Loading Belted Magnum Cartridges

[Petey308]

Loading Belted Magnum Cartridges​

TL;DR: Treat most belted magnums like a regular bottleneck: headspace off the shoulder with a light bump (~0.001–0.003"). Reserve belt headspacing for true H&H-style chambers. Prep brass carefully (same lot, anneal, mandrel/bushing, sort) to control SD/ES.

Background — why belts exist (and why they usually don’t matter now)​

The original belt (e.g., .375 H&H) provided a datum for headspacing when case shoulders were shallow or poorly defined. Many later magnums kept the belt for legacy/marketing reasons even though those cases have usable shoulders.
**Modern best practice** for most belted magnums is to size and headspace off the shoulder with a controlled shoulder “bump.” This usually produces more consistent internal volume and lower SD/ES than relying on the belt.

When to treat the belt as the headspace datum​

Use belt headspacing only when one of the following applies:
- You have a true H&H-style case with almost no shoulder.
- Your chamber was cut to headspace on the belt (some older/custom chambers).
- Load development or chamber checks show you must set headspace from the belt to achieve reliable chambering/cycling.

If none of those apply, prefer shoulder headspacing for best consistency.

Practical sizing guidance (how I do it)​

1. Full-length resize with a light shoulder bump — typically ~0.001–0.003" depending on chamber and brass. Size enough for reliable chambering/extraction, but not so much that the brass stretches forward and thins above the belt.
2. Don’t over-bump — excessive bump → forward stretch on firing → thinning at the web above the belt → risk of case head separation. If you see thinning or separations, retire the affected cases.
3. If shoulders stretch too much and you get a bulge near the base after firing: consider a bulge-removal die (Larry Willis sells a die for this) or switch to longer magnum brass that starts you closer to the chamber shoulder (e.g., Peterson long-mag options). Visit: http://www.larrywillis.com

Tools that make setup easy and repeatable​

- L.E. Wilson adjustable case gauges & micrometer setups — excellent for precisely setting die shoulder bump and headspace. Highly recommended for odd chambers or when you must headspace on the belt.
- Calipers / micrometer / case gauge — verify datum-to-base measurements and control bump consistently.
- Mandrel sizing vs bushing vs expander — mandrel sizing usually gives the most uniform internal neck diameter; bushing dies are fine when neck walls are already uniform.

Steps to lower SD/ES with belted mags (practical brass prep & loading)​

Belted magnums can show higher SD/ES because top-tier brass is less common. To get the best results:

• Use the same headstamp & lot number whenever possible.
• Buy the best brass you can (Lapua / Peterson / Alpha / ADG if available).
• Anneal the neck and shoulder (if part of your workflow) — this helps make shoulder bump and neck tension consistent across the batch.
• Sort by weight or internal volume (weights are quick; water-capacity is the true internal volume). Batch brass before development.
• Use mandrel sizing or turn necks if neck wall thickness is inconsistent. Turning before final sizing helps.
• Set a conservative shoulder bump and measure carefully; avoid bumps that require excessive stretching on firing.
• Trim / chamfer / deburr for consistent seating depth.
• Seat with a comparator (base-to-ogive) for consistent seating depth.
• Use match primers and a precise powder scale (0.02–0.05 gr). Choose a powder that provides consistent fill and burn for the case capacity.
• Test and log MV, SD/ES — if ES is large (20+ fps) you likely have a major prep or component issue; crimping is rarely the correct first fix.

More about headspacing and internal volume​

If you headspace on the belt, small variations in case length from datum to base change internal volume and pressure, increasing SD/ES. Headspacing on the shoulder standardizes that critical dimension and reduces volume-based variance — a primary reason to prefer shoulder headspacing when possible.

Special-case hardware & brass options​

- Longer magnum brass options (e.g., Peterson long-mag cases) let you start closer to the chamber shoulder and avoid excessive bump/stretch.
- Larry Willis bulge-removal die can fix base bulges after the fact: http://www.larrywillis.com.
- L.E. Wilson gauges / micrometer setups are excellent for setting exact bump/headspace.

Suggested workflow for belted magnums (step-by-step)​

1. Inspect brass; segregate by headstamp/lot.
2. Wet-tumble and dry.
3. (Optional) Anneal necks/shoulders to baseline hardness.
4. Full-length size with no or minimal expander ball; set shoulder bump ~0.001–0.003" (measure).
5. Run through mandrel (or bushing if necks are uniform) to set final neck ID/tension.
6. Trim, chamfer, deburr; uniform primer pockets if required.
7. Seat bullets to comparator OAL; record base-to-ogive measurements.
8. Start load development: ladder tests / node hunting. Log MV, SD/ES, group sizes.
9. Adjust prep steps if SD/ES or groups indicate variance (trim more, turn necks, re-anneal, change sizing strategy, or sort more tightly).

Warnings / pitfalls to watch for​

• Case head separation — usually caused by over-bumping and repeated stretching. Inspect the web above the belt and retire suspect cases.
• Bulge at the base — shoulder stretching forward too much; consider longer brass or smaller bump.
• Mixing headstamps/lots increases SD/ES — don’t mix during precision load development.
• Relying on the belt when you don’t need to creates unnecessary variability.

Quick checklist (printable)​

• Same headstamp & lot? Y/N
• Brass cleaned & dried? Y/N
• Annealed? Y/N (if part of workflow)
• FL sized with bump set (measure): ____"
• Mandrel/bushing size: ____"
• Trimmed & deburred? Y/N
• Primer pockets uniformed? Y/N
• Comparator OAL recorded? Y/N
• Load development log started (vel, SD/ES, groups)? Y/N

Final thoughts​

Most modern belted magnums are best treated like ordinary bottleneck cases: headspace off the shoulder with a small, controlled bump and spend your prep-time on uniform brass and consistent neck tension to lower SD/ES. Use belt headspacing only when technical necessity dictates (true H&H-style chambers or specific chamber cuts). Precision with belted mags is absolutely achievable — it just takes deliberate prep and the right tools.


Aaron Peterson
Hawkeye Ammosmithing