While there's a certain amount of variation in U.S. Krag chambers and probably somewhat more in new .303 brass, it's been my experience that most .303 cartridges can be chambered in my Krag using only a minor amount of brute force turning down the bolt handle. Try it with some new .303 brass and you may be surprised. (Shoulders are pretty far back on most unfired specimens I've handled.)

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Naturally, firing this combination isn't a recommended practice - but probably wouldn't produce a disaster since the Krag's breech configuration provides outstanding case support, even if the primary (forward lug) is compromised.