AR-15 Rate of Fire as a Function of Buffer Weight and Action Spring – Initial Comparison

I must stress that this is an initial comparison based on limited data points, but I feel that it will be fairly accurate once more data has been gathered.

Many rounds were fired through a number of uppers, but I spent the most time with a 20″ rifle gas upper; data can be seen below. The barrel used had a 5.56mm chamber and a .092″ gas port, which are standard M16A2/A4 features.

Although the BCM action spring and the “generic” action spring were identical in appearance, their performance was quite different. Given the same weapon, recoil buffer, magazine, ammunition, environmental conditions, and even number of rounds in the magazine, the BCM spring reduced rate of fire by approximately 70rpm compared to the generic spring.

Time in each component of bolt carrier travel was reduced, however, the most significant change occurred during the “hang time” where the bolt carrier group was at its rearmost point of travel; the BCM spring delayed forward movement by as much as 35%. In terms of real world performance, the BCM spring allowed 35% more time for the magazine spring to properly feed the “stack” of ammunition.

Even the Wolff reduced power spring slowed rate of fire more than the generic spring. All springs had less than 300 rounds/cycles “on” them prior to testing.

While the switch from Carbine to H buffers resulted in a significant (~50rpm) drop, going from H to H3 – double the weight difference of the carbine to the H – only reduced rate of fire by 6-13rpm. I have many theories, only some of which are grounded in reality. It may be that simply having any amount of tungsten in the buffer changes the way the buffer acts when it reaches the rearmost point in the receiver extension tube, for the average rearward velocities of the H and H3 buffers were nearly identical, whereas other times and velocities differed.

Although I am not ready to release data for other uppers (even on an “initial” basis), their behavior with the above buffers and springs seemed similar.

Before anyone asks, I don’t have sufficient data for the H3/generic spring yet.

Rates of fire were calculated on high speed video, which was calibrated prior to testing. High speed video of my uppers on registered, full auto lowers were used to calculate theoretical rates of fire for semi-auto lowers. Rate of fire reduction is not the only reason to select an action spring and buffer. Your individual results may vary based on dozens of factors.

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4 Comments

Filed under Tests

4 responses to “AR-15 Rate of Fire as a Function of Buffer Weight and Action Spring – Initial Comparison

  1. Matt Ho

    It would be interesting to see where the H2 and ST-T2 buffer data fit in this test. Great article.

  2. Nathaniel

    How does one tell what buffer they have in their rifle? I have a Colt 6920 with a stock buffer. It’s marked “H”. Is this an H buffer or a carbine buffer?

  3. Bob der Baumeister

    Very interesting. I’d be interested to see how Sprinco’s springs might change this equation.

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