Colt HBAR 16" cut to 11.5" without opening up the gas port?

[Five_Point_Five_Six]

There's a local guy selling an 11.5" Colt HBAR complete upper that was cut down from 16". It's 1:9 twist and he said he did not change the gas port size. He says it runs reliably and has no problems with stabilization with many different 55 and 62 grain loads. Does this seem plausible? I've only ran barrels in factory configurations with the exceptions of AK's which don't give a shit about barrel length.