Mechanics of a crush fit

Discussion in 'Reloading' started by woods, Apr 18, 2014.

  1. woods

    woods Well-Known Member

    Jun 4, 2006
    I often run across those who say that a case can not grow larger than the chamber, i.e. "how could it?"

    So I have a theory, and that's all it can be since what happens in the chamber is hidden from us. Sorta like astronomers inferring the existence of a black hole by other factors.

    So, 2 things make a crush fit possible:

    1. The ignition of the primer inside the primer pocket propels the case forward with force. In my tests and on other websites, an empty case fired with a live primer will have the headspace reduced by as much as .002"

    2. In a load firing, the shoulder gets pushed back by the primer explosion, the case walls expand to the chamber walls, the case head gets pushed back to the bolt face (case shoulder still hard against the chamber shoulder) and the case walls springback (allowing extraction). Upon ejection the pent up springback forces of the shoulder move it forward (it was pushed back now springback is forward)

    Perhaps this is common knowledge but I've never seen it explained so I'm looking for validation, rejection or discussion

  2. AZShooter

    AZShooter Well-Known Member

    Dec 12, 2005
  3. Mikecr

    Mikecr Well-Known Member

    Aug 10, 2003
    HS setback on primer firing only occurs with disposable case designs holding very low shoulder angles & high body taper (243, 270, 308, 30-06, etc). In these cases it takes little to nothing to wedge and vary the angles. An exception to this, there always is, something like a 223 where relative thickness to case area is high. So even though a 223 holds a low shoulder angle, it doesn't setback on primer firing. And with a rational load, you can crush fit it for all eternity. Not so with the same thickness in 308 capacity.

    Newer cartridges with 30deg shoulders are more reloader friendly.
    These also do not set back in HS on primer firing. The price(there always is)? They cannot be counted on to self-center on firing in chambers with excess clearances, and when loaded TIR is high.
    So they are way easier to make and maintain with precision, but also need to be (less forgiving).

    It's not easy to see brass gripping walls whether front-to-back, or back-to-front, until measuring variances from fully fire-formed. I imagine this dynamic again changes with case design & headspacing system.
    I know with fully FF'd cases of modern design, having very high shoulder angles and very low body taper, normal load pressures, enough barrel steel around chamber, and held within normal springback for clearances, no dimensions yield and sizing is no longer required at all.
    This is where I like to be.