Considerations for loading belted magnums


Well-Known Member
Aug 23, 2018
This thread is another collection of words by me I’ve saved to share when it might be useful. It’s not all inclusive, but hopefully someone will find it useful.

Loading Belted Magnum Cartridges

I was asked to talk about reloading belted magnum cartridges. I myself don't load for too many belted cartridges but the ones I do load for, I tend to essentially ignore the belt and load like any non-belted bottleneck cartridge and headspace/size the case off the shoulder. The only belted mags I wouldn't do that with are something of the H&H variety that really don't have much of a shoulder to begin with. That was the whole reason for the belt in the first place. The lack of a real pronounced shoulder meant that they had to create an area for the case to seal to the chamber when fired and prevent the gases from escaping rearwards around the case. Subsequent magnum cartridges just kept on with this method, even though most had sufficient shoulder area to headspace off of.

Sure, cartridges like the 7mm Rem mag, 300 win mag, 264 win mag, etc all have belts, but nowadays it's common practice to simply treat them like any other bottleneck cartridge and simply full length resize the cases with about .002" of shoulder bump. You're mainly after sizing just enough, and not too much. If you size/bump your shoulders too much, you can create a scenario to where your brass expands forwards (stretches) when fired and it'll cause the metal just above the belt to thin out and eventually cause case head separation. That's no fun to deal with. So, it's essential you're sizing your belted cases properly, and not overdoing it. On the otherhand, you can sometimes run into issues where the shoulders have to stretch too far and you can get a bulge near the base of the case that also needs tended to. Larry Willis makes and sells a die that will remove this bulge if you have this issue. It can be found at Also, some brass manufactures, such as Peterson, make longer versions of certain belted magnums that allow you to already start out closer to your chamber shoulders and avoid this issue altogether.

I've also used things like the L.E. Wilson adjustable case gages, which work very well to set up your dies and get your cases sized exactly like you want. I'd recommend that setup for new belted magnum reloaders. I'll provide a link to a video below showing how they work, and then another video showing another tool they make to go along with it that makes setting your precise bump easy. These are handy tools, but not required. You can get away with traditional methods just fine. I have one of those case gage setups for my 300wm, but I do a lot of 7mag loading too and don't use one for it. The gage and micrometer setup does make it extremely handy for those situations when you have a chamber that does require you to headspace from the belt, but still want to bump your shoulders slightly. You can ensure you're getting precisely the right sizing on your case once you get it all setup to your needs.

Some other considerations regarding magnums:

Belted case's can be a struggle to get low SD/ES, in general. One big reason is that getting high quality top tier brass can itself be a struggle, or even impossible, so then you're left to try to prep the brass you have and get it as consistent as possible.

If you're headspacing/sizing your brass off of the belt only, your shoulder bump might vary as a result. If your dimensions from the datum line to the base of the case vary, so will the internal volume of the case. Any variation of the internal volume will equate to variation in pressure when the powder is ignited, and thus your MV will also vary, creating higher SD/ES and higher or lower impact on target (particularly at long ranges).

So, in order to more easily lower SD/ES with something like a 300wm, 7mag, etc it really is best to headspace off of the shoulder rather than the belt. That'll ensure a more consistent dimension/measurement from the base of the case to the datum line. Like I said, when they made the 300wm, 7mag, 264wm, etc, they were based on the H&H magnums, like the 375 H&H, which needed the belt because it lacked sufficient shoulder area to headspace off of.

So, if you run as high quality of brass as you can get, and of the same lot, simply take the extra steps to prep them and sort them. Anneal the necks and shoulders to achieve more consistent bump/sizing and neck tension. Sort the cases by weight or water capacity into batches. Ensure you're using a powder that is an appropriate burn rate and that will fill the case, producing a more consistent ignition and subsequent burn, and preferably a high quality powder that is already proven to burn very consistently to begin with. Use match grade primers. Experiment with uniformingprimer pockets to see if it helps. Sometimes it does, sometimes it doesn't. Increase your neck tension a little and see if it helps. Sometimes increasing your entire neck's tension by .001" or more can help produce a more uniform tension on the bullet, especially from round to round, and produce a more consistent release of the bullet.

Anyways, these are the kind of steps I'd be taking to help you lower your SD/ES with the belted variety of cartridges, or any cartridge you're struggling with for that matter. Hopefully you find this useful.


Well-Known Member
LRH Team Member
Apr 18, 2018
Canadian, Texas
I've never considered any of that before. I've reloaded 264wm for 37 years and has been the only belted mag until 300wm about 4 years ago. Now by some are deamed dangerous as goes the new beltless mags? Are they more accurate? I don't know? I'm not a prs guy, but for what I do it's good enough accuracy wise staying belted.

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