Chamber Length

[mtang45]

SAAMI specs for the 7mm STW case are 2.850 max (at least the reloading manual states trim at this dimension). I just got one of those tools that measures chamber length in your rifle and mine measures 2.878 or .028 over max. Is this unusual in a rifle chamber or more typical?

Thanks

 

[fguffey]

2.850 is the length of the case, then there is the minus .020 thing, meaning 2.850 is the maximum length. Then there is the length of the chamber 2.877 with a 45 degree taper from the throat that gets the length of the chamber down to 2.870, anyhow, I form cases to determine the maximum length of the chamber, for what ever reason chambers have been throated for longer case necks, so I do not assume the chamber has any resemblance to a SAAMI chamber.

F. Guffey

 

[mtang45]

2.850 is the length of the case, then there is the minus .020 thing, meaning 2.850 is the maximum length. Then there is the length of the chamber 2.877 with a 45 degree taper from the throat that gets the length of the chamber down to 2.870, anyhow, I form cases to determine the maximum length of the chamber, for what ever reason chambers have been throated for longer case necks, so I do not assume the chamber has any resemblance to a SAAMI chamber.

F. Guffey

Thanks for the response! So its not unusual to have a throat much longer than max spec? And should I not be concerned about keeping the case length under 2.850?

 

[fguffey]

Not unseal? There is the case length, there is the length of the chamber, then you mention throat, the length of the throat is another measurement, I make transfers for moving bullet contact with the rifling when adjusting ‘off the lands’ to the seater die.

F. Guffey

 

[mtang45]

Pardon the terminology errors. I'm calling the chamber the area south of the rifling all the way to the bolt face. Included in that is the area where the neck of the cartridge fits, I refer to it (probably incorrectly) as the throat. I assumed by SAAMI specs that the chamber area (by my definition) is usually something slightly longer than 2.850 (max case dimension). I was surprised that it was actually 2.878. It makes me wonder where the extra .028 is in the chamber dimensions.

Maybe I should do a cast of this thing to find out what it really looks like, or am I making a big deal out of nothing?

 

[Joe O]

SAAMI specs for the 7mm STW case are 2.850 max (at least the reloading manual states trim at this dimension). I just got one of those tools that measures chamber length in your rifle and mine measures 2.878 or .028 over max. Is this unusual in a rifle chamber or more typical?

Thanks

I always find the chamber length,longer than expected when I measure with a Sinclair chamber length gage,so I never trim until after the first fireing,(unless a case mouth is oviously deformed).I then only trim to square the mouth,disreguarding the "trim to length" in the manual.(it's always too short).I even had a custom chamber done for a 20 Ackley Bee,sending dummy rounds.The smith still got the chamber neck too long (.042).Pretty common in factory barrels.
I would prefer .015.