On rim diameters, cartridge families and the Model 7

14 March 2006, 07:59

Recoil Rob

Years ago I bought a Rem Model 7 action and put a 700 Classic barrel on it chambered for .250 Savage.

I've been thinking about rechambering to .257 Bob so I got out my copy of Speers Reloading manual to check the dimensional differences between the two. I also checked against the .308 family for which the M7 was originally used.

As I understand it the .257 Bob is a necked down 7mm Mauser (7x57) but according to Speer the Bob has a rim diameter (RD) of .457 while the 7x57 has an RD of .473.

I also noticed that both the .308 & .243 have an RD of .457 while the 7mm-08 has an RD of .473.

What would be the reason for changing the RD for cartridges in the same family?

My gun has functioned fine even though the bolt face is cut for a .308 and the .250 Savage rim is .016 larger. Does Remington cut them larger than necessary since I imagine the bolt face diameter is not as critical in a push feed than in a CRF?

Will I encounter any feeding problems if I go to the Bob? The max. cart. length for the Bob is less than the .308.

Thanks, Rob

Thanks, Rob

14 March 2006, 08:46

Ol` Joe

Hmmmm.

According to RCBS load and Speers #13, I find ALL of the cartridges as haveing a RIM diameter of 0.473".

The case HEAD diameter of the 250 Savage is 0.469", 257 Rbt is 0.473", the 7x57 is 0.4729", and the 243 & 308 have a HEAD diameter of 0.4709". The head is the case area just in front of the extractor cut.
I`d suspect Remington and all other manufactores use the same bolt face for all of these cartridges. The 250 should feed just fine from your M7 although the 7x57 will be too long to function from the mag.

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14 March 2006, 08:59

vapodog

Functionally all those cases are the same diameter.

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14 March 2006, 09:17

Recoil Rob

Thanks Joe, of course you're correct, I meant rim diameter, not casehead diameter, I've edited my post.

I double checked my Speers #12 and it does say that the .243, the .257 and the .308 have a RD of .457.

I understand why I would have a feed problem with a 7x57 but the Bob is shorter. The action was originally for a .308 which has a max. cart. length of 2.81" and the Bob's is only 2.78". (I took those specs from the same Speers #12, is it wrong?)

Thanks for the help.

Rob

14 March 2006, 10:04

James C Scott

Keep in mind the only limits to COAL or COL are the depth of the throat in the barrel and the length of the magazine. Either one may set COL (cartridge overall length). But, if you want a repeating rifle the shortest COL rules.

(may not get a second chance if you jam one into the lands on that first shot)

I suggest making a dummy round in the longest combination you may want to use and see if it will fit in the magazine first.

Hell Rob, you know this