Investigation of transferred ammunition

Investigation of the Submitted Ammunition

Background Information

Along with the destroyed BespokeGun Specter .33XC rifle, two cartridges were submitted to the company as samples of the ammunition used in firing immediately prior to the failure.

Findings:

• neither cartridge was assembled by the firearm manufacturer;
• the ammunition was handloaded by third parties;
• the cases are not new (not zero-cycle); the actual number of reloading cycles is unknown;
• the history of case forming, sizing, and geometry inspection has not been verified.

Chamber Compatibility Check

The check was performed using a new test rifle with a fully identical Specter .33XC chamber.

Cartridge #1:

• the bolt does not close in the chamber at all;
• normal lockup is ruled out;
• a gross geometric mismatch with the tight chamber is recorded.

Cartridge #2:

• the bolt closes only with significant force applied;
• forced closure was deliberately not attempted;
• the nature of closure indicates that the geometry exceeds allowable tolerances.

Identified Geometric Deviations

Based on visual and functional inspection, the following indicators were identified:

• a thickened and/or deformed case neck;
• possible shoulder displacement or over-expansion;
• absence of guaranteed neck clearance for the tight chamber;
• overall geometric instability characteristic of cases subjected to multiple reloading cycles.

Firing with such deviations is strictly prohibited and life-threatening.

Relationship Between Ammunition Condition and the Failure Scenario

Incorrect case geometry in a tight chamber:

• restricts normal expansion of the case body and neck;
• causes a sharp increase in resistance at bullet onset;
• promotes an extremely rapid pressure spike.

In the presence of:

• an excessively fast powder or a powder mixture;
• possible firing from the lands;
• absence of neck clearance,

this combination of factors produces the failure scenario that occurred in the case under review.

Relationship Between Powder Burn Rate and Chamber Pressure

• 100% case fill with one of the fastest powders for this cartridge, VihtaVuori N570, yields a maximum pressure of no more than 570 MPa. (QuickLoad calculations based on a 300 gr Berger Hybrid OTM bullet);
• With the powder used by the rifle's owner, ReloadSwiss RS80, the maximum pressure in a correctly prepared cartridge does not exceed 520 MPa at 100% case fill. (QuickLoad calculations based on a 300 gr Berger Hybrid OTM bullet).

Therefore, when using a correctly assembled cartridge with ReloadSwiss RS80 powder, the rifle should have functioned normally, as it did with VV N570.

Ammunition Conclusions

• Both submitted cartridges are physically incompatible with the geometry of the Specter .33XC chamber.
• The cases show signs of use over an unknown number of reloading cycles and cannot be considered controlled components.
• The identified geometric discrepancies are a direct contributing factor to the excess pressure event.
• The submitted ammunition cannot be used to evaluate the structural integrity of the firearm.
• The condition of the ammunition directly correlates with the failure scenario that occurred.
• Had a correctly assembled cartridge loaded with RS80 powder been used, the rifle should have functioned normally.
• The most probable cause of the pressure exceedance is an error in the powder charge. The ReloadSwiss RS80 powder claimed by the user cannot produce damage of this nature even at 100% case fill.