Sodak, are you using wide or narrow neck polycarb liters? I've never seen the narrow necks fit into a GSI or Olicamp cup, they are a few hundreds of an inch bigger around the body than the wide mouths. The HDPEs are a little skinnier; I'm not sure about new, semi-squishy ones.
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-IronRaven

When a man dare not speak without malice for fear of giving insult, that is when truth starts to die. Truth is the truest freedom.