Chamber Throat Reaming with A Precision Touch

On more than one occasion, the use of a throating reamer has resulted in increased accuracy from the rifle I used it on. Moreover, the reaming allowed for a wider selection of bullets that could be used in the rifle. Nevertheless, if a common reamer is used to make the chamber, it may not result in the desired leade to fit the bullets being used. As a result, a special throating reamer maybe needed. Over time, I’ve developed a method for using a throating reamer that has proven to be very successful.

And I’ve adopted a few modifications to the usual technique that have been helpful in using the throaters for optimal precision. Let me start with a typical example of how I lengthened the leade on a short-throated .221-caliber Remington Fireball factory rifle chamber. I was unable to use the handload bullets that are commonly used in benchrest shooting without seating them very deeply into the case.

This, in itself, was a deterrent to best accuracy, and it significantly reduced case capacity, which affected the case/bore expansion ratio as well. I talked with Larry Weeks at Brownells, who was very helpful in providing me with good information and a throater to lengthen the chamber leade. This particular throater was a spiral five-flute t mer Reamers, Inc. Five spiral flutes ei- 221 Remington case, used as a reamer guide. Below that is the reamer stop, shown next to a test cartridge.

Note the darkened area on the ogive of the bullet used to show seating depth. bility of the flutes dropping into the rifling, which could cause obvious problems. As a means of precisely controlling the reamer’s advancement into the leade and at the same time promoting perfect reamer alignment, I used a modified, fired cartridge case as a bushing and T-handle stop. I drilled and bored a hole through the base of a fired cartridge case to the exact shaft size of the reamer.

The forward section of the reamer was guided by the case neck, as the bored out case head served as a guide for the reamer shank. This provided perfect coaxial alignment with the bore. As a positive stop for the throater entering the bore, I used a bushing between the case head and the Thandle into the leade section to the nearest thousandth of an inch. This stop was made by drilling a hole the size of the reamer shaft through a

Clymer makes a spiral reamer to prevent the flutes from entering the rifling. round length of plastic (or metal) and faced off squarely at each end. The length of the stop is determined by the span between the T-handle and the shaft of the reamer extending from the mouth of the bored cartridge case. This is usually about an inch in length.

Prior to setting up the throater, I loaded a match bullet into a .221 Remington Fireball case seated to overlength, and chambered the round to test how far the existing rifling would further seat the bullet. I coated the bullet ogive (curved point) with blackening material to indicate where land/ I bullet contact is made and as a means of showing how deep I needed to recut the rifling leade.

When setting up the throater for cutting the leade, I fully inserted the bored-out cartridge case and gently advanced the throating reamer to land contact. At this point, I put the Thandle on the reamer shank and tightened the laterally opposing screw on the T-handle. Then, by gently turning the T-handle, I could perceptibly feel the reamer land contact, which was freed by a dusting of metal removal from the lands. Further depth adjustment was accomplished by adjusting the stop.

At this point, a measurement is made at the juncture of the T-handle and the extended portion of the reamer shank. This is a very important measurement, which is used to determine incremental reamer advancement. It’s important to know that the maximum leade lengthening will be but 0.025 inches, so advancing the reamer a few thousandths at a time using the T-handle bushing stop is the best way to go. Cutting too deeply cannot be corrected short of setting the barrel back.

At this stage, I examined the leade with a Hawkeye borescope, and by seating a bullet I checked for land contact and overall bullet seating depth. Finally, I lapped the throat to eliminate the reamer marks which appear as a sort of sawtooth configuration. This is said to produce a plasma-like gas as copper material is shaved from the bullet. This material is deposited throughout the bore as copper fouling, sometimes requiring extensive cleaning to remove it.

Conclusions Lengthening the chamber throat on a firearm enables the shooter to use different bullets and loads. A special throating reamer does an excellent job, but careful attention is required in using it. Using the aforementioned technique ensures a good job with total control over the leade depth.