Gentlemen, there is a lot of advice posted here, and most is good, some is not well thought out, and some is useless! The question is, if I convert my 375 H&H double to 375 Flanged mag, can I still shoot the belted case in the chamber! The answer is NO!

The Flanged chamber will not accept the belt in the flanged chamber, and the extractor wouldn’t reach the rim, for extraction, even if it would.

I don’t have the drawings of the two cases in front of me as I write this, but if memory serves, the case of the flanged is longer than the belted case, though this is not a big deal. The 375 H&H belted Mag chamber can be modified to a flanged chamber. If done properly, it will work like a charm. The chamber end of the barrels have to be back bored just deep enough to remove the belt area of the chamber, and the diameter only slightly narrower than the rim of the rim of the flanged case, and cut fine threads in the wall. Then a bottom of the boreing faced off square in the bottom of the cylinder. Now make a cylinder threaded to fit the threaded portion of the chamber’s back boreing. The bore through the cylinder just under the size of the head area of the flanged case. Once installed in the barrel butt, cut the cylinder off flush with the face of the original barrels, run a Flanged chambering reamer into the chamber to cut the excess out of the bushing area, and cut for the extractor, then cut the recess for the rim of the flanged case, till the case headspaces in the rifle! The original extractor can usually be used because the rim on the flanged case will require enough steel from it be removed to get rid of the old pall area for the belted case. Some times a little throat lengthening may be needed. With the difference in the speed of the flanged can be adjusted with handloading, and may shoot factory with no adjustment in load at all.

Now I have a question ! Why do you want to do this? The work is involved, and expensive, and The brass, and ammo availability are dismal at best, and the 375 H&H isn’t generally used for dangerous game in a double anyway, and if the rifle ejects properly anyway, it is not a cost effective repair. With a double chambered for a cartridge like the 458 Win Mag, this makes sense, because the chambering reamer of the 450NE 3 1/4", removes enough of the wall to clean out the chamber area of the 458 win mag. Sometimes re-regulating is needed, but the work on the chambers is not a difficult operation, and many rifles do not even need re-regulating, and you have a better rifle at fairly low cost! The 375H&H rechambering is not only more work, but you must find someone who knows what he’s about to do it, and far more costly!