Bore Throating With Precision And Control

No single chambering reamer will form a leade to meet the needs of all handloaders and shooters. Bullet shapes and lengths, as well as rifle loading restrictions, dictate this. Therefore, it may become necessary to alter the existing bore throat or leade as shooting needs and conditions dictate. The typical chamber, along with its leade, will conform to SAAMI specifications. If it didn’t, you couldn’t use just any brand of ammunition to fit your rifle.

But this doesn’t provide much solace for the handloader who may wish to use a variety of bullets, or a specific bullet, to fit one’s needs. This handloader may find it necessary to seat longer bullets at a depth well into the case 5×47 Lapua has a 140 grain bullet seated well into the space that could be used for powder. These 6.5mm bullets for the .260 Remington cartridge range in weight from 95-140 grains. Throat lengthening may need to be considered. neck, even well into the powder column.

Boat tail and flat base bullets also can present a unique problem as well as bullet ogive configuration and makeup. It may be useful to explore leade alteration options. The rifle throat or leade will conform to SAAMI specifications in the vast majority of instances where a shooter handloads a given cartridge. But, if a handloader wishes to load a bullet longer, the bore throat may need to be lengthened. This can be accomplished with a special throater offered by reamer manufacturers.

These throaters are caliber specific with standard or custom bullet ogive configuration. Proper use of a throating reamer should be thoroughly understood, and here I shall provide some proven procedural help that has served me well. Throating Guides Here I shall describe a precision throating reamer guide that is fundamentally easy to make and use. For each cartridge throat you choose to lengthen, you will need a fired case for that chamber.

Be sure the fired and deprimed case fits freely in the chamber of the rifle being worked on and that a bullet fits freely into the mouth of the case. I use a body die to full-length size these cases. Drill the back end of the case out to the hole size of the throating reamer flutes. The pilot may be removable or solid, and is machined to precisely ride the lands. Use of the throating reamer to final size the hole to exact size is ideal.

Throating reamer shanks are normally 1/4" so reaming a case head to this size for calibers under .25 is fine. Though throating could be accomplished through the drilled-out case head as a guide only, the chambered case is used as a twofold purpose, including a means of precise depth control. The throater depth control stop consists of a collar that fits the quarter-inch shank of the reamer. The collar has a set screw to restrict throating entry beyond a precisely determined depth.

With the collar at pre-determined contact with the case head, it is firmly locked into position.

The finished hole will be the maximum diameter of the throating reamer which passes through the case neck. The 0.2560" diameter pilot fits inside the bore lands of a .260 Remington’s 0.264" groove, indicating the lands are 0.004" deep. A precise throating job tapers the lands without cutting into the rifling grooves. This alerts us to the challenge of performing a perfect throating job. Throating Procedure Set Up Here we are dealing with a procedure akin to the forming of the finest chamber.

As a bullet-seating control, have a seated bullet in a sized, unprimed case at the original seating depth and one at the selected, new seating depth you want. Measure these two cartridges for over-all length and carefully note the difference. At this stage I gently insert the throater into and through the chambered guide cartridge to determine throater bore land contact. Do this carefully so the reamer cutting flutes are making gentle contact with the throat lands and not straddling them.

Try this a few times until you get a precise feel for correct throater flute land contact. Then, as a means of determining the new throater reamer depth, place a feeler gauge between the case head and control collar and tighten the collar set screw. Double check this depth a few times to be sure the throater flutes are making contact with the beginning of the lands and not falling between them.

The space you see between the case head and locked collar will be altered throat depth and should coincide with the new bullet seating depth. Leade Depth Operation The rifle barrel must be held steady in a perpendicular or horizontal posi- The collar must be thin enough to leave room for the throating reamer "T" handle. Always be sure the control cartridge is fully inserted. tion. This method of throating could be performed in a lathe but it is not necessary when using the control guide as outlined here.

With a coating of cutting oil, the throater is inserted into the fully chambered case. You may feel the pilot engage the outer surfaces of the lands. No resistance should be felt here. Note the space between the case head and the stop collar. Advance the throater very slowly while turning it with feel to locate throater flute contact or resistance. Resistance tells you cutting is taking place. This is normal. Here I gently advance the reamer and make full turns of the handle.

A double check of the depth collar will show how far you may have advanced. If cutting offers much resistance, withdraw a few thousandths and resume the throating procedure. Don’t despair if the throating procedure appears to go too fast. The throater is essentially self-feeding and you are shaving off only the few thousandths of land metal. As you feel the stop collar come to rest it will turn freely and you are finished.

At this stage I withdraw the throating reamer and guide case, blow out the chamber with compressed air, and clean the bore. A bore scope will show the extent of the altered lands forming the new throat. It is not unusual to see a trace of remaining lands where the throater didn’t cut into the rifling grooves quite as deeply as the reamer throat before it. This will have little, if any, effect on the bullet.

The function of the feeler gauge, stop collar, and "T" handle are shown. This results in a most precision method of throating. Lap Polishing The new throat will reveal some reamer marks on the riflings, despite your best efforts, just as the chambering reamer did before it. This is normal.

I lap-polish the new throat with tight-fitting patches of 3M 30- Micron grade (400 US Mesh) lapping The cartridge case is inserted into the pre-reamed short chamber as the datum line of the case shoulder contacts the partially reamed short chamber. These cartridge-specific short chambers are used as a precise means of bullet seating for the newly throated bore throat. This leaves the new throat ready to shoot without the usual shoot-clean regimen. paper.

About 30 passes of a few tight patches and this lap polishing will do the trick. You won’t need further throat break-in but you will need to check the new bullet seating depth in the new throat. This changes the bore/case expansion ratio to some degree and the resultant powder charges—hence pressures. Expansion ratio is the result of dividing the volume of the powder chamber, with bullet seated, into the powder chamber and bore combined.

The handloader has some leeway in powder charge with bullets seated farther into the lands. This was quite apparent to me in loading some 6.5mm cartridges and is part of the web we gunsmith handloaders weave. Conclusions The observant handloader may need a deeper leade or throat with some bullet/cartridge combinations. I have experienced these benefits, particularly with the smaller 6.5mm (.264) cartridges that can use all the case capacity available.

This is particularly true of the 6.5×47 Lapua right through the .260 Remington and 6.5×55 Swede with longer 140 grain bullets. The method of altering the throat or leade as I have described herein is precise, safe, and stands to improve one’s shooting where the needs are well thought out. AG