Thread: My 260AI Load Development Begins

Originally Posted by outdoorlad

KG, can you explain the reasoning behind this?

How do you get it hard into the lands when you don't use a projectile? What difference does FF the case have with or without a projectile?

If the brass isn't hard onto the bolt face, when the firing pin hits the primer it pushes the case forward as far as the extractor or chamber will allow.

Then the powder ignites & the shoulder pushes forward & grips the chamber then the case head finds it has nothing behind it so slams back against the bolt, usually the primer has also backed out against bolt, then gets completely flattened looking a lot like an overpressure situation.

This sort of thing also happens when brass is too short for a chamber because of over sizing or sloppy Gunsmithing with long head spaced chambers.

This stretches the brass usually right where case wall meets the web.

With "proper" AI (Ackley Improved) chambers the neck shoulder junction is slightly shorter than the brass so it is reasonably firm against the bolt face.

Some brass is a bit short & some chambers are a bit long so this doesn't work sometimes.

There is a difference between AI & improved chambers.

A false shoulder is the way forward for an improved chamber.