30BR donut turning leaves thin spot on neck

[lrgoodger]

I have a new barrel coming which I am going to chamber in 30BR. Everything I have found on 30BR case forming says to simply turn off the bulge at the base of the neck caused by the old 6BR shoulder. I expanded my first case and measured the neck at .329 except on the donut, where it measures .335. Looking inside the case with my borescope reveals what I suspected. There is a groove inside the case under the donut. Now, it is a fact that when I turn that neck and remove the donut, the groove is still going to be there on the inside. That means there is now a thin-spot ring at the base of the neck that is .005 thinner than the rest of the neck. Has anyone experienced a neck cracking on this ring?

It seems to me that Lee could pick up some business by offering a collet neck sizer that would squeeze this ring in before neck turning. That should eliminate the thin spot.

 

[RGRobinett]

I have a new barrel coming which I am going to chamber in 30BR. Everything I have found on 30BR case forming says to simply turn off the bulge at the base of the neck caused by the old 6BR shoulder. I expanded my first case and measured the neck at .329 except on the donut, where it measures .335. Looking inside the case with my borescope reveals what I suspected. There is a groove inside the case under the donut. Now, it is a fact that when I turn that neck and remove the donut, the groove is still going to be there on the inside. That means there is now a thin-spot ring at the base of the neck that is .005 thinner than the rest of the neck. Has anyone experienced a neck cracking on this ring?

It seems to me that Lee could pick up some business by offering a collet neck sizer that would squeeze this ring in before neck turning. That should eliminate the thin spot.

While this - the thinner neck-base - was one of our original concerns, unless one cuts too deeply INTO the shoulder, it is not a problem: for my original 30BR chamber, thirty cases were used to fire 6,400 rounds through the barrel. The cases were never annealed - there were ZERO case failures, neck separations, or splits. The case-necks were turned for a loaded-round neck diameter of .328", and , from the beginning, sized with a .324" neck-bushing.

The best method for avoiding the 'bulge' is to fire-form prior to neck-turning (several methods are successfully employed). Cutting too deeply into the shoulder DOES result in case-neck separations - I have witnessed this, but, with several barrels and thousands to shots fired, have not experienced it. The last [registered] event fired using that original barrel produced a 500-27x score - for the conditions, a disappointing score, but not bad for 6K plus shots, at something over 200 firings per case - and a second place finish . . . RG

 

[lrgoodger]

Yes, I had thought of fire forming before turning as a way to keep from getting the thin spot, but I don't think I can do it without a special setup. The reamer I plan to use has a .332 neck. I just put a bullet into the case I expanded and it now measures .337 at the donut. I can't chamber that in the .332 neck chamber to fireform it.

Also, I think the guy who has the reamer got it before Lapua went to the "blue box" size. I turned the neck just enough to clean it up on a test piece and the loaded round measures .328. Would not a chamber neck of .330 be a better size for that round?

 

[RGRobinett]

Yes, I had thought of fire forming before turning as a way to keep from getting the thin spot, but I don't think I can do it without a special setup. The reamer I plan to use has a .332 neck. I just put a bullet into the case I expanded and it now measures .337 at the donut. I can't chamber that in the .332 neck chamber to fireform it.

Also, I think the guy who has the reamer got it before Lapua went to the "blue box" size. I turned the neck just enough to clean it up on a test piece and the loaded round measures .328. Would not a chamber neck of .330 be a better size for that round?

Good points. Many people fire-form the 6 BR brass [in a 30BR chamber] using "pistol powder", held in place with cream-of-wheat, Kleen-x, etc.: this seems to work well for them. To my knowledge, Jackie Schmidt, was the first individual simply run a chucking reamer in to an old 6MM BR barrel, and open the neck diameter to something like .335", then F-F using "cheap", light weight 6mm bullets and a "full" (loading manual) charge of appropriate powder. Assuming an old 6MM BR barrel, and access to a lathe, this method works GREAT: Jackie sent me some sample cases which measrue about 1.550" long, and, in every respect, appear perfect. There are many ways to skin the proverbial cat! It is very difficult to improve on the end result obtained by following the methods outlined by "SMOKIN' Joe" Entrekin, and Al Nyhus, on this forum's 30BR page.

Most of the competition rigs are chambered using .330" chamber neck diameters. RG

 

[lrgoodger]

I appreciate your responses, RG.

I have continued to research this issue. My friend confirmed that his reamer pre-dated the Lapua neck size change, so I am going to have the chamber done by Pierce engineering, who has the proper reamer on hand.

I found the Jackie Schmidt posts and I like what I am reading. I just happen to have an old 6mm barrel lying around, so I think I will get a fire-forming rig made from it. As an engineer, it just bugs me to know that groove is there inside the neck if I use an expander.

 

[North Fork]

RGROBINETT If you use a 324neck bushing ? What did you turn your necks to.when you turned your necks.

 

[nonliberal]

RGROBINETT If you use a 324neck bushing ? What did you turn your necks to.when you turned your necks.

From reading his post it looks like .010 since he had a loaded diameter of .328. .010+.010+.308=328. Then he had .004 neck tension with his .324 bushing.