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Posted: 6/20/2010 5:15:05 PM EDT
Prvi Partizan M193 2009
PPU’s M193 clone uses a lead core, copper jacketed 55 grain bullet with a cannelure. The base of the bullet has a deeply “rolled-over edge” design which creates a large hollow in the base of the bullet. Using a Mitutoyo blade micrometer, I measured the diameter of the bearing surface of the bullet. Several measurements gave an average diameter of 0.22415”. 
The cartridge case is constructed of brass and displays the annealing iris typically found on military ammunition. The case mouth and bullet have a small amount of asphalt sealant applied. The case mouth is crimped into the bullet cannelure using a collet crimp. The cartridge is charged will “ball” powder and the primers are crimped and sealed. The head stamp reads: “PPU 09 5.56 x 45”. The cartridges come packaged in little “sugar cube” boxes holding twenty rounds apiece. 
The U.S. mil-spec accuracy requirement for M193 calls for 10-shot groups of the ammunition to be fired from a machine rested, bolt-actioned test barrel such as those pictured below. 
The mil-spec testing is conducted from a distance of 200 yards and the dispersion of the shot-groups is measured using the mean radius. The accuracy spec requires that the average of the mean radii of all the 10-shot groups is not to exceed 2 inches. A mean radius of two inches at 200 yards equates to an extreme spread of approximately 6 inches or approximately 3 minutes of angle. I performed an accuracy evaluation of Prvi Partizan’s M193 load (lot # 0905-05) following my usual protocol. Shooting was conducted from a concrete bench from a distance of 100 yards (confirmed with a laser range finder.) The test vehicle was a 16” Colt HBAR with chrome-lining, a NATO chamber and a 1:9” twist. This is the barrel found on the Colt 6721 Tactical Carbine. The barrel was free-floated with a LaRue Tactical handguard. 
The fore-end of the carbine rested in a Sinclair Windage Benchrest and the butt-stock rode in a Protektor rear-bag. Sighting was accomplished via a Leupold VARI-X III set at 25X magnification and adjusted to be parallax-free at 100 yards. A mirage shade was attached to the objective-bell of the scope. Naturally, the wind conditions were monitored using a Wind Probe. The set-up was very similar to that pictured below. 
The Wind Probe. 
No animals were harmed during the testing of this ammunition! A 10-shot control group was fired using match-grade hand-loads topped with Sierra 55 grain BlitzKings. The extreme spread of that group measured 0.89”. 
Four 10-shot groups of the Prvi Partizan M193 load were fired in a row. The extreme spreads of those groups measured: 2.73” 5.45” 3.07” 3.74” for a 10-shot group average extreme spread of 3.75”. The four 10-shot groups were over-layed on each other using RSI Shooting Lab to form a 40-shot composite group. The mean radius of the composite group was 1.08”. For comparison, the 40-shot composite group of PPU M193 is shown below next to a 30-shot composite group of IMI M855 fired from the same barrel. The 40-shot composite group. 
Comparison to IMI M855 
The smallest 10-shot group of PPU M193. 
1:7" Twist Barrel After performing the above evaluation using the 1:9” twist barrel, I repeated the exact same evaluation of the Prvi M193 using a 20” Colt HBAR with a 1:7” twist. The four 10-shot groups obtained from the 1:7” twist barrel were also over-layed on each other to form a 40-shot composite group. The mean radius for that composite group was 1.01”; nearly identical to the results obtained from the 1:9” twist barrel. Both composite groups are shown below for comparison. 
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Molon,
Excellent choice of two popular mil spec rounds. Your range reports are second to none, not to mention the results of the last couple reports indicate my latest substantial donation to the IDF was a good choice. I believe all of us who frequent this forum look forward to your reports. Thanks for spoiling us, no matter what the results. 
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Here's a little refresher on the concpet of mean radius.
MEAN RADIUS Mean radius is the method of measurement used by the US Military for accuracy/precision testing of ammunition. It provides a more useful analysis of the consistency of ammunition and rifle accuracy/precision. The typical method used to measure a group consists of measuring the distance between the centers of the two most outlying shots of a group. This would be the "extreme spread" of the group. We are essentially measuring the distance between the two worst shots of a group. Take a look at the two targets below. 
Most people would intuitively conclude that the second target shown is the "better" group. Measuring the two groups using the "extreme spread" method, we find that both groups measure 2.1". Once again with the typical method of measuring groups we are measuring the distance between the two worst shots of the group. This method tells us nothing about the other eight shots in the group. So how can we quantitatively show that the second group is better than the first? (Yes, we could score the groups using "X-ring" count, but this does not give us any differential information about all those shots in the X-ring.) This is were the mean radius method comes in. It will give us that extra information we need to better analyze our groups, rifles and ammuntion. If I just reported the measurements of the two groups above using the extreme spread meathod, without a picture, you would assume that the two groups were very much the same. Using the mean radius method shows that the second group is much more consistent. It has a mean radius of 0.43" compared to 0.78" for the first group. Mean radius as defined in Hatcher's Notebook "is the average distance of all the shots from the center of the group. It is usually about one third the group diameter" (extreme spread). "To obtain the mean radius of a shot group, measure the heights of all shots above an arbitrarily chosen horizontal line. Average these measurements. The result is the height of the center of the group above the chosen line. Then in the same way get the horizontal distance of the center from some vertical line, such as for instance, the left edge of the target. These two measurements will locate the group center. "Now measure the distance of each shot from this center. The average of these measures is the mean radius." Once you get the hang of measuring groups using the mean radius it becomes very simple to do. While being very simple to do, it is also very time consuming. Modern software programs such as RSI Shooting Lab make determining the mean radius a snap. The picture below is a screen snapshot from RSI Shooting Lab using the group from the above target. The red cross is the center of the group (a little high and right of the aiming point). The long red line shows the two shots forming the extreme spread or group size. The yellow line from the red cross to one of the shots is a radius. Measure all the radii and take the average to obtain the mean radius. 
Mean Radius Demonstration Let’s say you fired a 5-shot group from 100 yards and the resulting target looks like this. (The X-ring measures 1.5” and the 10-ring measures 3.5”.) 
The extreme spread of the group measures 2.83”, but we want to find the mean radius (or average group radius.) In order to find the mean radius we must first find the center of the group. By “eye-balling” the target most people would see that the group is centered to the left of the "X-ring" and probably a little high, but we need to find the exact location of the center of the group. Locating the Center of the Group The first step in finding the center of the group is to find the lowest shot of the group and draw a horizontal line through the center of that shot. 
Next, find the left-most shot of the group and draw a vertical line through the center of that shot. 
Now measure the distance from the horizontal line to the other four shots of the group that are above that line. Add those numbers together and divide by the total number of shots in the group (5). 
2.50” + 1.03” + 2.01” + 1.30” = 6.84” Divide by 5 to get 1.37”. This number is the elevation component of the center of the group. Next we need to find the windage component of the center of the group. From the vertical line, measure the distance to the other four shots of the group that are to the right of the line. Add those numbers together and again divide by the total number of shots in the group (5). 
1.76” + 2.54” + 0.45” + 1.19” = 5.94” Divide by 5 to get 1.19” This is the windage component of the center of the group. Finding the windage and elevation components of the center of the group is the most difficult part of this process. Once that is done the rest of the process is a piece of cake. Using the windage and elevation components, locate the position on the target that is 1.37” (elevation component) above the horizontal line and 1.19” (windage component) to the right of the vertical line. This location is the center of the group! 
Determining the Mean Radius Now that we have located the position of the center of the group, the first step in determining the mean radius is to measure the distance from the center of the group to the center of one of the shots. This line is a single radius. 
Now measure the distance from the center of the group to the center of each of the rest of the shots in the group. Add the measurements of all the radii together and then divide by the total number of shots in the group (5). 
0.85” + 1.35” + 1.38” + 0.84” + 1.61” = 6.03” Divide by 5 to get 1.21”. This is the mean radius (or average group radius) of the group! Using the mean radius measurement to scribe a circle around the center of the group gives you a graphic representation of the mean radius. This shows the average accuracy of all the shots in the group. This demonstrates why the mean radius is much more useful than the extreme spread in evaluating the accuracy of our rifles and ammunition. 
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Wow, that's brings back a lot of fragmented memories of college statistics and math courses.
Kind of makes my brain freeze up a bit. The illustrations and your step-by-step explanations are excellent as usual. As a visual learner, I greatly appreciate the above post ! Thanks! |
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Quoted:
average of the mean radii of all the 10-shot groups is not to exceed 2 inches
Ok so this means that since the test shows the mean radii of 1.08", this is acceptable mil spec ammo & gtg? The U.S. mil-spec of a 2” mean radius for 10-shot groups is from a bolt-actioned test barrel in a machine rest fired from 200 yards. The mean radius of 1.08” that I obtained was from a semi-automatic AR-15 at a distance of 100 yards. An email from Prvi Partizan sent to UVvis stated that they obtained of a mean radius of 1.1” from 110 yards with another lot of their M193. |
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I've been lurking 'round this forum for ages just so I could begin to know what I don't know. I am, at the core, a visual learner but the logic of the process must be clear to me before I am able to accept the information as valid. Molon ALWAYS presents things in a manner that is understandable to me and also satisfies my AR (Anal Retentiveness).  Thank you, Molon, for another exceptional lesson! This one is getting permanently "Subscribed". |
I was able to eake out an ok group(for me) today using Prvi M193. Lot 0905-01. I recently built a new upper using a 20in SS Lothar Walther 1:8 twist, Wylde chamber barrel. Today was the 2nd time I shot it. I feel I could have done better with a better rest and more magnification on my scope. I had it set to the Max at 12 power. Distance was 100 yards. Someone else better than me I'm sure could have gotten a smaller group. Pretty good for Prvi M193 if you ask me. I also got similar groups using 69 and 75 gr Prvi Match as well as Q3131A1.
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Quoted:
I was able to eake out an ok group(for me) today using Prvi M193. Lot 0905-01. I recently built a new upper using a 20in SS Lothar Walther 1:8 twist, Wylde chamber barrel. Today was the 2nd time I shot it. I feel I could have done better with a better rest and more magnification on my scope. I had it set to the Max at 12 power. Distance was 100 yards. Someone else better than me I'm sure could have gotten a smaller group. Pretty good for Prvi M193 if you ask me. I also got similar groups using 69 and 75 gr Prvi Match as well as Q3131A1. http://i87.photobucket.com/albums/k124/FelixN/GandA/OT_PrviM193.jpg Dude, that's an excellent group for that ammunition! What did the group before it and the group after it look like? |
| That was the only group I shot using the Prvi M193. I was just feeding it with different ammo and doing 10 round strings using a single shot follower. Wanted to see how this barrel ate the different ammo I had. I think the Prvi M193 did have the smallest group however. |
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Quoted:
That was the only group I shot using the Prvi M193. I was just feeding it with different ammo and doing 10 round strings using a single shot follower. Wanted to see how this barrel ate the different ammo I had. I think the Prvi M193 did have the smallest group however. I occasionally get groups from "mil-spec" ammo like the one you posted, but when you average in the 10-shot group before it and the 10-shot group after it, your right back up to the much larger average extreme spread. 
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