HPT and MPI testing. Why?

[lysander]

Wow. I haven't seen information this detailed. Thanks.

I would say primary extraction happens when unlocking completes at about 2.3ms mark. End of piston motion means primary extraction completed and the bolt travels with the bcg at the same speed. That happens at 2.5ms mark. The displacement difference is about 0.8mm, or 0.031", or 1/32". Energy needed is 20*0.031=0.6 lbf-inch for brass and 9.3 lbf-inch for steel. You are right. My estimate was off by factor of 2.

Energy loss to extraction is indeed insignificant except for steel cased ammo. Looks like adjustable gas block is way to go.

-TL



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Nitpicky, but extraction starts at the end of piston motion (2.5 ms), when the cam pin bottoms out in the cam track. Only then does the bolt start to move.

What we have not taken into account is residual chamber pressure, depending on the timing of unlock there might be 200 psi residual chamber pressure. When the shoulder is uncorked from the chamber this will assist in extraction.

I normally ignore this as one, the force is low, and two, depending on the P-T curve used in the simulation, it sometimes isn't there.

[tangolima]

Nitpicky, but extraction starts at the end of piston motion (2.5 ms), when the cam pin bottoms out in the cam track. Only then does the bolt start to move.

What we have not taken into account is residual chamber pressure, depending on the timing of unlock there might be 200 psi residual chamber pressure. When the shoulder is uncorked from the chamber this will assist in extraction.

I normally ignore this as one, the force is low, and two, depending on the P-T curve used in the simulation, it sometimes isn't there.

Probably. But there really isn't any detectable slop in the slot for the cam pin to bottom out after unlocking position though. But if we define end of piston motion as bcg and bolt travel together, then you are correct.

There is one error I have noticed. The bcg speed is over 20fps, or 240 inch/s. It travels 0.36" in 1.5ms, which is way too long for primary extraction. The process must take shorter than that. The force plot shows distinct peak at 0.3ms for steel. Most likely it is when the casing breaks loose from the chamber. There is no peak for brass. The gauge probably was set to measure steel, and thus not sensitive enough for brass.

Distance traveled in 0.3ms is 0.072", or 1/16"! It doesn't change the conclusion. Energy loss to extraction is insignificant unless steel cased ammo is fired.

-TL

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[shadowspirit]

It's all about making sure everything's in top shape and reducing the chances of any nasty surprises popping up later on. Trust me, it's worth it for the extra peace of mind knowing your gear's solid.

Yes..

[shadowspirit]

Every so often check the end of the RE for any accumulation and clean it out. If you stand the gun on its butt CLP can migrate off the carrier etc and gravity never ceases, so a peek now and then never hurts. The carrier going back in there brings shit with it over time anyway.

Petroleum based lube or CLP does not migrate off things as readily as Breakfree CLP or other synthetic CLPs.

[Strikefirst]

Where do you buy your standard srainless springs from?

[1168]

The force plot shows distinct peak at 0.3ms for steel. Most likely it is when the casing breaks loose from the chamber. There is no peak for brass. The gauge probably was set to measure steel, and thus not sensitive enough for brass.

Distance traveled in 0.3ms is 0.072", or 1/16"! It doesn't change the conclusion. Energy loss to extraction is insignificant unless steel cased ammo is fired.

-TL

Can you (and Lysander) expound more on the extraction differences in steel vs brass?

Where do you buy your standard srainless springs from?

I buy Colt ones from Brownells, but I’m sure Colt or BCM ones would be just fine from BCM, Brownells, or Midway. VLTOR’s A5 spring is probably a rifle spring from BCM, so that’d probably be fine, too, in the appropriate receiver extension.

[lysander]

There is no peak for brass. The gauge probably was set to measure steel, and thus not sensitive enough for brass.

The test was specifically to measure the difference between the extraction force of brass cases vs steel cases. I will just quote the the report on why the difference in extraction force:

"During pressurization, the chamber of the gun expands radially in the elastic region and relaxes after the pressure dissipates to its original dimensions. However, the steel used in this analysis deformed plastically through most of the cartridge case during pressurization. Due to the level of the plastic deformation in the steel in the steel case, an interference fit occurs after elastic relaxation of the chamber. The radial force acting on the chamber due to the interference fit causes the steel case to stick in the chamber explaining the higher extraction forces. An example of this as related to the stress-strain curves of a material is shown in figure 14 . . . Switching to a steel with a higher yield strength may help solve this issue, but care must be taken to ensure the steel retains enough ductility to avoid case rupture."



"Pressurization and Extraction of a Steel and Brass Cartridge Case using Finite Element Analysis", Raymond Chaplin, ARDEC, Picatinny Arsenal, NJ, January 2017.

Also, friction between the chamber and case is greater for steel than brass. It should be noted that if the interference becomes to great, the case will stick fast in the chamber. Steel, as a case material, is much more sensitive to manufacturing tolerances than brass.

[lysander]

Oh, and the case-to-chamber fit does not actually have to be a hard interference (the case is larger than the chamber) in order to have high extraction forces. Here are the results from another extraction force study showing the extraction force required for a given case-chamber clearance:



You can see that with a clearance of 0.0025" (diametrical) it will take 40 to 45 pounds to extract a brass case (zero residual pressure).

[tangolima]

Oh, and the case-to-chamber fit does not actually have to be a hard interference (the case is larger than the chamber) in order to have high extraction forces. Here are the results from another extraction force study showing the extraction force required for a given case-chamber clearance:



You can see that with a clearance of 0.0025" (diametrical) it will take 40 to 45 pounds to extract a brass case (zero residual pressure).

There is clearance between casing and chamber before firing, as per saami, and force to extract a live round should be minimal. After firing, the clearance is gone as the casing expands under pressure. At zero pressure, the casing should spring back to restore part of the initial clearance to facilitate extraction. The amount of springing back is different for different metals. Brass is good. Mild steel is not so good. And hence rather different levels of extraction forces.

The chart for brass extraction doesn't seem making sense. Maybe I didn't read it correctly.

1. Why so much force required if there is clearance?

2. Why more force is required with zero pressure?

3. How can the extraction force be negative?

-TL

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[lysander]

There is clearance between casing and chamber before firing, as per saami, and force to extract a live round should be minimal. After firing, the clearance is gone as the casing expands under pressure. At zero pressure, the casing should spring back to restore part of the initial clearance to facilitate extraction. The amount of springing back is different for different metals. Brass is good. Mild steel is not so good. And hence rather different levels of extraction forces.

The chart for brass extraction doesn't seem making sense. Maybe I didn't read it correctly.

1. Why so much force required if there is clearance?

2. Why more force is required with zero pressure?

3. How can the extraction force be negative?

-TL

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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.