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Siamese Help


ken98k

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I'm getting ready to build a siamese and need some info.

I got my action without the barrel and it looks as if I will need a relief cut for the extractor on the breach end of the barrel.

Any photos?

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De Haas shows:

 

0.990 major thread diameter

14V

0.515 shank depth

 

The breech end face of the barrel is "flat", with a slot cut for the extractor. The slot is a full run on the right side of the face, running from the chamber to the outer threads, approximately the height of the extractor.

 

No depth or height dimensions are given for this slot. You'll need to use a depth mic from the receiver face and a calculator for the depth. Maybe 0.010 deeper than measured to prevent to extractor from hitting the face of the slot?

 

No idea on the slot tolerance.... +0.010 on top and bottom?

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I'm just reading what's in De Haas' book.

 

The Yugos barrels can be cut without the extractor slot and function fine, although the normal 98 is for rimless cartridges. Because headspace for rimless cartridges is set at the shoulder, there's some flexibility to what happens at the cartridge base.

 

The Siamese is for rimmed, and the breech face will actually be the headspace shoulder for the rim. The breech face presentation is absolute.

 

The length of the shank will be the receiver face to boltface, minus the rim thickness.

 

I don't see how get around some sort of slot with a rimmed cartridge. The extractor will need to sit forward of the rim to extract the case, especially with a 45/70, as it has no shoulder to headspace on. A cartridge like the 30/40, .303, or 7.62x54R has a shoulder and could be treated as headspacing on the shoulder (and should be re-sized as such).

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I'm just reading what's in De Haas' book.

 

The Yugos barrels can be cut without the extractor slot and function fine, although the normal 98 is for rimless cartridges. Because headspace for rimless cartridges is set at the shoulder, there's some flexibility to what happens at the cartridge base.

 

The Siamese is for rimmed, and the breech face will actually be the headspace shoulder for the rim. The breech face presentation is absolute.

 

The length of the shank will be the receiver face to boltface, minus the rim thickness.

 

I don't see how get around some sort of slot with a rimmed cartridge. The extractor will need to sit forward of the rim to extract the case, especially with a 45/70, as it has no shoulder to headspace on. A cartridge like the 30/40, .303, or 7.62x54R has a shoulder and could be treated as headspacing on the shoulder (and should be re-sized as such).

Thanks guys

I haven't done any measuring yet and the receiver is locked up for now. But as I recall the rim around the bolt face is even with the front side of the c-ring and the weird looking extractor goes well beyond the c-ring

Kenny

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On the receiver I have the rim of the boltface protrudes beyond the "C" ring as does the weird looking extractor. I never saw what the barrel looked like but on my receiver it looks as if the breech never touched the inner ring.

 

IPB Image

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  • 1 year later...
I'm getting ready to build a siamese and need some info.

I got my action without the barrel and it looks as if I will need a relief cut for the extractor on the breach end of the barrel.

Any photos?

 

 

 

Coming in a little late on this one. The barrel has an extractor cut and a very slight counterbore, like a Remington 700 only this one is only .020 or so deep. The counterbore would be the headspace dimension on this reciever because of the rimmed cartrige. I can post a picture of one if I can figure out how to get it posted.

Ken

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  • 2 weeks later...

I barreled my Siamese in 25 Krag AI today. The shank length was .490 after cleaning the front of the receiver. I left .002 between the innner ring and the shank. The bolt face rim is below the ring by .010. From the end of the shank to the portion of the bolt face where the case sits is .054. Minimum headspace is .064, so I recessed the shank .011 for the rim. I hope my math is good.

 

I let someone borrow my milling attachment, and he is to return it tomorrow. I will cut the extractor slot after I get the attachment back. I milled off the charger hump and the side rails in the B'port. I am debating whether to build ring bases or ue factory. I have a Leupold that fits fine without shimming.

 

De Haas called for a shank major diameter of .990. I cut the threads at 55*, but could not find a spec for them. At .977 I could hand screw the barrel almost in. I had to take another .010 off to get it to screw in all the way. The inner most thread must be tight. I ended up with 5 full threads.

 

The barrel is a Pac Nor three groove one in ten twist finished with a recessed crown at 26 inches. It is the same as I hung on my DHT Springfield in 257 AI. The Krag has more case capacity, so it will be interesting to see how they compare. LL

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  • 6 months later...

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