HPT and MPI testing. Why?

[tangolima]

The extraction force drops with increased clearance. At zero pressure, a clearance of 0.000", the extraction force is about 100 pounds, at a clearance of 0.005" the extraction force is nearly zero.

If there is pressure inside the chamber, it is pushing the case walls outward, but it is also pushing the case out of the chamber. The force pushing the case out of the chamber subtracts from the extraction force, if this chamber pressure force pushng the case out is greater than the friction force from the case-wall interaction, the extraction force will be negative.

I see it now. Thanks.

It is still hard to understand why 2.5mil clearance still needs 40lbf. The 2 objects are out of contact already. There is air between them. Even with 5mil clearance (more than thickness of printer paper) the force is still non-zero. Where does the resistance come from? Carbon residues filling the clearance?

That aside, the graph gives insight to the difference between steel and brass casing extractions. The force vs. time graph you posted early indicates 20lbf for brass, and there is no peak in the curve. The brass case springs back so much that there is about 3mil clearance to the chamber wall. On the other hand, steel casing has little or no clearance.

-TL



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[prepare]

Here is some interesting unscientific data on actions springs that I have tested numerous times with my spring tester.

The Tubb Flat Wire in the chart is incorrect. It's only 10 lbs in condition 2. Not 16 lbs
Screenshot 2024-04-07 at 11.59.54 AM.jpeg

[the AR-15 Junkie]

Here is some interesting unscientific data on actions springs that I have tested numerous times with my spring tester.

The Tubb Flat Wire in the chart is incorrect. It's only 10 lbs in condition 2. Not 16 lbs
Screenshot 2024-04-07 at 11.59.54 AM.jpeg

No difference between sprinco blue and sprinco red?

[prepare]

No difference between sprinco blue and sprinco red?

Nope.

I was surprised how little difference there was between all the AR15 carbine springs until you get to the Sprinco Green.

Of course the .308 caliber recoil springs are heavier...

[1168]

Here is some interesting unscientific data on actions springs that I have tested numerous times with my spring tester.

The Tubb Flat Wire in the chart is incorrect. It's only 10 lbs in condition 2. Not 16 lbs
Screenshot 2024-04-07 at 11.59.54 AM.jpeg

Great post. Makes sense, given that I have a hard time telling some of Sprinco’s springs apart without the paint on them. Let me know if you’d like me to send you a Hot White, Colt rifle, and a Tubbs .308. A standalone thread with the op having a pretty complete list of springs tested on a single rig would be super cool. I’m sure someone has a G$ braided and a Strike flatwire floating around, too.

[Disciple]

Great post. Makes sense, given that I have a hard time telling some of Sprinco’s springs apart without the paint on them. Let me know if you’d like me to send you a Hot White, Colt rifle, and a Tubbs .308. A standalone thread with the op having a pretty complete list of springs tested on a single rig would be super cool. I’m sure someone has a G$ braided and a Strike flatwire floating around, too.

Have you seen this?

This topic is relevant to my interests. As a mental exercise to satisfy my own curiosity, I had measured a few springs to try and understand how spring parameters might translate into AR15 operation and perceived recoil. In concept, the spring stores the recoil energy and the reciprocating mass changes the harmonic frequency of the spring-mass system. In my case, I just wanted to look at the spring variable.

I used a digital trigger scale to roughly measure the force, in pounds, at bolt closed position and after (x) inches compression. I did this by installing the respective spring into a lower receiver with the corresponding buffer and measured the force needed to push the buffer away from the buffer detent horizontally. Then I used an extension of known length (x) to push the buffer and tried to get the force after (x) compression. I averaged three to five measurements each time. Those numbers are F1 and F@x. Due to the limits of the trigger scale, I had to calculate the bottom-out force of F2 at 3.5" of bolt travel.

I also measured other spring characteristics like coil count, free length, coil diameter, etc. and used spring formulas to back-calculate linear spring rate (k) and stored energy (1/2 kx^2). The below table is the data I've collected so far. I haven't had any time to do any analysis beyond a brief review of it and I'm sure there are many problems with it.

The flatwire springs with (Calc) in the description are calculated A5 extension values using measurements in a carbine extension as a sanity check to compare with actual A5 measurements. They seem to correlate for the most part.



There are some general observations I made from the data, although not precise due to the limits of my measurement system. I think there are valid general trends and physical principles.

• Flatwire springs have a higher F1 due to the longer free-length and more initial compression. The flatwire allows the longer spring to fit within the confines of the buffer and receiver extension at full compression.
• The A5 rifle spring follows a similar principle by using a longer free-length but instead, uses a longer extension to accommodate the longer coil spring.
• Even at the same spring rate, longer springs with more initial compression can store more energy within the same 3.5" of bolt travel. This will reduce the bottom-out impact and therefore, felt recoil until it prevents full bolt travel and proper function.
• Flatwire springs make a grinding sound and generally have more friction than coil springs in the same receiver extension.