andrés]

I disagree with the idea of making research harder. If you do that you'll never get them before the enemy has phased shields and will never be able to exploit their special damage type.
IMHO PPB should be balanced to make them useful ONLY when you can exploit their special ability and become obsolete once phased shields come into play. A solution to acieve this should involve rebalancing not only PPBs but also SGs and PSGs (I recall complains about shields being too weak)

I had the idea of finding a formula relating all features of a weapon, and then modding all weapon to get the same balance result.
Something like
AverangeDamage/Size/ReloadTime
but making it more complex to involve things such as range, to hit bonuses, weapon type, special damage type, cost and research cost.

I had made a spredsheet with that but could never find a formula because the role of most of those factors, and their relative importance will vary with the situation.