Firearm Chamber Design

[eljefe]

Firearm Design

Firearm Chamber Design


Cartridge Chambers are the most important details in the design and manufacture of small arms barrels. Bullets, projectiles and shells are larger in diameter than the bore and groove diameters of barrels. The cartridge case that holds the bullet must be the same diameter inside its neck as the bullet, and therefore the outside of the neck of the case is considerably larger in diameter than the diameter of the bullet or than the bore or groove diameter of the barrel. In addition the body of the case is often made of still greater diameter to hold the desired powder charge . Therefore the bore of the barrel, originally drilled and reamed to a stated diameter, must be enlarged or reamed out at the breech to contain and fit the cartridge or shell. This enlarged portion is termed the chamber.
The exact shape and dimensions of the chamber with respect to the cartridge or shell to be used is extremely important. On these dimensions depend the safety of the shooter, the normal and smooth operation, a long accurate life, and the accuracy potential of the firearm, and these dimensions are one of the factors which determine whether the barrel will be accurate or not. The chamber must be a proper fit for the cartridge.
Cartridges of a given caliber and model are never absolutely uniform as to dimensions because it is impossible to make them so when produced in quantity. Therefore ammunition specifications prescribe maximum and minimum dimensions for cartridge case, bullet, and loaded cartridge. The chamber must be of such dimensions that the maximum cartridge will enter it without undue force, it must fire with safe pressure, and the fired case must extract without undue force. Likewise the chamber must not be so large that a minimum cartridge would misfire, or the case rupture, or gas escape to the rear, and the minimum cartridge must give satisfactory accuracy.

To give an idea how individual cartridges of the same size may differ, there are some where the differences between maximum and minimum are as much as .004 inch for the outside of the neck of the loaded case, and .009inch for overall length of case. These would seem to be very small variations, but they are all important when working with a gas pressure of over 50,000 pounds per square inch, and particularly with a gas as hot as that generated by smokeless powder.

In an overloaded condition that gas exceeds the temperature of an oxy-acetylene gas cutting set. I have viewed barrel Knox forms -that’s the thick end near the receiver- that look like they were made of butter and a hot knife had melted cuts in the butter. In reality of course the hot escaping gasses from a ruptured case had cut the high chrome carbon steel as though it were butter. So be warned the safety factors in firearms are massive but arrogance and ignorance can defeat any safety factor.



Variations as large as the above will seldom be found, even when examining large batches of cartridges, except perhaps with military cartridges made in great quantities and speed in war, for manufacturers have ideal or standard dimensions to which they try to adhere as closely as possible and with very considerable success, particularly in these days of modern precision machine tools. But the manufacturer who makes his arms for public sale must make them, including the chamber and bore, so that they will be safe and function perfectly with both the maximum and minimum cartridges, because he can never tell when the ultimate user of the arm might fire such a cartridge. The technical rifleman who hand loads his own ammunition can, however, afford to use a chamber with closer dimensions because he can inspect the components which go into his cartridge and throw out all that do not gauge within his established dimensions.

As most custom barrels are rifled prior to chambering, and cartridge choice is selected at a later date, leade is usually just a space, (not tapered), for the full diameter of the bullet to sit in.
Handloaders for the best result seat the bullets so that there is a very little space, less than .020 or no space at all between the front of the bullet and the commencement of the rifling. This space is called the Leade. Consideration has to be given for this technique as if there is no gap at all between the bullet and the rifling that could increase internal pressure within the cartridge case so an equivalent decrease in the powder charge should be considered, which would not necessarily decrease velocity in fact due to no space, even with a reduction of the powder charge, the smaller space to fit the gas expansion could still increase pressure and increase velocity.


Notes on Free Bore or Leade

The gun itself can cause greater pressure. This changes with the amount of use the gun has received. A new gun will often cause higher pressure because the tool marks create added friction to the bullet. Gun bores are smoothed with use. Used long enough, the hot gas will erode the bore near the chamber, to produce more freebore. Freebore is the distance the bullet moves before it engages the rifling. A common practice for large capacity cases such as 300 Weatherby is to bore out the rifling ahead of the chamber. This permits the bullet to get a running start before engaging the rifling, which keeps pressure safe. An opposite condition is more likely if the gun has had many high pressure loads through it. The end of the chamber can become heat crazed. Tiny cracks form, which when magnified appear as a dried out mud flat. These cracks cause extra grip on the bullet which in turn requires more pressure to get the bullet moving. This can increase the pressure significantly. Acquire the habit of checking the primer for signs of pressure. It could save your day.

Cases that are too long will pinch the bullet and cause excessive pressure. Just because the cartridge easily enters the chamber it does not mean the case is not too long. The clearance created by the crimp allows the case to chamber. However, when the bullet tries to open the case mouth it cannot, because the mouth is beyond the end of the chamber into the start of the bore, which due to the rifling lands is a smaller diameter.Remember Bullets fit the grooves not the bore. Once the brass is pinched by this it increases pressure and makes extraction difficult. Of course accuracy will be non existent and un predictable. Be sure, to always check and trim your cases, if needed for overall length.

(C) Owen Guns

[Vikram]

Highly interesting,Jefe.Some very important basics for a re-loader and the amateur enthusiast.Thanks for sharing.

Best-
Vikram

[timmy]

Excellent information, El Jefe. Thanks for sharing this with us.

[cottage cheese]

..ees good 'jefe ... mucho gracias.

[MoA]

And then you have the custom chambers...

The crush fit chamber, no turn, turn et al.

Most people forget to clean the chamber too...

[lazybones]

A good read eljefe. Thanks for sharing.

Ashok

[eljefe]

MoA
am not decided whether my new 6.5x284 for F class is going to be crush or jump...

[MoA]

I would go with a standard chamber...

But would go with either a .260, .260 AI or 6-6.5x47 Lapua either way would prefer a throat for the 139/140 gr bullets for the 6.5 caliber

[eljefe]

MoA
guess the new maddco 31 incher is going to be a jump fit too late now, still have a barrel to go, maybe in its next avatar?

[MoA]

Let us know how it works out...