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If a difference of around 3 gravity points (between primed and final gravity) will carbonate a beverage (yes, depending on style), how many gravity points will make a bottle bomb?

No, I'm not trying to make one - nor am I trying to trying to carb with residual sugar and the difference between measured and target gravity. I'm just one more paranoid n00b, curious about the effects of missing my final gravity and wondering how much wiggle room there is.

Let's say I'm bottling with 12oz reusable longnecks from LHBS, and storing at 68F.

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There is no hard and fast rule on when bottles will go bang. You can calculate the amount of CO2 produced in bottle fermentation from the gravity drop just like you can calculate the amount of alcohol produced, but when it goes bang depends on several other factors, of which the pressure rating of the bottle and the amount of head space are the two main ones. (Less head space means more pressure from the same amount of CO2 produced.)

When in doubt, err on the side of caution when priming or bottling early, leave ample head space, and use heavy duty bottles that come with a decent pressure rating.

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  • I don't know if I agree with you regarding head space. Correct me if I am mistaken, but the CO2 in the beer and in the head space will reach some equilibrium with the head space pressure the same whether there is a lot or a little space. I would also add to your erring on the side of caution: the colder you keep the bottles, the better, because that will reduce the pressure in the bottle as well.
    – Dave
    Nov 20, 2018 at 14:52
  • You are right: the closed system that is the bottle will tend toward a static equilibrium. However, temperature fluctuations will disrupt that equilibrium. Since air is compressible and water is not, a temperature increase will have a greater probability of causing the bottle to fail as head spaces get smaller.
    – Frank van Wensveen
    Nov 21, 2018 at 7:05
  • Personally I’ve only encountered this when infected with Pediococcus. Keep an eye on the caps. If it starts too bulge a lot (too much) then it probably time to open and check.
    – Martin
    Feb 11, 2019 at 20:11
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In my experience, 0.010 SG above terminal gravity is asking for bottle bombs.

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