The Structural Core and Its Fit
The receiver is the structural component that houses the primary operating parts, such as the bolt, the hammer or striker, and the trigger mechanism. Because so many parts locate against it, its dimensions and the tolerances on those dimensions determine whether an assembly fits and functions. Small errors here cascade through everything that mounts to it.
Nominal Dimension and Tolerance
A nominal dimension is the target size a feature is meant to be, and the tolerance is the allowable band above and below that target. A pin hole specified as 5.00 mm with a tolerance of plus or minus 0.02 mm may finish anywhere from 4.98 to 5.02 mm and still pass. Tolerances exist because no machining process is perfectly repeatable.
Clearance, Interference, and Transition Fits
When two parts mate, the relationship between their tolerances defines the fit. A clearance fit always leaves a gap so parts slide, an interference fit is intentionally tight so parts must be pressed together, and a transition fit can land either way. A trigger pin that must rotate uses a clearance fit; a bushing pressed permanently into place uses an interference fit.
Datums and Reference Surfaces
A datum is a reference surface or feature from which other dimensions are measured. Consistent datums keep tolerances from compounding, because every feature is located from the same origin rather than measured part-to-part down a chain. Receivers rely on well-defined datums so mating parts index to the same reference.
Why Ranges, Not Single Numbers
Specifications state ranges because insisting on one exact size is impossible to manufacture and needlessly costly. A well-chosen tolerance is tight enough to guarantee function but loose enough to produce economically. This balance is the core discipline of dimensioning.
Worked Example
Suppose a receiver's bolt raceway is specified at 12.70 mm wide with a tolerance of plus 0.05 and minus 0.00 mm, and a bolt is specified at 12.60 mm with plus 0.00 and minus 0.05 mm. The tightest possible pairing is a 12.70 raceway around a 12.60 bolt, leaving 0.10 mm clearance; the loosest is 12.75 around 12.55, leaving 0.20 mm. The design guarantees the bolt always slides yet never rattles beyond the intended band.
A Common Misconception
People often assume a tighter tolerance is always better. In reality, tolerances that are needlessly tight raise cost and reject rates without improving function, and over-tight mating parts can bind as they heat and expand or collect fouling. Good engineering specifies the loosest tolerance that still guarantees reliable operation.