Rate of Fire: What Determines it and How to Change It

Published on January 17, 2025
Duration: 12:20

This video from Forgotten Weapons, hosted by Ian McCollum, provides an in-depth analysis of what determines a firearm's rate of fire and how it can be manipulated through design. McCollum, a recognized expert in historical firearms, explains the fundamental principles of bolt cycle time, bolt mass, and bolt velocity, and how these apply to different operating systems like simple blowback and locked breech. He illustrates these concepts with examples of various historical firearms, including the M11A1 and Chauchat, and discusses design modifications for rate adjustment.

Quick Summary

The rate of fire in firearms is fundamentally determined by the bolt's cycle time, which is influenced by the distance the bolt travels and its velocity. In simple blowback systems, bolt mass is key: heavier bolts decrease rate, lighter bolts increase it. Locked breech systems adjust rate via gas piston/rod mass or gas port size.

Chapters

  1. 00:00Introduction to Rate of Fire
  2. 01:21Fundamental Determinants of Rate of Fire
  3. 02:25Simple Blowback Systems and Rate of Fire
  4. 03:41Locked Breech and Gas Systems
  5. 05:21Case Study: M16 in Vietnam
  6. 06:01Case Study: M11A1 vs. Chauchat
  7. 08:19Design Choices for Changing Rate
  8. 09:24Dual Rate Systems: DS-39
  9. 10:34Mechanical Delays in Rate of Fire

Frequently Asked Questions

What are the primary factors that determine a firearm's rate of fire?

The rate of fire is primarily determined by the bolt's cycle time. This is influenced by two main factors: the distance the bolt must travel during its cycle and the average velocity of the bolt as it moves, including its acceleration and deceleration.

How does bolt mass affect the rate of fire in simple blowback firearms?

In simple blowback systems, increasing the bolt mass decreases the bolt's velocity and thus lowers the rate of fire. Conversely, decreasing the bolt mass increases its velocity and leads to a higher rate of fire.

Can the rate of fire in gas-operated firearms be adjusted?

Yes, the rate of fire in gas-operated firearms can be adjusted. Designers can alter the mass of components like the gas piston and operating rod, or change the size of the gas port to modify bolt carrier velocity and the resulting rate of fire.

What are some examples of firearms with adjustable or dual rates of fire?

The Soviet DS-39 Degtyarev machine gun is a notable example, featuring a dual rate system (600 or 900 rpm) achieved by engaging a buffer and switching between different gas port sizes. Some designs also use mechanical delays to control fire rate.

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