I'm new to forums and reloading so excuse any ignorance I show. I inherited a Rem 721 300 H&H along with dies. I've messed around with reloading for a number of years but thought I would see how well I could tune a load for this rifle. Found I could get a nice .75" group at 100 and a 2.0" at 200 yards.
I had purchased a 100 pieces of brass from eBay and only gave them a quick look. On the range the other day one shell wouldn't feed. Turns out about a third of the brass is resized 300 Rem Mag.
Is there a problem with using this brass? It seems to shoot fine. Reading various threads there is talk of using other brass and resizing or FL? the brass.

I speak under correction here (no reference books at the office), but isn't 300H&H a whole lot longer than the Win mag? The H&H is essentially a necked-down .375H&H (72mm or 2.83" case length). It could be worth your while to check that.

You can re-form 300 Weatherby brass to 300H&H, but the "short mag" (300Win Mag, 7mmRem Mag etc) brass will be way to short as these were designed to work in 30-06 length actions.

don't know what a 300 rem mag is..
if it is a 300 RUM, with no belt, there's not way it will go into dies/gun/etc

if it is a 300 winmag, it won't close the bolt in a 300HH, as it has far less bodt taper. You can't resize to 300HH as it is .35 too short

jefe