I experimented with priming 3/4 of a gallon of WGV Pale into a Big Time Brewery Beer Box.

The box has a 1 gallon plastic type bladder inside that is fitted with a plastic spigot. I primed it at about 2 volumes and fermented it at ambient temperature for 12 days before we set it up to drink. I vented it once on day 10 and once on day 12. I was scared it was going to explode and make a giant mess! It puffed out quite a bit, but it did fine. Upon pouring it had some carbonation. It worked... kinda. I wanted a little more carbonation, but it might be risky if done again in the future. I think it was a bit lackluster. I may try it again with the Brown Ale. But, it's more likely that I'll try the same thing with 1 gallon of beer in a 2 gallon bucket fitted with the same spigot.

Any ideas on the priming rate in respect to the extreme headspace?