Carbonation in bottles

No matter how much sugar is added to the recipe (which will increase OG and possibly FG), the main problem is over carbonation in the bottles. Two carbonation drops per bottle is ok for 750ml bottles. Cooper's mention: "The dosage required is 1 drop per 345-375ml bottle or 2 drops per 740-750ml bottle".

So it is possible that fermentation was not completely finished, causing the over carbonation. What was the FG? Fermentation could have been stuck for 3 days due to temperature or other factors, but doesn't means it was finished unless the FG was reached. Coopers will specify the estimated FG in their instructions.

That being clarified, your beer will still be good even if some has escaped the bottle.

So you used 750ml bottles .With the Mr Beer kits 2 carbonation drops work well. My self I like to use table sugar or the small suger cubes, cubes no measuring involved. I've always used 12oz bottles a little messy. I avoid the chance of over Carbonating.

The original post reads as though you used almost 1kg of different sugars for priming the beer for carbonation. But from your comment I see that is not the case.

Without knowing the size in g of the carbonation drops I can't say if 2 is to much, but for the rest of the answer I'm going to assume it is not.

As you have taken 3 gravity readings and they where similar or the same there are some things that could still be wrong. One thing that could lead to over carbonation is if the yeast stopped converting sugars to alcohol to soon. This is usually caused by pitching too little yeast. This again can come from the yeast being to old or the yeast being stored to hot or to cold for a long period of time.

Under pitching is also a common problem in bigger beers but the amount of sugars used here it is not the problem. Large beers also have higher alcohol content which can stop the yeast early if the strain used can't handle high alcohol content. But this is not likely here.

An additional source of the problem could be an infection, this could be wild yeast or it could be bacterial. This could happen on bottling day if it is in the air (unlikely) or if the hose and or rakking cane has the contamination. Both bacteria and wild yeast eat sugars and produce carbon dioxide.

Finally using carbonation drops is in general not a great way of carbonation, you have to touch the drops which can transfer infection, and you have less control over how much the beer gets carbonated. A better method is to use a second fermentation bucket or carboy, sanetize it boil x amount of sugars in a small amount of water. Then pure this into the second vesel. Then transfer the beer to this new vesel and let it sit for a short while before bottling.

As far as letting some of the gas in the bottle escape this is fine. Ideally you should not need to but I have felt the need to do this myself and done so. It does perhaps give a chance of some oxygen to enter, which is not good for long term storage, but not very much.

What size batch was this? 600g of DME might be reasonable for a 15 gallon (~57L) batch but if you also added in 400g of other sugars, it's not surprising that you had an over-carbonation issue. It's almost surprising it took 4 weeks to have issues. If the batch was smaller (especially if it was 5 gallons/19L) even just the DME would be to much.

We're you possibly using quantities from a recipe? If so, double check that you appropriately scaled it for your batch size.

In the future, it may be safe to use a priming calculator such as the one below to figure out how much would be appropriate for your batch:

https://www.brewersfriend.com/beer-priming-calculator/

Keep in mind that although the calculator gives amounts of table sugar, corn sugar, and DME those quantities are meant to be equivalent substitutes for one another, not all used together.