Brownells - Jack Rowe, Master Gunsmith Series, Bore Size and Proofing, Part 2 of 6

Published on May 30, 2012
Duration: 9:51

This video delves into the complexities of firearm proofing and its implications for gunsmiths and owners, particularly concerning bore size tolerances, chamber modifications, and conversions. It highlights the legal and safety ramifications of altering firearms, emphasizing the need for reproofing after significant modifications to ensure continued safety and compliance with regulations, especially in the UK and Europe.

Quick Summary

Firearm proofing regulations, particularly in the UK and Europe, dictate strict tolerances for bore size and require firearms to be reproofed after modifications like chamber lengthening or conversions from non-ejector to ejector. These rules are in place to ensure the firearm can safely handle increased pressures, such as those from longer shells, and to maintain structural integrity.

Chapters

  1. 00:08Bore Size and Proofing Intro
  2. 00:311925 Rolls of Proof - 12 Gauge
  3. 01:1212 Gauge Bore Size Range
  4. 02:0112 Over 1 Marking
  5. 02:2611 Gauge Limit
  6. 02:52Dangerous Barrel Modifications
  7. 03:281989 Rules of Proof & Chamber Lengthening
  8. 04:03Chamber Lengthening Invalidation
  9. 04:362.5 vs 3 Inch Chambers
  10. 05:02Pressure Differences (2.5 Inch)
  11. 05:25Pressure Differences (3 Inch)
  12. 06:01410 Gauge Pressure Differences
  13. 06:45410 Gauge Caution
  14. 07:16Non-Ejector to Ejector Conversion
  15. 07:49Extractor Hole Weakening
  16. 08:50Screw-in Chokes and Proofing

Frequently Asked Questions

What are the acceptable bore size tolerances for a 12-gauge shotgun under the 1925 Rolls of Proof?

Under the 1925 Rolls of Proof, a 12-gauge bore marked '12' had a nominal size with a tolerance of 10 thousandths of an inch. This allowed for a bore diameter ranging from .729 to .739 inches. If it exceeded .739 inches, reaching .740, it would be marked '12 over 1'.

What happens if a firearm's chambers are lengthened in the UK or Europe?

In the UK and Europe, lengthening a firearm's chambers invalidates its existing proof marks according to the 1989 Rules of Proof. The firearm must then be re-proof tested at an official proof house to ensure its continued safety and compliance.

How does lengthening chambers affect the pressure a shotgun can handle?

Lengthening chambers significantly increases the pressure a shotgun must withstand. For example, a 3-inch shell generates approximately 3.5 tons per square inch of pressure one inch from the breech face, compared to about 3 tons per square inch for a 2.5-inch shell, a substantial increase.

Why is converting a non-ejector to an ejector gun potentially dangerous?

Converting a non-ejector to an ejector gun involves enlarging the extractor hole. This modification can weaken the barrel lump and reduce the distance between chambers, potentially compromising the structural integrity of the firearm and requiring reproofing.

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