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RCBS Precision Mic

 
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  #15  
Old 03-03-2010, 06:18 PM
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Re: RCBS Precision Mic

Tom,

First I would say everthig Kevin said is exactly correct. To try and help clarify more for you, the idea behind neck sizing in the first place is to get a tight fit to your chamber (headspace –wise). As you know the closer your sized cases are to the dimension of the chamber, the better alignment to the center of your bore (as the theory goes), and better accuracy. In contrast, what everyone is saying is that for a hunting cartridge , you don’t’ want to get too tight (headspace-wise), after repeated neck-sizing of cases.

Having said that , after repeated firings of cases that you are neck-sizing only, eventually the shoulder (or headspace) will begin to extend beyond the walls of the chamber, causing unreliable (stiff bolt etc) feeds or not allow you to chamber a round at all. And, when hunting dangerous game, that is potentially a life threatening situation.

Once you realize your cases are getting stiff to extract from the chamber , its time to bump the shoulder (which can be achieved by proper adjustment of a FL sizing die, or a separage “body die”). When this happens, the RCBS mic is extremely handy in measuring the headspace at this point because you now know the limits of your cases based on your rifle’s chamber. On my gun, stiff extraction comes around +.001” headspace (+.001” over SAAMI spec). I use a body die to ‘bump the shoulder’ back to about -.001 or 2. (the body die only sizes the body, doesn’t touch the neck). At this point I may bump the shoulder of the case here on out after every reloading or every other reloading. I will have to use a separate neck sizer to size the neck after body sizing.

One other thing, the case overall lengths will tend to grow as you reload for same cases, but when your headspace is tight, they will grow less, hence, less needs to be trimmed. You will need to trim your cases from time to time so be aware of this.

Hopefully this helped..
Good luck.
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  #16  
Old 03-03-2010, 06:37 PM
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Re: RCBS Precision Mic

Tom,

You've got several separate issues and/or questions going on here, and they're independant of one another.

1) Yes, we're talking about bumping the shoulder back, but the FL dies will also size the body diameter down a bit as well; a neck die touches only the neck.

2) Yes, sort of, on the neck. This really depends on the dies itself and the expander ball. A standard FL die is going to size both neck and body, and give the finshed I.D. to the neck via the expander ball. This is probably how you're setting your neck tension now, correct? In the case of bushing dies, you have far more control over this, and can set the dimensions to whatever you need them to be. Much more versatile, and gives you the option of using no expander at all (if you so choose), and still get the correct neck tension. Like I said, a much better option there, IMHO.

3) Inconsistent neck tension? Refer to #2.

4) Yes, the only disadvantege to neck sizing only is that at some point you won't be able to chamber that ammo in your rifle. Personally, I consider ammo that won't chamber in my rifle to be a pretty major inconvenience. ANd don't forget what I said about Murphy. I can almost personally guarantee you that you'll never have a problem with just neck-sizing, until it's really important that your rifle work . . . and then it won't. Murphy's a real weenie that way.

5) Again, the brass is expanding in all directions. At some point, it will just be difficult to downright impossible to chamber the round, or possibly to extract a round that you did manage to chamber. The gages will allow you to measure some of these changes and set the dies properly to correct this, but it's a potential problem that you really don't need to experience in the first place. Why go there? The "advantages" of neck sizing are very much over-hyped, and ignore the very real disadvantages inherent to this mode of resizing.

Hope this clears it up a bit, gotta bail for now.

Kevin Thomas
Lapua USA
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  #17  
Old 03-03-2010, 06:40 PM
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Join Date: Feb 2010
Posts: 9
Re: RCBS Precision Mic

I use a full length sizing die to bump back the shoulder. I start with a fire formed brass, insert into the press and then run the ram up. Then I screw in the sizing die till it sits on top of the fire formed brass and I can't screw the die in any more. Then I gradually screw the die down a little at the time, measure the case after each time until I get the desired size case.
I currently don't hunt any dangerous game, but for reliability and consistency for each shot, I push my first brass back 0.002 from whatever it was fire formed and then make the rest this same length. My rem 700 doesn't seem to mind just neck sized brass, but my encore will close but not cock unless I push the brass back a few hundred thousands.

Matt
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  #18  
Old 03-04-2010, 06:02 AM
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Re: RCBS Precision Mic

A F/L die turned down fully will push the shoulder back, so that rounds will feed easily. But, care must be taken not to move the shoulder too much, creating excessive headspace. You gotta size a case slightly, try it in the rifle. If too tight, turn die down about 1/8th turn, try again and repeat this until the case feeds with very slight resistance into your rifle.

The Precision Mic is fine for checking shoulder bump, but I agree the fake cartridge to check OAL is a waste. The Hornay guage and others are much better for that purpose.
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  #19  
Old 03-04-2010, 08:07 AM
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Re: RCBS Precision Mic

I think it's a good idea to mention how a new rimless bottleneck case fits the chamber when it's loaded and fired. This should help folks understand what happens to the case dimensionally.

When the round's chambered, it is pressed as far forward as it can go by an in-line ejector in the bolt face such as Remington and the newer Winchester 70's have. The case shoulder centers in the chamber shoulder; this aligns the case neck and the bullet it holds to the rifling. It doesn't matter how much clearance there is between the chamber neck and case neck. If the case neck's not centered on the case shoulder, it won't be centered in the bore. Ejectors external to the bolt such as the Mauser or pre-'64 Winchester 70's don't put any force on the loaded round, so it just sits someplace at its front end. But the case head is a short distance from the bolt face.

At the back end, the spring-loaded extractor pushes the case against the chamber typically at the pressure ring (about 2/10ths inch in front of the case head). Depending on the difference in case and chamber diameters at this point, the back end of the case will be some small amount off center in the chamber. The case does not rest at the bottom of the chamber as is so often stated in media print.

When fired, the firing pin strikes the primer driving the case hard into the chamber shoulder. If the front of the case wasn't well centered in the chamber before the firing pin struck, it certainly is now. That force sets the case sholder back a thousandth or two (sometimes more) and the case head is now a bit further away from the bolt face. At this time the primer's detonation starts burning the powder. As pressure builds up the bullet gets pushed out and the thinnest part of the case body just behind the shoulder expands against the chamber. As pressure increases, more of the case body presses against the chamber and the back end of the case stretches back. This case expansion shortens the case neck. At peak pressure, the case head has stopped against the bolt face and the outside of the case is pressed hard against the chamber walls. When pressure drops and the bullet exits, the case shrinks back a thousandth or so from the chamber. It's now a thousandth or two shorter in case length than when new.

In resizing this fired case to reload it, most folks want the loaded round to fit the chamber good enough to shoot accurately. As long as the sized case ends up with its neck well centered on its shoulder, that will center the bullet well with the bore. But getting the neck centered on the shoulder seems to be the issue.

When the case is neck only sized, there's nothing to keep it centered on the case shoulder. If a fired case is neck only sized a few times, its diameter at the shoulder will soon start to interfere with the chamber. This happens as neither case or shoudler's perfectly round and at some point, depending on how the case is indexed in the chamber, interference will happen. The case has to be sized to prevent this interference. If the case headspace is greater than the chambers, the bolt will bind when its closed on such a case and it won't be at the same place for each shot; a known cause of accuracy problems. Expecially when the bolt face isn't squared with the chamber axis. Accuracy is the reduction of all varialbles to zero or as close to zero as possible.

A recent poll on one of the benchrest shooting forums showed most of them are moving away from neck only sizing and full length sizing for best accuracy. The don't reduce case diameters nor set case shoulders back much, but they do get consistantly great accuracy that way.

Full length sizing that reduces fired case diameters and sets its shoulder back no more than 2 thousandths guarangtees there'll be no interference. The case is free to center perfectly up front and the back end will be a tiny bit off center. How much this makes the bullet crooked to the chamber is less than 1 thousandth of bullet runout would cause. But it doesn't matter as every case is aligned the same amount and direction for each shot; that's consistant. Redding and RCBS make full length sizing dies that use bushings that can be had about 2 thousandths smaller than loaded round neck diameter. These dies don't use expander balls; those darned things that bend case necks and make them off center on the shoulder. One should decap then clean cases before lubing and sizing them in these dies. Sierra Bullets uses the Redding ones for all cases they're available for when tesing their bullets for accuracy. I doubt anyone shoots those bullets as accurate as they do. And they're fired in standard SAAMI dimensioned chambers, too.

RCBS Precision Mic's and Hornady LNL case gage (formerly Stony Point) are great. There were a few custom make gages made by a few folks years ago. I got my first one back in the 1960's. After lapping the neck in a few RCBS .308 Win. full length sizing dies for different neck wall thicknesses, I could get 30 or more reloads on cases using maximum powder charges.
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  #20  
Old 03-04-2010, 08:48 AM
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Join Date: Jul 2009
Posts: 219
Re: RCBS Precision Mic

Kevin,

When I asked if the only advantage of bumping the shoulders back over neck sizing was to reduce the likelihood that a round wouldn't chamber, I was only asking for clarification. At lot of this is new to me. I wasn't trying to be short.

Bart B,

I read your post about three times. Thanks for the detail.


To all that have responded to this post I now see why it's important to bump the shoulders back. I shoot a lot and only neck size and I only on very rare occasion run into a round that is difficult to chamber. Never have run into one that is difficult to extract. But I now understand that I've been playing with fire.

So the next step for me at this point is to figure out what I need to do to switch over to bumping as opposed to neck sizing.

I reload primarily with Lee Reloading Dies. I buy the deluxe sets. (I know many of you will say they are junk, but with a Ruger MKII in 300WM I can shoot 3/4" groups regularly. I'm shooting sub-moa with a new Winchester M70 in .30-06)

Can I use my Lee FL die to bump the shoulder back as MMOSER described in his post?

Bart B referenced dies that use bushings instead of a ball to size the neck. I believe he says he cleans the brass then sizes them. Do you have to clean the brass with a bushing type die? I don't own a tumbler so that would be an expensive change over to have to buy a tumbler, media and new set of dies.

Can someone explain how a bushing die works? When I hear bushing, I'm thinking they are removeable and so maybe there are many different sizes that one could use. Would that happen to be true?

Do you have to lube the cases to use a bushing die? I hate the lube. Especially when you have to lube inside the case necks.

I got the RCBS Mic in the mail yesterday. The oal guage, which is the primary reason I bought it, sucks. You can't get the screw tight enough so that when you attempt to extract the tool form the chamber it will come out in one piece.

I'm going to see about converting over to bumping instead of neck sizing so I'm going to keep the mic.

I'll be creating another post to address a problem that I've been having with my Hornady OAL guage. I do have one of those.

Thanks again to all. Really appreciate the long posts.

Tom
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  #21  
Old 03-04-2010, 09:31 AM
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Location: Sedalia, MO
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Re: RCBS Precision Mic

Morning Tom,

My aplogies if it sounded like that, but I was just trying to get out the door, and didn't mean it to sound like I thought you were being short.

As far as the bushing dies go, we could describe them for you, but instead, I'd say just pop on over to the Sinclair international or Redding web sites, and there's some pics of these dies. And as we know, that pics worth a thousand words. You'll see what they are immediately, but it really amounts to nothing more than a standard die, with interchangable bushings that drop into the top of the die (and are secured by the decapping rod) which allow the user to select the exact size O.D. they wish to use in the neck area. These bushings are available in .001" increments, allowing total control over this part of the resizing process. Very cool.

Case cleaning is always a good idea, for a variety of reasons. Longevity of the dies/bushings is just one of them. There's other ways of cleaning cases than the tumblers, they just take a bit more effort and energy. The tumblers are nice, but not a neccesity. You'll make the decision to buy one whenever it seems right to you, like most any item of reloading equipement.

Lube is something you'll use with any bottlenecked case, so there's no way around that. Dillon used to (still may, for all I know) make carbide sizing dies for both 308 and 223s. Unlike carbide dies for pistol cartridges, you still had to use lube with these. The carbide was used in those dies only to provide longevity for commercial loaders. Very expenssive (a few hundred $ for a size die alone) and you still had to lube. No, no getting around this one. Some lubes are easier to use than others, and they each have advantages/disadvantages in comparison with each other. Play with some different types, and see if you can find something more to your liking. Personally, I prefer Imperial (now Redding) sizing die wax fro heavy-duty resizing and case forming. I tend to use the spray lubes for my bulk reloading since it saves time and keeps me from having to handle 1000 or so cases individually for this process. I even lube pistol cases which will be going through carbide dies in my RL1050, simply because it makes the press cycle sooooo much easier. We're only talking the faintest trace of lube on those cases, so little you can barely feel it. But you wouldn't believe the difference it makes in how easily the press cycles.

Glad to hear you have the RCBS Mic in hand now. As I said, you start using it and a lot of this stuff will become clear to you very quickly. You're off to a good start here!

Kevin Thomas
Lapua USA
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