45 acp brass sizing

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bjm42

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After resizing & loading 45 ACP, my measurement @ mouth (light taper) is .471(manual states .473). Measurement just forward of head is .473 (manual states .476). Brass is mixed-stamp range pick-up. Is there a problem, or am I just being OCD?
Foster CO-Ax press; RCBS carbide die. Appreciate advice.
 
I don't know that I have ever measured my 45ACP handloaded ammo at the case mouth. I'm not that OCD :D. If your ammo is "plunking" (dropping freely) into your firearm's chamber without resistance; and your bullets are being held firm, you should be good to go. I would caution over-thinking this.

45ACP was the first non-revolver chambering I ever loaded (late 1984). There was no interweb and I'm not so sure I read the chapter on taper crimping (I likely didn't). I didn't even know there was a difference between taper and roll crimping; yet somehow, I managed to swerve into doing it right. Point is, getting the crimp right on 45ACP must be easy because I'm not that lucky. ;)
 
bmj42,

The way SAAMI gives dimensions is with unilateral tolerances, so the number you see published is actually at one end of the range of values considered acceptable. For 45 Auto the number is 0.473" with a tolerance of -0.006", so the actual range is 0.467" to 0.473". The average is 0.470". So, if you aim at 0.470" you are in the middle of the range and that gives you the most room to err in either direction due to bullet tolerances or different cases having different neck wall thicknesses, etc.
 
Is there a problem, or am I just being OCD?
Click to expand...

Neither, really. There is no problem, just a misunderstanding about the spec. The .473" and all the other specs on the usual drawings are maximum limits, and your slightly smaller ammo is within the lower limit specs, which one usually doesn't see in the manuals.

When you see only one number given as the dimension, assume it is the maximum.
 
Some brass is thicker than others and that could be leading to your concern. It's been awhile but I seem to recall that Remington brass was on the thin side..

Tony
 
When I was new to loading, I measured a half-dozen factory rounds, commercial and G.I., and the average was .473", so I started there.
Ultimately, I settled on .470", as every round plunked and fed in every gun.
If you have a single .45, you could tailor OAL and crimp for that gun.
 
44 AMP said:
When you see only one number given as the dimension, assume it is the maximum.
Click to expand...

I need to add that this is true for cartridges. The opposite is true for chambers, where the smallest number is given.

The origin of the above is what is called a critical dimension. This is generally a number at one end of the acceptable range of a dimension for which going outside it (greater or smaller, depending which sex a part has) will prevent the assembly of a mechanism. So, if a cartridge (male) is too big for a chamber that meets dimensional standards, it won't allow you to close the gun (the final assembling of the working mechanism), and so the critical cartridge dimensions are maximums. If a chamber (female) is too small, a cartridge that meets dimensional standards won't fit inside, again preventing closing the gun for operation, so the critical chamber dimension is a minimum.

The only clue the drawings give you that you are dealing with a critical dimension is the tolerance is unilatereal (in one direction only) instead of the familiar plus or minus. The convention with a critical dimension is to provide the critical limit and provide the tolerance only in the other direction.
 
Unclenick said:
The only clue the drawings give you that you are dealing with a critical dimension is the tolerance is unilatereal (in one direction only) instead of the familiar plus or minus. The convention with a critical dimension is to provide the critical limit and provide the tolerance only in the other direction.
Click to expand...
The Ordnance Department blueprint drawings for the M1911A1 are a good example. Most of the dimensions have a tolerance and, in most cases, it's a unilateral tolerance.

I have a running argument with a friend who persists in citing M1911A1 mid-range dimensions as if they are the optimum. I maintain the stated dimensions are the optimum, and the mid-range dimensions just represent a part (or a gun, if everything is made to mid-range dimensions -- which is what he does with his sketches) that's pretty much okay and not in danger of wearing out of spec the first time you shoot it.

http://www.sightm1911.com/blueprint/m1911a1_blue.pdf
 
Yep. If you want a nice tight gun, you want everything on those critical values. The only exception is keeping your cartridges small enough to feed reliably.
 
In the 50 years I've been loading, I've never mixed cases because of the significant difference in dimensions. There is enough variance in one brand that has kept me away from mixing.
 
GeauxTide said:
In the 50 years I've been loading, I've never mixed cases because of the significant difference in dimensions. There is enough variance in one brand that has kept me away from mixing.
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But what are you reloading for?

If I were shooting benchrest competition or NRA Bullseye, I'd be picky about mixed brass. For IDPA blaster fodder that's going to be mostly lost to me after the match ... I'm not picky. In fact, I save my Winchester brass for ammo I care about, and I use everything else for the IDPA blaster fodder.
 
You are OCD. Sizing dies are made to ensure even cases with the thinnest case walls are sized down enough. Expander dies are designed up increase the case ID, over the range of bullet seating, to be 0.001-0.002" SMALLER than the actual bullet diameter.
 
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