2.5 id is the sweet spot and it still is going to be a little crammed when you add that top cap so make sure you do a test fire with a wet wick and the top cap on to ensure no shorts or hot spots prior to putting the whole tank back together or your throat will be in for a world of hurt....99% of people think they are getting a burnt hit when in fact its a hots spot burning the cotton due to their build shorting on the rba. so always test it at all phases of the build.