How much extral room is allowable before a spacer is needed? Would say, a Mod. 70 is 300 RUM being rebarreled to 338 Dakota need a spacer? What about a CZ550 in 416 rebarreled to 338 Lapua? What cost should I estimate for fabrication and instalation of such a thing should it be needed? There a lots of used 300 RUM's around at good prices that I have no use for except to rebarrel to something more useful.

DOC

Doc, I wouldn't worry about the spacer until I measure the COL required to put a bullet close to the lands. Once you know that dimension, and assuming it's quite a bit shorter than the mag length, then you can start calculating spacers and follower length. - Dan

Some magazines (Sako comes to mind) are substiantally longer than the standard length of the cartridge. Sako uses a 3.6 inch magazine box even for the .30-06 (at least they did in their long actions before the new model 75). That makes it about .3 inches longer than a standard factory load, but this has no detrimental effect on feeding, etc.

I don't really think a magazine block is necessary until the cartridges are so short that they mis-stack in the magazine and screw up feeding.

I've been shooting .308 out of my long-action 700 Rem for years without mishap. It has a stripper guide cut into the rear receiver ring and five .308s go into the rear section of the standard mag quite happily. Standard follower, too.

I've seen many .308 M1 rifles with a small block in the forward section of the mag well, but have no experience shooting one long-term.

On the M1 magazine block, it was put their to prevent loading of 30-06 ammo on rifles that had been converted to 7.62NATO.