determining reamer freebore

[Charles E]

Just a check on my thinking:

OK, you know the bearing length of a bullet (full diameter portion).

You decide how much you want in the neck. The remainder is the "freebore candidate."

To spec the reamer, subtract from that the clearance of the case in the chamber, say .010.

Additionally, subtract the length taken by the 45-degree "mean little shoulder."

In concert, for a .30, if you subtract about .050 from the aforementioned "freeebore candidate," you have about the right amount to spec for the reamer freebore length.

Sound OK?

 

[pbike]

Charles,

You may be right. Last time I talked to Kiff about this he said that its easiest to think of the freebore as a measurement from the boltface.

Seat the bullet in the case to the length you want it seated, ie where is the boattail or flatbase in the neck. open calipers to the size of the bore, and make a mark (scratch) on the bullet then measure from the base of the cartridge.

Kiff and JGS show their reamer prints with the freebore measurement from the base.

Paul

 

[Charles E]

Yeah, but I don't use PTG or JGS for reamers. In any case, showing it from the base is just a matter of addition. If you run a reamer in short (lor long) the freebore length stays the same. The guy grinding it needs one number, but in the final analysis, the user is after how much of the bullet is in the neck.

 

[pbike]

Charles,

I don't grind reamers, and I didn't stay in a Holiday Inn Express last night, but if I did.

I would have a start point for all my measurements, that start point would be the base of the cartridge. This way all cuts would start with a measurement from the same point, a Datum if you will. Once you have that measurement and you draw up a reamer print, simple Addition and Subtraction will give you the number you are looking for.

 

[Cheechako]

Charles

Throat angle and bullet ogive will play into it. Why not make a couple of dummy samples for your grinder to use. He has the optical equipment necessary to make precise measurements.

Ray

 

[M.D.Spencer]

I like the use of a dummy round

When I worked for Manson. A lot of guys sent in dummy rounds. We tried to get as close as we could.. Grind wise

 

[JerrySharrett]

Charles, if I were dealing with two unknowns, like amount of bullet in the case and what should the freebore be, I use QuickLOAD and keep changing the seating depth and load density based on the bullet and powder I think I want. (Notice I said "think I want"). That is what it is all about, I have an idea I want to try.

After that you can seat the bullet and pretty much "scale" the remaining external bearing length with a dial caliper. I seriously doubt that changing the freebore by a few thousants will change anything and you can get a reading within a few thousants with the method above.

As an aside, Jim Borden uses (or did) a CAD system and sets up the freebore and leade of the desired angle and lets the CAD system do a graphical solution.

 

[Charles E]

Thanks Jerry, Steve Shelp has a CAD program, maybe I'll use that. Or just be real conservative; I have a good throater in .30. The problem is the case is one of those I swore I'd never do again, a wildcat, with the body to be blown out and the shoulder to be pushed forward. I know the bullets I plan to use, they are quite long. But having been bit before, suppose I want a shorter bullet, like the 185 Berger LRHPBT? If I set the reamer up for that, then I can throat it out for my old standby, the 187 BIB, or one of the newer 210s.

Actually, the way the math works out, if I get it just right I could try both with one freebore length -- I'd have .120 of the bearing surface of 185 LRHPBT in the neck and .290 of the 210s in the neck. I was hoping someone could get me within .010 or so, but I suppose it would be better to just specify even less freebore and throat it out to get that seating.

 

[Cheechako]

And then, after all the agonizing and measuring and fretting, everything will change starting with the first shot, and get progressively worse as those white-hot gasses eat away at the throat.

Merry Christmas.

Ray