Measurements

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  • Last Post 24 July 2016
Wineman posted this 20 July 2016

I have always considered myself capable of understanding/fixing/repairing most things mechanical. A talented machinist or gunsmith I am not. That said it is never too late to learn and I have become enamored with the world of “tenths". I can read dial and digital caliper and a mic with a vernier to get those tenths. I would love to hear advice or methods from the many more talented folks here.

Lets start out with something small and not too complicated: 30-06 case neck tension. I have a carefully cast, lubed and LEE push through sized bullet at 0.314” (it's a long story why 0.314), cast of Lyman #2 recipe. I want to have a neck tension of 0.002” so I manage by LEE Collet and M-Die to have a case with a supposed ID of 0.312". My question is how to check this on a case by case basis (pun intended)? I have 0.312 + and - Pin Gauges; a Starrett Hole gauge; a Dial Indicator and Mandrel to get wall thickness and then use case neck OD; bullet seating feel; and finally, just wing it.

I assume a 0.312” Pin Gauge should not go into a 0.312 hole, if it does it is larger than 0.312". I'm not sure I understand the + or - aspect (these are ZZ gauges). The Hole Gauge seems to drag at first and then become loose so my assumption is that the entire neck is not sizing the same. The Wall thickness as measured by a dial indicator shows 0.015” +/- 0.00075” (swing of 0.0015")

Trying to learn, so any comments are welcome.

Thanks to the group.

Dave

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frnkeore posted this 20 July 2016

Pin gages come in two tolerances, -.0002 is the most common but, you may also buy them in +.0002. You box should be marked as to where they are + or -. Be aware that the longer you hold that pin or the case neck in your hand, the larger that pin and neck will become. For close measurements, the temp has to be controlled closely. 20C (68 deg) is the standard.

Your collet die, should make the ID the same even with uneven case thinkness. Keep in mind that dirt and lubrication, take up space, also.

Your uneven .0015 case necks, need to be outside turned. Your pilot needs to fit with a little push, to ensure they are even after turning.

For even tension, you also need to aneal all the case necks, so that they have close to the same number of firings.

Frank

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Ken Campbell Iowa posted this 20 July 2016

for a home machinist the best plan is to work with designs that don't require tolerances less than about 0.0003 inch.

down about 0.0002 , everything becomes pretty nit-picky .... you have to specify part temperature and need to check your mics against standards ( at spec. temps ) often .

if you are really into small distances stop by a swiss screw machine or grinding shop ... these guys work in tenths of tenths ( 0.00001 ) ... it is fascinating the methods they use to keep those tolerances .

i am still impressed with taking a box of ... nosler .... bullets and finding no detectable difference in base diameters using a hand mic ...

ken

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Wineman posted this 21 July 2016

It is a facilitating craft. How the Manhattan Project came to fruition in four years, is just mind boggling. The tolerances, science, physics, metallurgy, chemistry, and hubris to take a theory and make it work was amazing. In my Golden years I really want to get good at splitting 10'ths. How some of you are able to turn metal into art is fantastic! Just to get back on point, if you set a cast for 0.002” under a cast bullet OD, how do you ensure that each case has that tolerance?

Dave

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billwnr posted this 21 July 2016

You didn't say what brand cases you were using. Some aren't as uniform as others. Lapua seems to be the best for consistent neck thickness and overall case weight uniformity.

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onondaga posted this 22 July 2016

 more important than your case dimension numbers is checking a dummy load using a  kinetic bullet puller to get a bullet out and then measure the diameter of the bullet in the area that it was in the case. The pulled bullet should measure .312 in your application. If it is smaller your case sizing is suspect and your bullets are being swaged down in diameter.  If you need a .312 bullet to fit the throat of your rifle anything smaller will shoot less accurately

Gary

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Tom Acheson posted this 22 July 2016

Dave,

I heard someone explain the neck tension thing this way.

You need to have uniform case neck walls (Lapua gives you the best shot at this but turning/thinning the necks is even better). Add the diameter of the bullet to 2X the neck wall thickness. This should be your calculated loaded round diameter. Pretend you want 0.003” of neck tension. Subtract the 0.003” from the loaded round diameter. That would be the size of the neck sizing bushing you use in your neck sizing die.

When the bullet is seated into the neck sized case it has to overcome the resistance of the drag on the bullet as it enters the case. 

But watch out for two things:   Deformation of the bullet as Gary notes above.    Maintaining adequate clearance around the perimeter of the loaded round's neck once it is in the chamber. Some people say that 0.0005” is the bare minimum. Has to be room for the brass to expand while releasing the bullet during ignition. Most common is 0.001” to 0.0015". This second thing is a safety detail.

I'm not sure this helps.

Tom

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billwnr posted this 22 July 2016

I think the military rifles had generous chamber and neck dimensions. Neck turning should not be any benefit at all, or marginal at best if other than Lapua cases are used.

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John Alexander posted this 23 July 2016 

             This thread and its discussion of precision measurement has been interesting.  The kind of stuff I enjoy reading and I always learn something.

   However, if I understand correctly, the issue expressed by Wineman in the original post to this thread was about getting precise measurements so uniform tension of the bullet from “case to case” could be achieved.      It seems to me that the discussion would be incomplete unless the usefulness of getting uniform case to case neck tension (Wineman's basic concern) is questioned. In other words why do it? There is excellent experimental evidence as well as solid theoretical support that worrying about the UNIFORMITY of neck tension from one cartridge to the next has NO effect on either the uniformity of muzzle velocity or the accuracy of the load so is a total waste of time.    Some questions about too much, or too little, neck tension for ALL the loads in a lot of ammunition, such as the issue mentioned by Gary, are important. But, achieving small case to case variation in tension is of no value. Anyone doubting this should read the results of experiments done by Gerry Bottiger and reported in Fouling Shot #223 and my article in Fouling Shot #240 which looks at some of the reasons why variations in case to case neck tension are unlikely to have any effect on either velocity or accuracy. 

John             

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Wineman posted this 23 July 2016

John,

Thank you for the information. The original question was how to measure to a gnat's rear end but also unasked was “does it really matter” to a gnat probably, to the rear sight on a M1903A3 where each notch probably has 10 gnats worth of play not so much.

Just to fill in the blanks: Fired case OD 0.343-344". Bullet OD 0.314", pulled bullet OD 0.314", loaded round OD 0.341", case wall thickness 0.014-0.015". If I assume an average wall thickness of 0.0145"I get 0.341-0.029 = 0.312” for the ID. The M-Die sized case will accept a 0.312+ and a 0.312- pin gauge without any effort so I would guess that the average ID has to be larger than 0.312". From my reading a hole will not accept a pin exactly the same diameter (interference fit?) so the ID is about where I wanted it to be. The load I was using did not perform very well, I am at a loss to understand why as past loads shot very well using the same components. Just trying to not make that mistake again and see if I can determine why things went awry.

Thanks again.

Dave

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John Alexander posted this 23 July 2016 

             Wineman wrote:

"The load I was using did not perform very well, I am at a loss to understand why as past loads shot very well using the same components."

Ah, the curse of the cast bullet shooter.  Maybe when we better understand what is happening to the bullet when the primer pops and what role variation in bore condition plays and a couple of other things we will be able to avoid this type of event -- most of the time.  To do that will take a lot more careful and well designed experiments than most cast bullet shooter seem interested in doing. These types of mysteries can't be solved if we keep on believing that doing things more carefully and weighing and cleaning everything in sight ever more precisely is all that is needed. John

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Wineman posted this 23 July 2016

Kind of like the episode from I believe “The Outer Limits” TV show, where the criminal is killed and ends up in “Heaven” after all the easy women, inability to lose gambling, the free meals, glad handing etc., he finds his angel and wants to go to the other place. Informed that he is in the other place the show ends. The other is the selfish Smallbore shooter who goes to “heaven” and finds no wind, a club with all the Anschutz rifles he could ask for, whiskey, cigars, no shadows on the targets, 70 F, shade on the shooter and all the copper plated 22 LR HP you can shoot, they get it by the truck load...

Dave

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JSH posted this 24 July 2016

Is the gun worthy of all the case prep? I turned a lot of brass for guns that really did not respond to it. I guess what I am saying is if you match prep brass it should be a match chamber. Neck tension has a bearing on results. But neck thickness can be looked at like a bushing in effect. I neck turned some 223 brass for a factory rifle and groups got worse, same for an 06. YMMV Jeff AddedWhen the 300 whisper came out in a factory TC barrel I had to have one. 221 brass was terrible. 223 required a lot of work. 222 was just right and my best lot of brass. 

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Premod70 posted this 24 July 2016

Neck tension works on jacketed bullets but the same tension will undersized lead so why mess with it. I expand my necks to the size of the bullet and let the chips fall where they may.

Dale Flinchum

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gpidaho posted this 24 July 2016

First, let me say that I'm speaking only of my single shot Handi's, T/C Encore's and bolt guns for obvious safety reasons. My recent tinkering's have left me in agreement with Premod 70, using an expander plug that matches the bullet size and relying on the small spring back of the brass to hold the bullet works best for me. This has been made easier to accomplish for me by the size bushings and expander plugs available at NOE Bullet Moulds. Al N. knows what one thousands of an inch looks like. (At least his micrometer agrees with mine) and I recommend his business for quality products and service. (Not a paid spokesman, lol) Gp

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