TBAC BACK PRESSURE MEASUREMENT METHOD

Introduction

TBAC has developed a way to characterize "back pressure" on a semi-automatic rifle platform in a way that utilizes direct physical measurements, is repeatable and consistent, produces a metric that correlates positively to other "back pressure" indicators, and has a strong basis in fundamental science.

This work has been in progress for several years and was the basis for the SPIRO development.

"Back Pressure" is a term that has been applied to the effects of sound suppressors on semi-automatic firearms, including but not limited to: how much total gas is sent to the rear of the rifle (ie, through the action); how much "gas in the face" the operator experiences; the total amount of particulates and/or undesired gasses are emitted near the operator's face; and the entirety of its effects on the action's dynamics, including but not limited to: cyclic rate, bolt speed, unlocking time or speed, ejection pattern, feed rate.

This method used an AR-15 as the host platform because it is the most universal platform for semiautomatic rifles and select-fire assault rifles. The same method could be adapted to other rifles and operating systems, with minor adjustments, but this document is in reference to the AR-15.

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Objectives

The measurement standard needed to satisfy the following requirements or objectives:

Uses Direct Physical Measurements - To the extent possible, the metric must rely on directly measuring a physical phenomenon that is either identical to or directly related to the gas flow itself. This means not using "derivative" or "inferred" metrics that do not necessarily follow from the gas flow itself.

Is Repeatable and Consistent - Given the same suppressor, host, and ammunition, the method must produce the same results between test sessions, or produce the same results given an identical setup at another site.

Be Consistent with Other Back Pressure Indicators - It is already known that things like cyclic rate, bolt group speed, ejection, and operator-perceived "gas in the face" are influenced by back pressure. This metric should not produce counter-intuitive results or counter known data with regard to those other factors.

Strong Basis in Fundamental Science - The principles and measurements used should be based in fundamental physical laws

Allow relatively "quick" testing - The test protocol should be relatively "quick" in order to be able to run many test runs in a given day. This is in contrast to some other methods, such as particulate "count", that have extremely long reset times. Our goal was to get a 5-shot dataset on a setup in about 5 minutes.

Quantitative - The metric must be numeric.

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Theory of Experiment

We measure the actual pressure near the breech from before the shot is fired until after the action opens. This timeframe is used to capture the entire firing event, including the time when the action is open.

Since measuring at the base of the chamber is difficult, we use a normally-positioned chamber pressure test location. (See next sections for detail.)

This pressure data is used for flow-rate and then total flow because flow rate is proportional to the pressure drop across an opening. In this case, the bore (or breech) is an opening between the suppressor and the atmosphere at the upper receiver. The pressure-vs-time data shows how much gas is flowing out of the rear of the barrel, and thus, out of the action.

The raw pressure data has some minimal post-processing done and then it is integrated over time to create a metric that is related to the total amount of back flow.

FIGURE 1. PRESSURE, ZOOMED TO 1200 PSI AND LOWER, VARIOUS MUZZLE DEVICES

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Experimental Setup and Procedure

The pressure sensor is at the case mouth seal, as described by page 116, drawing 12-(5.56mm)-3 of NATO AEP-97 EDA V1 E.

FIGURE 3. SETUP DIAGRAM

The data-acquisition system is a BK 3052, sampling at 262KHz, PCB 119C12 piezo sensor, and PCB 422E5 charge converter. The voltage to psi conversion is calculated based on the calibration data provided by PCB.

Host weapon- The lower used is a Daniel Defense DDM4 lower with the factory spring and an H2 buffer. The upper used has a 10.3” barrel with a carbine length gas system, a 5.56 NATO chamber, and a 0.070” gas port; the bolt carrier group is from Aero precision. The ammo is GGG M193.

FIGURE 4. RIFLE SETUP WITH SENSOR

Maintenance considerations- To prevent errors caused by changes in cyclic rate due to fouling, the bolt carrier should be wiped clean with a solvent and re oiled every 50 rounds of testing. To ensure that there is not excess fouling impeding the sensor, the sensor should be removed, and the weapon thoroughly cleaned every 200 rounds of testing. 10 fouling rounds need to be fired after maintenance to ensure consistency for the data.

Data Post-Processing

Signal Clamping: Signal clamping forces the voltage to stay in a desired range for signal processing. ICP transducers can dip below zero PSI after a rapid pressure change, like a gunshot. This negative range can mask things happening at low pressures (relative to the peak). This is commonly done using analog hardware, like a diode (Ref 1), but we chose to do it digitally. We created a Simple Positive Clamping Circuit by finding the minimum pressure level of the signal, and shifting the entire data set up, so that the minimum is at 0 PSI.

Moving Average: The data is processed with a 100-point moving average. Raw pressure data is very noisy and hard to interpret without some type of filter. This creates a much cleaner signal that is still representative of the high sample that it was collected at.

FIGURE 4. MOVING AVERAGE VS RAW

Total Gas Flow Calculations: The comparative gas flow data was calculated by computing the area under the curve of the pressure plot. This was done using a trapezoidal summation from 1000psi to 0 psi. The calculated value is in PSI*Seconds, the value is multiplied by 1000 to convert it to PSI*Milliseconds. In other words, the Y-values (pressure) are integrated along the X-axis (time) starting at the point where the pressure first drops below 1000 psi and continues until the instantaneous pressure drops to zero (0). This generally happens after the "Bolt Fully Open" timeframe (see next section).

FIGURE 5. TRAPEZOIDAL INTEGRATION

FIGURE 6. ACTUAL INTEGRATION

The 1000 psi ceiling was chosen based off the initial data set of suppressors that included traditional baffles and low back pressure designs. The plots for all cans deviate below 1000 psi, while the pressure data collected above 1000 psi does not deviate between different designs and is not affected by the suppressor. This led us to start the analysis at 1000 psi to focus on the window where the differences are visible and provide large enough differences to develop the best product.

FIGURE 1. PRESSURE OVER ENTIRE EVENT

FIGURE 2. PRESSURE, ZOOMED TO 1200 PSI AND LOWER

Ref 1: https://www.allelcoelec.com/blog/The-Basics-of-Clamping-Diodes-and-Their-Circuit-Configurations.html?srsltid=AfmBOor4AVgDYS7dBGa5zysxR8-llHS53oW_fJ6qSLbUisDqUXuuMYU-

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DISCUSSION

The pressure is considered starting when it drops below 1000 psi and then the integration is continued until the pressure drops to zero (0). In the figures below, major events including "Bullet Passes Gas Port", "Bullet Exit Muzzle", "Bolt Unlocks", and "Bolt Fully Open" are highlighted. This puts the timing into perspective and shows that integrating the pressure during this time window does capture the back flow of gas.

FIGURE 1B. PRESSURE OVER ENTIRE EVENT -- TIME MARKS

FIGURE 2. PRESSURE, ZOOMED TO 1200 PSI AND LOWER -- TIME MARKS

Integration does not stop at "Bolt fully open"-- it continues until the breech pressure is zero.

Frequently Asked Questions (FAQ)

Why ignore the gas tube?

The gas tube determines when the bolt opens, and once the bolt opens the flow through the gas tube vs through the bore will be proportional to each other as they are two openings in the same pressure vessel. With the bore having a larger cross-sectional area, as much as 10x larger depending on the gas port setup, the pressure in the bore will be the largest contributor of the flow into the upper.

Why measure pressure vs time?

Flow rate is proportional to the pressure drop across an opening. If we look at the bore as an opening between the suppressor and the upper receiver, higher pressure will lead to higher flow. Time and pressure show how much gas is flowing to the chamber for how long, showing the amount of gas that is coming out of the chamber into the receiver and affecting the user.

Why did you focus on the pressure after the pressure dropped to 1000 psi?

Initial analysis of unsuppressed vs suppressed, using multiple different suppressors with varying designs, showed that the plots did not deviate from one another until the pressure was below 1000 psi. This makes sense for several reasons: at very high pressures, the suppressor does not provide a large amount of restriction to the gas flow; the suppressor has no effect on the gas flow until the bullet has left the muzzle (on a 10.3” 5.56 that is about 18,000 psi); and, the flow out of the chamber is blocked while the case is still seated. Once the pressure has dropped low enough and the chamber has opened, the suppressor begins to affect the flow.

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COPYRIGHT Notes

This document and its presentation, along with the linked graphs, are © COPYRIGHT 2025 THUNDER BEAST ARMS CORPORATION (TBAC), ALL RIGHTS RESERVED, and may not be reproduced without written permission from TBAC.