I received a box of random parts as part of a payment and in the box is an upper receiver and a BCG. The upper isn't marked except for a brand (real helpful, PSA) and the BCG isn't marked at all.
The previous owner is pretty sure they're both 5.56 but they also said they'd dabbled in 300 BLK and 9mm so there's a non-zero chance these aren't 5.56/.223 parts. They're not 9mm (the upper has a gas system and the BCG doesn't have a 9mm bolt face) but I can't guarantee they're not 300 BLK.
How can you check to be sure unlabeled parts are what you think they are aside from putting them on a gun with a string on the trigger and hoping the shrapnel doesn't hit you?
EDIT: I should have been clearer. The upper receiver is a complete upper receiver with a barrel and gas system.
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