Head space Gauges.

[Sunray]

Better to have 'em and not need 'em than need 'em and have to find 'em again.
Like ikesdad says, head space gauges are 100% required when installing a barrel. The bolt and barrel must be properly fitted. Neither are drop in parts.
You're going to need a barrel vise with bushings and the correct action wrench too. Installing a barrel is not just screwing the thing into the receiver. Fail to use the proper action wrench can leave you with an oddly shaped paperweight when the receiver is twisted.

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[USGI]

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