Scale of permissible pressures and confirmed safety margin

Pressure Rating Scale and Confirmed Safety Margin

This post describes the calculated, operational, and empirically confirmed allowable pressure levels for the BespokeGun Specter .33XC rifle and ammunition loaded on Peterson brass. It also documents the confirmed safety margin of the locking assembly, barrel breech, and receiver.

In the video — Pressure test of the Specter 33XC carbine with an overpressure cartridge.
In the first photo — Internal ballistics of the Specter 33XC carbine with an overpressure cartridge.
In the second photo — The base of the test cartridge after firing.

Operational and maximum pressures declared by the manufacturer
For the .33XC caliber, David Tubb (creator of the .33XC) has established the following operational pressure value:

• Operating range: approximately 450 MPa — standard operating mode.

When firing at this pressure level, the rifle demonstrates stable operation of the locking assembly, no signs of plastic deformation of components, and preservation of chamber geometry.

At the same time, David Tubb warns: when pressure reaches 600 MPa, the Peterson case undergoes irreversible weakening in the head area; therefore, any further increase in pressure during ammunition loading may only be done at the shooter's own discretion and risk.The value of 600 MPa is accepted as the maximum conditionally safe pressure for the .33XC cartridge.

This pressure level corresponds to proof testing intended to confirm structural integrity with a standard safety margin. At these pressures, plastic deformation is not permitted.

As part of BespokeGun factory testing, the Specter .33XC rifle was subjected to firing at pressure levels of approximately 650–700 MPa, which significantly exceeds both the operational and maximum pressures declared by the manufacturer.
As seen in the graph above, to achieve a substantial pressure excess (700 MPa), the test cartridge uses the faster-burning powder VihtaVuori N560 with 100% case fill, as a correctly prepared load using N570 powder does not allow such pressure to be reached. (QuickLoad calculations based on a 300 gr Berger Hybrid OTM bullet);

Based on the test results, the following was recorded:

• no case failure;
• no lug imprints;
• no plastic deformation of the bolt head;
• no damage to the bolt body;
• no plastic deformation of the receiver;
• no bulging or change in geometry of the chamber or barrel.
• In all cases, when using ammunition with geometry conforming to David Tubb's technical documentation, no failures or plastic deformations were recorded.
  
  Factory test results confirm the actual safety margin of the locking assembly and barrel breech at no less than 600 MPa when using ammunition with geometry conforming to Tubb's technical documentation.
  
  This level should be regarded as a confirmed engineering structural safety margin, and not as a permissible operational mode.
• Pressure Mode Classification
• It is important to distinguish between the following modes:
• Operating mode — ~420 MPa.
• Maximum pressure mode permitted by the manufacturer — ~600 MPa.
• Proof testing mode — ~700 MPa.
• Emergency mode — approximately 900 MPa and above (data on this will be described in subsequent parts).
• The emergency mode differs fundamentally from the proof testing mode in terms of the nature of the load and the rate of pressure rise, and does not fall within the confirmed safety margin.
  
  BespokeGun Specter .33XC has a confirmed safety margin of no less than 650 MPa with correct ammunition. The recorded catastrophic failure occurred outside the confirmed and normative operating modes and is attributed to emergency pressure levels exceeding the calculated values.