I believe the drop rate is about 1/3 (or at least that is what I have seemed to experienced)
If that holds true, it would be much cheaper to farm for stun guns rather than buying in the shipments.
---incoming math---
50 crystals for 120 refresh
120 energy / 3 drop rate = approx. 4 stun guns per refresh
50 guns required / 4 per refresh = 13 refreshes (rounded up)
13 * 50 crystals = 650 crystals per 50 stun guns
---
The break-even to be indifferent between spending crystals to buy refresh and directly from the shipment is approximately a drop rate of 15% I believe.
I would do the calculations again, but I'm currently doing this on my phone and it makes it a hassle.
This is similar to stun cuffs as well.
I hope this was clear enough for you to understand.