For me never completely reliable. Too many unknown factors, quality of components, flaws in manufacturing of both gun components and ammunition, weather dust exposure, moving parts wearing, fouling, oxidation, corrosion, over lubrication, under lubrication..... I've had three seperate rifles fail, one brand new lever action .22 never fed rounds into chamber always jammed, side by side 12g hammer shot gun fired uncountable rounds 4000+ , handed to me from my grand father failed, worn mushroomed firing pins and loose latching shears. Ruger .233 mkII not so much a fail but an example of becoming unreliable after 200+ rounds scope mounts came loose, something minor but crucial to reliability, so I've had failures at each end of the spectrum. By the time you've proven above doubt you can rely on the guns manufacturing and quality you've rendered it less reliable as you have been wearing it out, detensioning springs, hot cold cycles loosing fixtures. Maintainance obviously prolongs but not all can be maintained or prevented.Very pessimistic approach but simple and obvious.