Neck Turning
We get tech questions on neck turning all the time here at Sinclair. Here are my replies to some of the most common questions:
Should I neck turn for better accuracy? This depends on the size of the neck area in the chamber and the thickness of the brass you’re using. If your brass is thin and the rifle has a SAMMI spec chamber, my answer is “no.”
Example: The chamber neck diameter for 308 Winchester is .343″ and the typical outside diameter of a loaded Winchester case is .333″. This means .010″ clearance, and the brass is worked to about its maximum. My suggestion is to buy brass with a thicker neck and check the variance of those necks. If the necks are within .002″ or less, turning is not needed.
Another Example: For neck thickness of .013″ maximum and .015″ minimum, you can take the average measurement of .014″ and multiply by 2 = .028. Add .028 to the .308″ bullet diameter and you get .336″. When you look at .343″ as the chamber size and loaded round size of .336″, you’ve cut the difference to .007″, which is only .0035″ per side of the case instead of .005″ per side with the thinner brass. With these tolerances you may not want to turn the necks. You have good clearance for bullet release.
Should I turn new brass? When you buy brass, check the neck wall thicknesses. If they are over .002″, neck turning can be done to make the brass uniform in neck thickness. Example: The neck wall’s variance is .012″ on the low side and .015″ on the thick side. I suggest setting a neck turning tool to .012″ and turn all cases to that measurement. This will clean the necks to about 90% of the neck wall. There may be some spots on the neck that are not touched at all by the neck turner, but you have cut the tolerances down to a minimal amount.
How do I determine neck wall thickness for a tight neck chamber? This is a tough question if you don’t know the chamber size or have the reamer. However, the gunsmith will usually stamp the neck size on the barrel. Example: Tight chambered 308 Winchester stamped .332″. Subtract .002″ from .332″ and that makes the loaded round .330″. .330″ minus .308″ equals .022″. Divide this by 2 and you get .011″ for the neck thickness. If you do not have this info, the next best way is have a chamber cast made and get the measurements from the chamber cast. By following the info, above you can get info for that measurement.
I use the following Sinclair tools for getting the measurements discussed in this article:
- 59-1100 Case Neck Sorting Tool and 26-308 Pilot. Use for factory rifles.
- MIC-3 Case Neck Micrometer, for the tight neck brass to get exact measurements for tight chambered rifles.
- CERR Cerrosafe to make a chamber cast of the rifle
- 52-2300 Starrett 1″ Micrometer
- MIC-14 Digital CaliperCH1-30 Neck Turning Kit
Ron Dague
Reloading Technician
NRA Certified Reloading Instructor
rond@sinclairintl.com