[Issue] Units can incorrectly fire caseless AC ammunition and other ammunition from the same AC by expending all non-caseless ammunition first

[GiovanniBertuccio]

Brief Description *

A unit that carries both caseless and non-caseless ammunition can expend all non-caseless ammunition, then fire the caseless ammo from the same weapons used to fire the non-caseless ammo.

ACs should be designated as caseless or standard before play, and ACs designated as caseless can only use caseless ammo, while ACs not designated as caseless can never use caseless ammo.

In MM, when both types of ammo are on a unit, all types of ammo are visible in the systems tab (and can probably be critted with a critical hit), but only the non-caseless ammo will show on the tooltip, and only non-caseless ammo can be selected in the weapons tab. When the non-caseless ammo is gone, the caseless ammo shows in the tooltip and can be selected and fired. This is incorrect behavior because the ACs are allowed to fire both types of ammo.

The intended programming behavior seems to be to have all ACs on a unit be standard or caseless, with the assumption that a unit with any standard-capable ammo has all standard ACs, and units with only caseless ammo having only caseless ACs.

2008-2010 TacOps page 352 "CASELESS AUTOCANNON AMMO" says

Which limits ACs to one class of ammo or the other, and seems to allow units with multiple ACs to pick on a per-gun basis.

Ignoring programming realities, the most RAW way to handle this would be to add a configuration option that allows users to set individual ACs to standard or caseless and carry both types of ammo.

Accepting programming realities, restricting units to carrying either all caseless or all non-caseless ammo would be a close, but not RAW implementation.

Marked as medium. Being able to fire both ammo types is fairly powerful, but it's easy for players to not intentionally do, and easy to fix Princess's loadouts in the lobby to prevent it.

3. Steps to Reproduce

1. In the MM lobby load only caseless ammo on a unit with standard autocannons and multiple ammo slots - I used a Devastator tank.
2. Before play, still in the lobby, mouse over the unit to see that the caseless ammo shows in the toolip
3. Change one of the ammo slots to any non-caseless ammo, leaving some caseless ammo
4. In the lobby mouse over the unit to see that only the non-caseless ammo shows in the tooltip
5. Begin play
6. Check the unit's system tab to see that both kinds of ammo are still present
7. Mouse over the unit to see that only non-caseless ammo is listed
8. In the weapons phase in the weapons tab check the ammo selector to see only non-caseless ammo is listed
9. Expend all non-caseless ammo
10. Check the tool tip and weapons tab to see that caseless ammo is listed and can be fired normally

Attach Files

No response

Medium*

Medium

MegaMek Suite Version *

50.12 2026-02-09

Operating System *

Windows 10

Java Version *

Java Vendor: Eclipse Adoptium     Java Version: 17.0.13

Final Verification

• I confirm this is a single, unique issue that hasn't been reported before
• I have included all necessary information and files to help reproduce this issue
• I have asked on MegaMek Discord about this issue
• I have confirmed this issue is being opened on the correct repository: MegaMek, MegaMekLab, or MekHQ