O ne of the facts about firearms and ammunition is that the cartridge case and the rifle chamber are never exactly the same size.
If they were the same size, the rifle manufacturer and the cartridge companies would have zero allowable tolerances and ammunition and firearms would cost a larger fortune than they do now, if they could be made at all. Therefore, every time you fire your rifle, the cartridge is to some extent being reformed to the exact dimensions of that particular rifle’s chamber.
However, when a reloader and experimenter speaks of “fire-forming” he is talking about firing a cartridge that has a significantly different shape so chamber pressure will reshape it to conform to the rifle’s chamber.
This is what happens when I fire a .243 Winchester cartridge in my .243 Ackley Improved rifle. What allows me to do this without blowing up my rifle is that the “Ackley Improved” chamber and the original Winchester cartridge have the same headspace. That is, the distance from the point where the shoulder meets the cartridge neck to the rear face of the cartridge case is the same on both cartridges.
What happens when I fire a factory cartridge in my Ackley Improved chamber is that the walls of the cartridge are blown outward to have less taper from head to shoulder, and the shoulder angle is changed from 20 degrees to 40 degrees (See photo).
The result is that the cartridge is reformed (fire-formed) to have greater powder capacity; the straighter case walls stick to the sides of the chamber better, which lessens thrust against the bolt face; and the sharper shoulder angle prevents brass from flowing forward to lengthen the cartridge case, and decreases the need to trim the case neck as often, or at all.
This is the simplest form of fire forming and works wonderfully well if the chambering job is done correctly. Sadly, not all gunsmiths have the knowledge, tools, and skills needed to perform such a chambering job.
If the chambering job is done correctly, the improved chamber will actually be a few thousandths of an inch shorter than the factory cartridge from head to the junction of shoulder and neck. This means that when you chamber a factory cartridge it should require some effort to close the bolt on the new cartridge. This prevents excess headspace on the fire-formed round, which could cause case head separation the next time the cartridge is reloaded and fired.
If, however, your rifle is not perfectly headspaced, it is still not a lost cause. There are ways to fire-form the case without danger to the shooter.
I have a .222 Magnum Ackley Improved that has this problem, the chamber is just a couple of thousandths too long at the shoulder/neck junction. The solution is to load the factory .222 Remington Magnum case with a stiff, but not maximum, charge of powder, then seat the bullet in the case so that it is far enough out of the case to firmly engage the rifling.
This keeps the cartridge case firmly against the face of the bolt. When fired the cartridge expands to fit the chamber and the headspace is perfect. I can then neck-size the cartridge and reload it.
Seating the bullet to engage the lands and grooves is often mandatory in a wildcat cartridge that moves the shoulder of the case forward to increase powder capacity, such as the old Gibbs line of cartridges. In these cartridges the parent case is the .30-06, but the shoulder is moved substantially forward to increase powder capacity.
In the smaller calibers such as the .240 and .25 Gibbs, necking the case down to the desired caliber forms a false shoulder at the correct place for proper headspacing. On the .30 Gibbs, however, it is necessary to seat the bullet out to engage the lands and grooves, keeping the cartridge firmly against the bolt face, or extreme headspace is likely.
Some hardy souls just shoot .30-06 cartridges in the .30 Gibbs chamber, but this is a very dangerous practice. I did this once before I knew better.
When I was a teenager an older friend asked me to fire-form a bunch of military .30-06 cartridges in his .30 Gibbs. Loving to shoot, I thought this would be a wonderful way to get some free practice, so I took it to the range and started shooting.
Every shot fired produced a ruptured case. It didn’t take long for me to figure out that there were safer pastimes, so I took the rifle and ammo back to him and told him I thought he was doing something wrong.
What he should have done was pull the bullets and reseat them so they engaged the rifling. It’s a miracle he didn’t get an ignorant teenager blown up. Now when you read “fire-forming” in one of the shooting magazines, you will know exactly what the writer is talking about.
Email Steve LaMascus at ContactUs@fishgame.com