7.5MM Swiss Bore Size

[tkcomer]

Contemplating getting a K-31 Swiss rifle in 7.5MM. But when I look for load data, I get conflicting info. Some say the “normal” bore diameter is .306. Some say .307. My old Sierra book says to use .308 bullets, but to slug the barrel to make sure the .308 bullets will fit. I know I'll have to slug the barrel when I get it, but does anyone know what the bore is supposed to be? I want to shoot low power cast loads through it using store bought bullets. With a .307 bore, I can find .309 bullets. But a .306 bore would be rough to find bullets for that. Any help would be appreciated.

 

[Cosmoline]

The round we know of as 7.5x55 Swiss is based on GP11 ball, .3086" bullet size for the 1911 and K-31. Modern .308" bullets will shoot, though your best accuracy is going to come from very closely matching GP11 ball.

The 1911 series rifles do have a slightly tighter bore than the K31's I believe, but it's not enough of a difference to matter much if at all. Both fire GP11 and .308" diameter bullets.

Only the earlier GP1890, 7.5x53.5 was in the narrower .304" diameter for the old 1889 series rifles, and it was paper patched to create a final diameter closer to 8mm.

http://www.swissrifles.com/ammo/#7.5

 

[tkcomer]

.3086 bore. Wow. I know I'll have to slug this barrel when I get one, but a .308 bore opens up a lot of possibilities for cast loads. Thanks.

 

[Cosmoline]

Keep in mind that's the listed bullet diameter, and those are unique match-grade bullets. For reloading, folks use .308" esp. the spitzer boat tails in the 165-175 grain range. There's not really any need to slug the bore, esp. on a K31. They were built with extreme precision. Probably the most precisely tuned standard issue military rifles ever made. Both the rifles and their ammo would have been considered sniper-grade in any other military.

The Swiss rifle site and forum have tons of good info on these topics

http://www.swissrifles.com/sr/pierre/data.html

 

[Chasing Crow]

Like Cosmoline said, .308. Watch your overall length. Mine have to be held to 2.770 or they won't chamber. Great rifle!

 

[918v]

http://www.surplusrifleforum.com/vi...&start=0&sid=d19be7d11bdeaa39933bf6d4b2849553

I've slugged all of the seven K-31's I've owned and while I don't remember the exact measurement of any of them (and I'm too lazy to walk to the shop to check my records), none of them had a groove diameter of over .3075" or so. I recall one in particular that slugs .3069"!!

 

[Cosmoline]

Keep in mind those guys are slugging to test bore wear, not to see if it's OK to shoot .308" bullets out of the K-31. It is OK, it's done all the time. As Pierre explains, just work up slowly with the loads until you find your rifle's sweet spot.

 

[tkcomer]

But since I'm going to try cast bullets, I'll need to know the exact groove size. A .306 will cause problems. That's why I'm trying to get a heads up on what the “average” groove diameter is.

 

[918v]

You need to cast the chamber. There are other considerations like throat diameter and length you need to consider before selecting a cast bullet profile.

 

[Cosmoline]

Ah, I got you now. I thought you were thinking commercial .308" bullets might not fit.

Some discussion on casts in the 7.5 here:

http://castboolits.gunloads.com/showthread.php?t=72704

 

[tkcomer]

Does anyone know what size their expander ball is? I “assume” the die manufacturers have figured out what the average bore diameter is.

 

[qajaq59]

I shot the commercial .308s in mine and the K31 shoots cast just fine, if you work at it a bit. And don't go crazy with the velocities. I think the Lee manual has some loads for it.