223 Case Weight vs H2O Capacity & Velocity

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Harrier
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223 Case Weight vs H2O Capacity & Velocity

#1

Post by Harrier »

warning to phonites... another long post, get ready to squint...

I don't know about you, but I typically pick up brass I find laying around at the various shooting sites I visit. Last month I found 125 A-USA once fired cases just laying in a wash. This results in accumulating a bunch of mixed headstamps. I typically sort cases by head-stamp and use a lot of the odd-ball stuff for practice or to identify a specific load. The common stuff like LC, RP, WCC, FC etc is used for bulk loading of a particular load.

In a previous post (this one) I mentioned trying to find a standard M193 practice reload. Over the course of numerous range sessions, I noticed velocity ranges out of position for the load- some over the top pressure wise while other of the same charge exhibited normal pressure. Since this load will be used with a variety of headstamps, I speculated that it might be case capacity differences and I might need 2 or 3 loads depending on brass weight (which presumably equates to internal volume).
How much actual volume change in a 223 case is enough to affect pressure & velocity?

While looking in my database of previous case measurements I noticed some discrepancies where a heavy weight case had more water capacity than a normal case. Eventually the only difference I could see between them was that the heavy-hi-vol case was neck sized rather than FL sized. I wondered if the difference in neck vs full length sizing accounted for the changes in pressure & velocity I was seeing in my range loads, so i devised a test. I love test cuz they require going to the range and... shooting.
Hopefully It will indicate how case weight and sizing type affects velocity for a given powder charge and if the load needs to be altered depending on case weight.

Cases were selected by head-stamp previously determined to be in a Light, Normal and Heavy category, and not weight matched , so variations would be as expected bulk loading by brand. For the light cases, I used HB (Singapore milsurp) which averaged 93gr , for the normal category I used (5) R-P and (5) Speer, at an avg weight of 96.2gr and the heavy cases were Magtech CBC head-stamp at a hefty 108.4gr avg.

They were all previously processed, FL sized w/neck bushing .242" & trimmed to 1.750" (+-.001) so any OAL shrinkage after firing is due to body expansion.
I loaded 5 each of selected head-stamps wit 2 charge weights, leaving all other aspects of the loads the same.
All 30 rounds were shot from the same milsurp 20" AR barrel so they would be from the same chamber.
(click to zoom)
Chrono test.jpg

I measured each fired case OAL, dry empty weight and total H2O weight (level with mouth). Individual measurements indicated the .2 charge spread didn't make any difference, so all 10 cases of a set were processed together. Fired primers were left intact throughout. For each set, the same cases were used in the same order and all measurements were repeated to indicate any differences sizing and trimming operations might have. After one set of measurements, I emptied the watered cases and let them sit a few days in a hot 100F+ garage to evaporate residual moisture, so I could weigh them again for the next test.
A drop of soap in the water helps reduce surface tension clinging and air bubbles.

Processing classifications are as follows:
o- (FAU) As fired, unsized and untrimmed- just as they came out of the gun were measured first.
o- (FNT) Neck Sized with a Lee neck size collet die and trimmed to 1.47" .
o- (FST) FL sized with RCBS Neck bushing die .242" and re-trimmed to uniform length as the last set.

After processing, I noticed the weight of all cases in the FNT set was uniformly .1 higher than the FAU batch. I reasoned that since the body wasn't sized and the necks were lengthened only .001" including some brass removed chamfering the mouth, they shouldn't weigh more than the FAU batch. There could still have been some residual moisture inside, likely the primer area... I let the next FST batch sit a few more days until they looked dry inside. For this evaluation I decided to just compared FAU and FST. Their deltas are far enough apart that .1gr here or there won't make much difference in the conclusion.

Subtracting empty case weight from water filled weight allows you to determine Total Case Capacity (TCC).
This is a Volume to Weight relationship since 1CC of water volume = 15.432 grains weight. TCC is a Gross capacity to the case mouth. However since we have to stuff a bullet in there we have to remove some powder and calculate the Net Useful Case Capacity (UCC).

This will vary with the bullet selected. A short flat base bullet seated .23" will take up less space than a long boat tail seated .45" into the case so when calculating your cOAL, you will have to consider the bullet seat depth too... don't forget the boat-tail (which requires additional calculation). Also, since bullets are given in inch diameters and take up volume, a conversion from Cubic Inches to CC will be required. For my example I am using a .224" bullet and .26" seating depth to arrive at my UCC. I have it set up in excel so once I measure the basic info, I can calculate Ucc for any given bullet and seating depth.

My Excel formula is: UCC= ((AE90-(((3.14*((0.224/2)^2))*AF90)))*16.387)*15.432
(Note, this case's data is on row 90, a different one is on row 89), the UCC formula is located in column AG on row 90.

cell AE90 =case volume in Cubic Inches: converting grains weight to cc then cc to cubic inches.
1 cc = 15.432 gr
1 ci = 16.387 cc
(Tcc/15.432)/16.387= v.ci

cell AF90= Bullet seating depth (you can adjust this based on bullet OAL and the cartridge OAL and enter it directly) used is .26
Here is a table of the test results,

Case Weight.jpg

Summary of findings- From reviewing the various data I came to the following conclusions:
o- 3/4 of the light and normal case loads are very close in volume and velocity despite processing changes, and I think they could used interchangeably without issues.

o- The heavy cases are substantially different, not only from the lighter case sets but also from each other depending on processing. They have a 1.14 gr spread in UCC volume between unsized and full-length/trimmed condition, which can account for variance in velocity. I think they need load reduction in order to match the target velocity and avoid excess pressures.

o- chrono data shows the lighter charge to be essentially the same as the .2 gr heavier charge, so I will use the lower change and won't waste powder.
o- With this charge weight, only the heavy cases make milspec velocities over 3200fps from this barrel... oh well....


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Re: 223 Case Weight vs H2O Capacity & Velocity

#2

Post by shooter444 »

Man-O-Man Harrier, you make my life long OCD affliction feel like child's play! :D

I thought that installing a torque wrench on a single stage press, to assure consistency of crimp pressure from case to case with a Lee Factory Crimp die was going over the top.

You make me look like a lazy piker!

thanks! :clap:
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Harrier
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Re: 223 Case Weight vs H2O Capacity & Velocity

#3

Post by Harrier »

I've found that sometimes little details can make a big difference. I like to know how certain things affect the end result so I do a fair amount of testing, some of it turns out to be meaningless but at least then i have an opinion about it.

OTOH, I probably don't do enough testing sometimes to prove something definitively, but I usually get a pretty good idea what is happening... but not always and that is what makes it fun for me. It also gives me a reason and goal to go back out to the range (not that I really need one).

I shot the above test with a set of loads (25.9 & 25.7gr) thinking I would find an overlapping load that worked in all case weights for both a 16" & 20" barrel. That didn't materialize so I decided to look at the above numbers in a different way, which led me to a entirely new conclusion...

In a few days, I will post on how this new evaluation went...
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Re: 223 Case Weight vs H2O Capacity & Velocity

#4

Post by Desert Rat »

I have been told I get into too much detail!! You did a great job there, :bow-blue: I am not sure which is scarier, the level of detail that you went into, or the fact that I was able to follow along and understand your process! :shock:
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