Not the definite answer, but weigh the bottles to get and idea of how much glass is there. For a ~12 oz bottle, more glass will be stronger, but there are always shape to consider and surface imperfections that can make the bottle weaker. Some are in the 195 gram range, for one use then then to be crushed and recycle the glass type bottles. Some older ones like Martin talks of are in the 280+ range. Some of the Belgian bottles I brought back years ago are 350 grams. Those are designed to hold more pressure as the beer they held was lively.