K-31 bore diameter?

[AbitNutz]

Any good folks here have knowledge of what the correct bullet diameter to use in a K-31 7.5x55? I suspect it's .308 but I ran into some who insist it is .310.

[AggieEE]

It's .308", back in 1903-06 time frame the army wanted to try a different bullet for the 30-03 ( fore runner of the .30-06). The 220gr. RN kicked the new recruits too much and I think the ranging wasn't too good either compared to the 7mm spritzers. The army got some 150gr. spritzers from the Swiss that they used in their 7.5x55. The .30-06 was born.

[RPRNY]

The Spec for the GP11 7.5x55 cartridge around, which the K-31 was designed, is a .3087" bullet. You may use .308" bullets. K-31 bores can, and often are slightly tight, at as much as @ .2956", so larger than .309" bullets should be avoided unless you have a very late model K-31. Definitely worth slugging.

[John Boy]

.297/.308

[P892Blakely]

1957 K31 .2960/.3075 if I measured correctly!
Gary

[madsenshooter]

.294/.3055 1950 SIG made. Some of these were sold with a tag denoting the 7.47mm bore, mine, purchased from Wideners didn't have the tag. I size cast bullets .308 for it. I've got a bump the nose down die that Smokin Joe made for me that allows me to make use of a wide variety of US made bullets, but usually use the Eagan MX3-30-AR. which is much like the RCBS 168gr silly-wet bullet, but with a bit more taper to the nose. The Eagan is a perfect fit as is.

[WineMan]

I never have heard about 150 grain bullets from the Swiss. When the 30-03 Begat the 30-06 the Swiss were still using GP 90/03 with its 190 grain round nosed bullet. The US Ordnance dept looked at the 7.92x57 with its 150 grain spitzer and came up with the original 150 grain FB M1906 round. After WW1 we did use Swiss 174 grain BT bullets from their GP-11 (1911) to come up with a round that did better at max range and eventually got the M1 round with its 172 grain BT bullet for long range and Machine Gun fire. Apparently National Guard units felt that round kicked too much and also shot too far for their range safe zones and wanted something more like the original M1906 round. That got us the M2 Ball. The other side branch was the 168 grain AP (discounting incendiary, tracer, blank, grenade cartridges etc.) of WW2 which seemed to have the best blend of range and accuracy of all of them.

Dave