March 6th, 2012

Optimizing Neck Tension with Bushing and Non-Bushing Dies

Redding Titanium TiNi BushingsWe are often asked: “What size neck bushing is best for Lapua 6mmBR brass in a ‘no-turn’ chamber?” The questioner planned to purchase a Redding Type ‘S’ full-length sizing die with neck bushings. The quick answer is that one should probably get both 0.265 and 0.266 bushings and see what works best. With the current “blue box” Lapua brass, a loaded 6BR round with an unturned neck will typically run about 0.268″ (depending on the bullet). A 0.266″ neck bushing, after springback, will give about 0.0015″ tension which can work well in a bolt gun. In a gas gun, we recommend running .003″ (or more) neck tension.

Alternative to Bushings — Honed Full-Length dies
Conventional, non-bushing full-length sizing dies can create ultra-accurate ammo with very low run-out. For some applications, we prefer a non-bushing FL die over a bushing die — so long as the neck tension is correct. But many FL dies have an undersized neck diameter so you end up with excess neck tension, and you work the brass excessively. Forster offers a simple, inexpensive solution — honing the neck diameter to whatever size you want.

If you purchase a Forster non-bushing, full-length sizing die, Forster will hone the neck dimension to your specs for about $10.00 extra. This way you can have a FL die that provides the right amount of tension for your particular load. Forster dies are relatively inexpensive so you can afford to have a couple of FL dies with necks honed to different diameters — such as 0.265″ and 0.266″ for a no-turn 6mmBR. The die itself is very affordable — currently Sinclair Int’l charges $33.95 for a Forster 6mmBR full-length sizing die (item FP6BRFL).

Forster FL dies, necks honed to .265″, .266″, and .267″.

Steve Rasmussen of gave this a try. In fact, he had three dies made — each with a different neck dimension. Here’s his report: “My original Forster 6BR FL die sized the necks down a lot, less than 0.260″, .256″ if I recall correctly. I sent my die in and asked if they could supply two more FL dies (for three total) to have the necks honed to 0.265″, 0.266″, and 0.267″. The cost was $10 for my supplied die and $38.50 plus $10.00 honing fee for each additional die. Return shipping was $11.00 via USPS Priority Mail.”

The table below shows the neck diameter range of ten (10) sized cases using each die. Brass springback after sizing runs 1 to 1.3 thousandths. Steve was using the older, “brown box” Lapua brass with thicker necks so he needed the .267″ bushing. The older Lapua 6mmBR brass measures about 0.2695″ with bullet in place. Steve explained: “My loaded rounds are running 0.2697-0.2699 using [brown box] Lapua 6BR brass. So far the dies are working well. I sized 80 cases with the 0.266″ necked die. The shoulder is running 0.4582″ and 0.300″ up from the base is 0.4684″. I spun 20 of ‘em and 16 had a runout of one thousandth (0.001) and the other 4 at 1.5 thousandths (0.0015).” That shows that these honed Forster FL dies produce exceptionally straight sized cases.

Die Diameter Sized Brass Springback Neck Tension
0.267 Die 0.2683-0.2684 0.0013 0.0014
0.266 Die 0.2672-0.2674 0.0013 0.0025
0.265 Die 0.2659-0.2660 0.0010 0.0039