Neck Separations, What to look for and Why.

Reloading info for the 5.7x28mm

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Neck Separations, What to look for and Why.

Post by panzermk2 » 18 Dec 2017, 11:28

I am re creating a thread concerning neck separations.
http://www.fivesevenforum.com/viewtopic ... hilit=ring

It started out as post about another question.

Due to Photobucket hosting bullshit all images I posted are no longer linked or showing in the above linked post defeating any purpose too it.

Neck Separations Why and What to Look For

This is a compellation of posts I have put up concerning this matter. I decided to combine them all into one informative post.
Some things related to Neck Separations.

Temp and Case Stretching

Because of the high pressure the 5.7 operates at you will see it affected by temp allot more. Also due to its long thin diameter it will heat up pretty fast in the chamber.

The high temp though will cause pressure to spike about a millisecond to soon. This causes the primer to start flowing and the neck to stretch to SOON, yes we are talking milliseconds here, and the brass does not grip the inside of the chamber and it starts stretching to soon also. Blow back operation causes this phenomenon. You can see a gold ring on the inside of the case just below the shoulder when this is happening.

This stretching will cost you up to 500FPS sometimes more. It is very consistent and hard to catch unless you know what you’re looking at and what to expect.

This is how guys blow their hands off. You start your test loads and when you hit this Plateau you think WOW the FPS has stopped increasing. Half the folkes will stop there, the other think heck I can load it hotter.

Then they blow their gun up.




I cut this case open from a load development test lot. I have tried to capture this ring. When it is formed most of the combustion is over and it's stays bright gold/brass color while the surrounding brass is sooty.


I thought I would share some of these pics. This is what the ring looks like. You can see the fracture point very clearly and see just how close this case is to letting go.

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The X-factor here is the blow back operation. When you start to hit max pressure on the 5.7x28 the case starts expanding at too great of a rate. This is why when guys load 1x brass hot you get neck separations. The case can't stretch enough to stay intact.

The 5.7x28 case has an internal taper at the base. This internal taper supports and keeps this section of the case from blowing out when it is fired. When you fire the 5.7x28 the case "grows" and 1/4 of it actually leaves the chamber and is suspended in mid air relying on the thicker section of the case to contain the pressure as the bullet travel down the barrel. Keep in mind that the pressure inside the case is evenly applied in all directions. So if the case did not have a taper it would blow out. This is why the internally straight walled 5.7x28 blank cases made by Starline for hollwierd can't be used in the PS90/P90 and FsN pistol.

When you get near the limits of the case at the wrong TIME (very important) it cannot contain this pressure and you get a velocity drop. Think of how when a hydraulic line starts to fail. Instead of the hydraulic force being applied at the end of the line, the line expands and absorbs some of this force.

It comes down to WHEN this spike happens. This is why EA can load 5.7x28 to the point of drastically moving the shoulder and yet not blowing out the primer or ripping the neck off and why when some of you who tested Lil'gun, (again on paper should be perfect for the 5.7x28) at light loads your where getting neck separation and primers blowing out.

this is what the section of neck looks like when stuck in chamber. A bore brush with some CLP ran through from ether end of the barrel will push/pull it out.

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I do know from experience that chronographing 5.7x28 and EA's stuff even more, can be very tricky. I have had to make blast shields to get accurate readings at anything close then 15 feet. You may be measuring muzzle blast and if your chrono was closure then 10 you would be.


Sorry for the long explanation. I don't want to sound like I am pontificating or being a show off but this is very critical info to understanding the 5.7x28.
Jay Wolf
Pr. Elite Ammunition

"Engineers, the oompa-loompas of science!"

Be'ein Tachbulot Yipol Am Veteshua Berov Yoetz
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