We now send 1/4" liners with our Dickert barrels. The 5/16" works too, but we lean towards being prepared for mishaps these days. By using a 1/4" liner, if the need ever arises to remove and replace it, and something goes wrong, there is still enough barrel left to install the bigger 5/16". We do drill out the actual fire hole to a 1/16" once it is installed.