Peruvian 1935 rebarrelled or rechambered? How to tell without slugging the barrel?

A seller has a nice Peruvian 1935 model FN Mauser converted to .30-06.

A number of these conversions were done by FN rebarrelling from the original 7.65x53mm caliber. Many others were done by Peruvian arsenals by just rechambering the original barrel.

The problem is the two processes produce markedly different results. The rebarrelled guns have an excellent reputation for accuracy. The rechambered guns do not, due to the original barrel diameter being .312" instead of the rebarrelled guns .308".

The question is how to tell which caliber conversion method was used without slugging the barrel?

Try a bullet in the muzzle. If it's 7.65 it'll disappear right to the brass.

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Originally Posted by MrNatural

The question is how to tell which caliber conversion method was used without slugging the barrel?

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Measure the groove diameter at the muzzle end with calipers.

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Originally Posted by browningautorifle

Try a bullet in the muzzle. If it's 7.65 it'll disappear right to the brass.

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If it has good rifling right to the end it won't. A .30-06 round in the muzzle of a 7.65mm Argentine 1891.

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Originally Posted by vintage hunter

If it has good rifling right to the end it won't.

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No, it probably won't. Many weren't that good, you guys probably got all the good ones...