I'm currently working on a wildcat based on 300 RUM. I have shortened it to 2.1" and formed a .458" neck. The wall thickness on the RUM case in area where the neck should be on the wildcat (1.7-2.1� from base) is about 0.015� to 0.020� (.38-.51mm). My question is whether I should make the chamber reamer neck larger than a normal .458� neck or should I make it normal and then reduce the wall thickness instead when making the cases?

Tomjac---

It's customary to keep neck diameters a standard dimension. You'll probably have to thin the brass some for the necks.

Neck turn the O.D. over a tight mandrel. Or make up a inside neck ream die and ream the inside of the neck.

Be sure to have enough clearance in the chamber on the neck area. For a loaded round, in a normal hunting chamber, around 0.005 to 0.007" total is OK, gives good feeding. For increased accuracy, go tighter, but beware, a snug chamber neck will spike pressures. Match chambers usually have about 0.002" total,or 0.001" per side, some of the braver guys use less.

And be aware of the "dreaded doughnut", this is a ring that forms at the junction of the neck and shoulder of a neck turned case after a few firings. If not taken care of, this will also spike pressures and get you into an unsafe condition.

Here is what I do in designing a wildcat. First make a case that matches your design criteria . This can be done with forming dies, machining a case etc. Then section it and carefully measure the brass thickness at the neck. You'll have to deceide if the brass is appropriate at the critical points. It can vary from about .012-.020 depending on the brass source you used for the case. Load the bullet you intend to use into the case neck and make sure its a tight fit. I ususally compress the neck brass about .003 to insure the bullet will be a reasonably tight fit. 5C collets often work well for this purpose! Now measure the case diameter at the extractor groove and at the shoulder as well as at the neck OD at the shoulder/neck junction and at the case mouth. You typically want your reamer to cut a chamber about .001-.002 inches larger than these measurements. Unless your after a tight necked chamber for which you will have to neck turn your cases to fit. Unless this is a varmint gun, the approach of giving your cases some room in the chamber is a pretty good approach IMHO.as it will keep you out of major pressure problems and sticky cases. I hope this answers your question.-Rob

[ 03-29-2003, 00:02: Message edited by: Robgunbuilder ]