So you spent 6 hours in front of the casting pot. Your shoulder is a bit sore, your back is a little stiff, and you look down at your days work only to see a bunch of wrinkly bullets mixed in with perfect ones. You, being the perfectionist, want to throw away all the bullets with the slightest defect, but the pains of your labor scream out and say ,”It’s just a small wrinkle, is it really that bad?” Thus the internal struggle between practicality and perfection begins.
How you set your quality standards is up to you. It should be balanced between what is required, I.E are these pistol rounds being shot at 10 yards, or will they be paper patched bullets for use in Black Powder Target Rifle Competitions? Determining the end goal will go a long ways to set your own personal standards.
When I cast bullets, my primary purpose is to produce a bullet that is inexpensive and can be used for general range fodder. Thus my quality standard allows for some wrinkles in my bullets. As I am not producing these bullets at a commercial scale or for the market, I’m not out to please anyone but myself.
The Visual Inspection
You can do a visual inspection at any point during the sorting. If you pick up a bullet, take a moment to do a quick once over. I generally look for these defects;
- Wrinkles that disturb the base
- Large voids, or wrinkles that would not pass weight inspection.
- Flashing along the mold seam
I like to lay out all my bullets in a box and give them a once over. I pick out pieces of lead and obvious defects before dumping the lot into a bucket for a weight inspection.
Sometimes I come across bullets that otherwise look ok but haven’t filled out the mold fully. This is evident by edges which have a generous radius as shown in the photo below. I’ll keep the bullets and if they make weight, then they are good enough, if they don’t they get scrapped.
Bullets that look otherwise normal but have some wrinkles are not always defective. They might not be Instagram perfect, but also may not fly any differently then the picture perfect bullets. If only defect is some wrinkling, but the base is intact, and there is no other defect then I allow these bullets to go on to weight sorting.
If I was planning to gas check these bullets, then some of the minor base defects I would probably allow. Since the gas check covers up the base of the bullet it eliminates the risk of upset as the bullet passes the muzzle, and the possibility of gas cutting that could lead to leading in the bore.
The Weight Inspection
Ever since I began casting, over ten years ago, I did some sort of weight inspection. My very first standard was this a general tolerance of +/- .5gr for a rifle bullet and 1.0gr for a pistol bullet. The mean weight of the sample was taken from 10 bullets. Unknown to me at the time this standard was more stringent then some jacket bullets standards. As my knowledge of ballistics and Quality Control has grown, I have done things differently.
A wrinkle in a bullet is an area where lead could occupy but doesn’t. Lead is a pretty dense element, so it doesn’t take much of a wrinkle to result in a measurable difference in weight. If the wrinkle is large enough for the weight to dip below spec, then it’s scrap. That provides a firm objective standard, as oppose to a subjective “That wrinkle doesn’t look too bad” standard. Wrinkles are voids, and are only one type of void, as there can be internal voids or voids that are created at the base of the bullet when the sprue is cut.
I am more concerned about voids I.E Wrinkles, in a bullet then I am about an out of round bullet. An out of round can be fixed in the sizing die. Since all of these bullets will be sized we can make an additional allowance for bullets that might be a bit heaver then the mean. Which is why I set the upper spec to +3 grains. Voids can cause a bullet to be out of balance, and can cause accuracy issue. Voids don’t go away and can’t be fixed, a bullet with voids will always come under weight, we set the lower limit to -1 grains.
For setting the tolerance we use the mean measured not what was intended by the mold. Changes in alloy can affect the mean weight of the lot, so we do not want to rely on the mold. In addition there could be errors on the mold, for example the NOE Mold was listed as a 155gr mold but dropped bullets at 170gr.
Thus my newer weight standard is -1gr and +3gr from the mean of 25 bullets for both pistol and rifle bullets. The weight difference isn’t enough to cause substantial velocity or pressure differences, so this standard is more about how well the bullet was formed in the mold. Since I am a visual person, I do this in excel and I apply some basic QC Tools to visualize how my lot of case bullets did.
This is time consuming, I do recommend you find a comfortable place to set up a scale and bins for scrap/good product. For reference it took me about an hour to sort through the 230gr bullets.
Quality Analysis
I weighed the bullets before sorting to generate a data set of 100 points to do an analysis on, and then weighed another 100 bullets post sorting to see the difference. I do not usually do this, I do the presort weigh in, but not the post sort weigh in and analysis. If I was to start making these bullets commercially and wanted to measure and cut down on scrap, I would be doing this types of analysis as part of regular QC checks. As well as working on process improvement programs to cut down on scrap product.
Looking at the Individual Moving Range Chart (IMR) we can see that our casting process is capable of producing bullets between 235.6 to 224.6gr. Which is an 11gr spread. This is most definitely outside of what we want from a quality control stand point. If we look at the reference lines of where our -1/+3 grain tolerance is, we most definitely will have some scrap to deal with.
Remember the Upper Control Limit (UCL) and the Lower Control Limit (LCL) are statistical calculations that are three times the standard deviation above and below the mean. It is desirable to have a process that has spec limits that fall outside of the UCL and LCL limits as is a process that has a very low probability of producing a defective product. As evident by these graphs, my casting proficiency is not quite up to Six Sigma snuff.
Our CPK value for the 230gr mold is 0.18, with a predicted scrap value of ~34%. The only way to verify this is to go ahead and weigh all of our bullets and see what we come up with.
Fortunately I did not have to count these buy hand, I had a scale that had a weighing function. The total number of 230gr bullets cast and made it to the weight QC was 632. We scrapped 140 or 22% of the total. The QC model predicted 214 would be scrapped. So was the model wrong? No necessarily, we scrapped some bullets before we made it to the weigh in. I actually didn’t keep a record of how many of those were, 20 or 30 a minimum, and some of those would have passed the weigh in as they had small defects on the base, others would have failed by large margins.
This sort of analysis is done in manufacturing all of the time. Looking at what the process is capable of, introducing QC or a new process and see how it changed the outcome. In our case we know that the casting process produces variability. Without any sort of QC the bullet weights would be all over the place, with QC we can sort the bullets so they fall within our spec limits, even though the control limits allow for bullets to fall outside of our spec limits due to normal process variation.
If you were a receiving inspector doing inspection on bullets coming in and you saw these results. Where the spec limits were inside of the control limits and the capability histogram ended at the lower and upper spec limit, this is indicative of QC that is being done on the supplier side. Their manufacturing process is not tight enough to produce a product that always fits within their spec limits, so they have a busy QC department that scraps a substantial amount of their product.
The NOE Mold produced bullets that were overall more uniform, despite having some issues. This is indicated by the CPK value of 0.31 presort. Also the Individuals chart looks more uniform then the presort Individuals Chart did for the 230gr bullet. I am stopping short of saying that the Lee Mold is an inferior mold, the bullets cast in the Lee Mold are long, while these pistol bullets in the NOE mold are short and fat. It is likely that the longer Lee mold wanted to be run hotter to get a better fill out then the NOE mold.
The Process Capability index predicts about a 17% scrap rate for the NOE bullets. We did our sorting and found that we weighed 1519 bullets. The number of bullets we scrapped was 228 which gives us a scrap rate of ~15%. A little bit lower then what the model predicted, but close enough that I am happy with the results.
The risk of having a supplier whose process in not “In Control” meaning the specs limits are outside of the upper and lower control limits (UCL and LCL) is that you might get some bad product from them. This product slipped through the QC process and shipped out to you. Unless you inspect every piece of product you may never find it. In my case, I accidentally put at least one over weight bullet into my good bullet bin. These mistakes happen in the real world too.
This is why companies often want manufactures who have a processing CPK of 1.33 or higher. They want to ensure that the probability of getting defective parts from that manufacturer is very low. That lowers the risk to the business, defective product going out the door is a liability to the company as it can affect everything from opinions of the company to the safety of their product.
Wrapping it Up
When you start making you own stuff, you get to be the judge of what is acceptable quality. We live in a world where consumers expect perfection, no matter the price point. Wrinkled and pitted bullets that might otherwise shoot fine, are scrapped because visually they don’t meet customer expectations. I have boxes of bullets that were sold to employees as factory seconds, with nothing wrong other then a little blemish.
You should have some sort of standard in place when doing your own reloading and manufacturing, and then have some way to see how are you doing. This inevitably leads to improvement, and then to a better quality product. Many of us strive for perfect in ammunition that doesn’t require it. This leads to wasted time, and frustration. Set some spec limits, check and see how well you are doing and then make some changes to see if it improves.
Hopefully this write up shows some of what I do when I inspect my cast bullets, and how apply some fairly basic tools to check the quality of my work. We used MiniTab 17 to do the analysis but this can be done in Excel as well.
Now that we have a good batch of bullets, let try out this Epoxy Stuff everyone talks about!