You understand wrongly. There were incorrect sizings, but this was not due to any metric-imperial conversion. They were due to the measurements being conducted improperly. A conversion to 8mm Mauser would have had the same bad result due the extremely shoddy workmanship that the Gladiator factory was putting out.
The contract specified that all systems operate in metric. As soon as that clause was put into the contract, the error becomes no different from any other software bug.And yeah, it's a simplification, and other mistakes had to be made in both cases. But you know what would make it a lot harder to screw up like that when dealing with conversions between imperial and metric parts, designs, measurements, or whatever else? If we used the same units on both sides.