To summarize all reloading info, the shellacking and why

Reloading info for the 5.7x28mm

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To summarize all reloading info, the shellacking and why

Post by Ppostma1 » 11 Feb 2015, 22:39

I have been going through this forum and finding bits and pieces of this new concept to me, and am trying to put it all together to understand what and why this is done. Please correct me if I am wrong. I am going into detail to understand and help others comprehend what the issue is. Yes, I engineer.

Blowback design: Works fine in short barrels like pistols. However, in longer barrels this concept fails. The round is ejected before the case is relieved of pressure and causes numerous problems. Semi/Auto rifles have changed ejection/chambering schemes by adding ports at a point in the barrel so the ejection process doesn't start until the bullet reaches a certain point, delaying the ejection process.

The ejection problem is not an issue in the Fiveseven handgun because it has a short barrel.

The designers of the PS90 decided to go with the blowback design because of long-term maintenance and durability as well as the simplicity of the design. The problem: how to slow or delay the ejection process so the case pressures dissipate before ejection starts...

They chose to shellack the shells. The coating is a soft surface that glides easily, but when the case is expanded (within tolerances) during the high pressures of the firing process, the coating sticks the round against the chamber walls preventing it from moving until the pressures are reduced by the bullet exiting the barrel and the shell contracts and is blown back by the remaining pressure.

Failure to slow the ejection of the shells (stripped or bare cases) allows the shell to leave the containing and supporting chamber while the pressure is still high, and causes the shell to bear the full pressure on it own. This results in bulging, splitting, or fracturing of the cases. The slide group also breeches sooner (while under pressure) exposing the gun and operator to excessive blowback (popping mags) and other risk as well as dropping the muzzle velocity of the round by releasing the pressure out the back.

Some have over come this problem in reloading by:
  • Using case cleaning mechanism that dont harm the coating (sonic) and they are good for a few uses
    Stripping and reapplying the coating on reload to fully extend their life
    Leaving the case dirty and the buildup helps slow the ejection and they are just lucky
    They havent and are just talking to feel like they have successfully accomplished something in their life :facepalm:
In theory, the cases can be cleaned gently with a standard tumbler, and reloaded to standard or light load, and used once more but with risk. And the cases are then too stressed for a 2nd reload. Particularly at the neck of the case. The shot will also suffer performance set backs as the blowback breeches and drops the pressure too soon. So muzzle velocity, distance, and energy are lost.

Some aspects help reduce this complication:
  • the shorter 10.3" barrel version allows the round to exit sooner reducing pressure against the case
    lighter loads reduce pressure
    lighter rounds exit the barrel quicker
    a heavier slide group group and/or spring slow extraction
    Slower burning powder reduces wear on the case, but increases post-ejection blowback.
So someone using an aftermarket 10.3 barrel (the inside diameter is .001" wider, causes less friction, and allows the round to accelerate faster) with light loads and the lightest rounds could have more success with reloading and not coating the casings. (Me : )

But the standard chrome lined 16.4" barrel is going to be much more fickle about properly reloaded ammo. (the one I'm reloading for)

Also some rounds are labeled "for handgun, fiveseven only". These are uncoated cases are cannot be used in the ps90 do to this complication.

Once this issue is over come, the reloading is:
standard small rifle primer (inter-changable with small pistol magnum primer from CCI, and small pistol will work but not seat fully)
5.7 is considered small pistol and shot or pistol powder should be used.
round is standard .224 diameter available from midway and others
cases will quickly start cracking at neck if overloaded

Do the cases need to be lubricated before sizing?
: yes

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