In his next article on what to look for in a custom rifle, John Stewart of Kiote Corp explains the right way and the wrong ways to cut a chamber and throat for a rifle…
The number one problem with most rifles both factory rifle and also some custom rifles as well, is the manner in which the chamber is cut.
Factory guns are made with reamers that get used on hundreds and hundreds of chambers. Over time, the reamers become dull, chipped, and dimensionally smaller due to wear. Scoring, gouges, too loose, too tight are all problems commonly seen in factory and less quality custom guns.
Just recently, I was asked to correct problems with chambers on two rifles that you’d expect to be near flawless: one Kimber 84L and one GA Precision HRT rifle. Both exhibited excessive scoring caused by chips not being removed during the reaming process. Both exhibited longitudinal gouging caused by the reamer being forced into the chamber. These are irreconcilable errors on guns that the owners paid in excess of $3000 for.
When I cut a chamber, I never use a reamer that has been used on more than 4 other rifles. Once that reamer completes its fourth rifle, it either gets sent back for re-conditioning or it gets tossed in the recycling bin. This ensures that my tooling is sharp and dimensionally sound before it ever cuts steel.
I also never cut more than 0.010” in depth in one pass. This ensures that I’m not overheating my tooling Even with oil or coolant applied, heat is generated which creates problems. More importantly, this ensures that I don’t have excessive chip build-up which could cause scoring or gouging inside the chamber while it’s being cut.
My lathe does not have integrated coolant nozzles. Even if it did, it wouldn’t really work for chambering because I don’t want to push/flush chips into the chamber. I’ve found that using ISO 32 machine oil on my tooling allows for both proper cutting and ease of chip removal during the cut, allowing the chips to slide down the flutes of the reamer and out of the chamber. I also use a floating reamer holder from Manson Precision that ensures that my reamer stays inline and parallel with the bore. Even if my barrel were to be a half-thousandths out, the chamber would still be cut properly.
Once I’ve achieved proper headspace, I use a honing brush from Flex-Hone to ensure a near-mirror finish on my chamber walls. I don’t necessarily believe that a true mirror finish is necessary. In fact, I prefer not to have one because the walls should provide a slight resistance to the brass during expansion so that everything stays in line with the bore during the burn process of firing.
Another problem is that all factory rifles are chambered with a long throat. This is primarily done to ensure that no matter what brand of ammo the consumer is using, it will fit in their rifle. While it’s convenient, it’s also detrimental to both accuracy and life-expectancy.
The longer the jump is between the ogive of the projectile to the start of the bore, commonly known as “free bore”, the bigger the chance is for that projectile to get off-center, commonly referred to as “sideways”, before entering the barrel. This is exaggerated by out of square, off-center chamber cuts commonly seen in most rifles.
The long throat also allows for quicker throat erosion. With the projectile essentially banging against the throat of the bore, it erodes microscopic pieces of the metal with forces in excess of 40K PSI found in most center fire rifles used for hunting or competition.
Several well-known custom have led the consumer to believe that they are required to have a longer throated chamber now so that they can set the projectile out farther to achieve greater velocity. Time and time again, history has proven that most calibers are more accurate and have longer life expectancy by setting the projectile a specified length off of the lands, typically 10-20 thousandths of an inch.