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Downchambering a rifle barrel: if a cartridge or chamber size reduction is warranted, here's what you'll need for secondary barrel refitting and chambering.

Perhaps you won't find the word "downchambering" in the dictionary or as a commonly used gunsmithing term, yet this is precisely that which will be undertaken here. In this somewhat unusual gunsmithing endeavor, the rifle barrel chamber will have been downsized from 6.5-284 to .260 Remington. The reason for this change was sound enough. While the 6.5-284 is a line long range target and hunting cartridge, it is highly erosive and recoil is quite pronounced. The .260 Remington yields more than double the barrel life with reduced recoil. Though accuracy is quite comparable with these two cartridges, the smaller .260 Remington cartridge costs less for both cartridge cases and loaded ammunition, and it is more readily available to the shooter.

Revision Up Close

As such a cartridge transformation is made, many precise measurements must be taken. Where a larger cartridge is being rechambered to a smaller one, the entire larger chamber section must be removed to the shoulder. This normally requires the larger chamber cut off at the shoulder datum line or where the case head of the smaller case would permit reamer clean up. It is important to double check all measurements before beginning the work. It is an absolute requirement to have enough remaining barrel shoulder at the receiver ring juncture after forming the barrel tenon.

For this particular rechambering I went from a Model 700 Remington to a Model 112 Savage with 0.863" x 1.50" x 20 thread per inch. Most rifle barrels have a major diameter of sufficient length for this to be accomplished rearward of the barrel taper. In the revision being made here, the 6.5-284 chamber was cut to the case datum line involving a length of 1.640 inches. The tenon cut for the new barrel thread left a full 1.240 inch shoulder for a safe and solid barrel-to-receiver joint.

For this work, a Bartlein 416 stainless with 1:8 twist and Number 5 contour was used. This is a size between a varmint and sporter weight, suitable for both target and hunting. On removal of the old chamber the barrel was precisely centered in the lathe in preparation for turning, threading, and chambering. The barrel is faced and precisely turned to diameter. Here the Savage 112 receiver has a 20 TPI pitch with a full 0.750 thread purchase beyond the interspacing recoil lug.

On fitting any replacement barrel to a Savage 110 style receiver for switch-barrel use, I seldom use the Savage barrel lock nut headspacing system. I pre-headspace the barrels via the barrel shoulder against the interfacing recoil lug for a most solid and precise barrel-to-receiver joint. The barrel would be going on a rifle that has two other barrels.

As the threading process nears completion, the receiver is tried onto the barrel until there is a smooth fit of the threads. The chamber is then reamed while the barrel remains positioned in the lathe. Here too, the receiver is turned onto the barrel with the Go headspace gauge in place to determine the remaining depth to cut the chamber. Carelessly reaming a chamber beyond headspace is a mistake that should never happen. Always hand ream the last few thousandths of a chamber in attaining final headspace.

There were a hundred once-fired Nosier custom cases in .260 Remington available from a different chamber. These cases, along with the Go headspace gauge, were virtually interchangeable, so things were working out nicely.

Finishing Touches and Testing

In keeping with tradition at my test range, load evaluation would be in my arena. Following barrel polishing and stamping, handloading and testing were next. Following leade polishing and a brief shoot-clean routine, a number of loads were tested, including factory ammunition. All went well and accuracy was most excellent. The 1:8 rifling twist would stabilize bullets from under 100 grains to over 142 grains.

Barrel length versus velocity is a factor to be considered when downchambering a barrel, as was done here. Expansion ratio, a function of barrel length and chamber (cartridge) volume, play a part in this equation. This effect on a modern-day centerfire rifle cartridge will involve about 25 feet per second/inch of barrel shortening, so an expected velocity loss of approximately 50 FPS was shown here, which is quite negligible. The resourceful handloader is usually able to compensate for this difference while not courting excessive chamber pressures.

Conclusion

Downchambering a rifle barrel can be a most practical alternative for many a target or hunting rifle. Following careful measurements, it becomes a routine gunsmithing procedure at a modest cost. Magazine alteration is normally not required with typical hunting cartridges. The revision can be akin to having a new rifle, so further encouragement shouldn't be necessary.

Caption: Below: Careful measurements are important in the chamber downsizing process. Here the 6.5-284 case is being replaced by the .260 Remington. The barrel must be shortened by 1.740" here. Below right: The barrel has been shortened by 1.740" to accommodate the downchambering process from 6.5-284 to .260 Remington.

Caption: Right: The barrel tenon is being machined to exact dimensions here for rechambering from 6.5-284 to .260 Remington. More than enough shoulder will remain.

Caption: Below left: With threads cut and shoulder turned to precise dimension, a case is tried in the chamber. Note entire case rim protrudes beyond tenon of typical push-feed chambering system. Below right: Both headspace gauge and cartridge case are used in determining headspace for the .260 Remington in the downchambering process.

Caption: Above right: With threads cut to proper depth and pitch, the chamber is reamed for .260 Remington. Note tape on the reamer flutes as a means of determining approximate maximum depth.

Right: In the threading and chambering process, the receiver is frequently turned onto the barrel to test for thread fit and preliminary headspacing.

Caption: Above left: It's all pictured here in downchambering from 6.5-284 to .260 Remington. Note how much was cut off the barrel for chambering transition. Barrel thread pitch from 16 to 20 TPI is required in going from the 700 Remington to 112 Savage action. Note absence of typical Savage head-spacing nut as direct barrel shoulder is used. The recoil lug is not shown.

Above center: Downchambering from 6.5-284 to .260 Rem. with new barrel in place on Model 112 Savage rifle. Shown is the cut-off chamber making this procedure possible. Above right: Finished barrel tenon shows barrel/receiver surface to be a full 1.242". Precision here is paramount to best accuracy.

Caption: Below left: A few very excellent groups shot with the newly chambered .260 Rem. on a Savage Model 112 single shot rifle. Upper left group is eight shots. 120 grain Berger bullets were used for the groups.

Below right: Completed downchambered rifle from 6.5-284 to .260 Rem. with Sightron scope shows a well-balanced, older Savage Model 112 rifle suitable for sporter or target. Talley Picatinny mounts are used to attach this scope.
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Title Annotation:TECH TOPICS
Author:Johnson, Norman E.
Publication:American Gunsmith
Date:Mar 1, 2017
Words:1165
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