Casting weight variations

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  • Last Post 08 July 2014
fa38 posted this 03 July 2014

Since it was raining today I decided to cast some .25 cal bullets for the Summer ASSRA match at Beeson's range near Etna Green Indiana which is shot Thursday thru Sunday the weekend after the 4th of July.

The mould is a Doc Brewer two cavity with a different shape bullet in each cavity. I threw 4 or five bullets back into the pot as I was casting and wound up with 259 from the heavier cavity and 256 from the light cavity.

Light cavity                          Heavy cavity 105.5- 4                                108.5- 20 105.6- 7                                108.6- 17 105.7- 12                              108.7- 65 105.8- 101                            108.8-71 105.9- 126                            109.9- 35 106.0- 7                                109.0- 34                                          

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joeb33050 posted this 03 July 2014

My records of just under 30,000 bullets cast and weighed shows that the mean standard deviation of sets of bullets is .155 grain. Your Light cavity standard deviation is .140 grain, the Heavy cavity standard deviation is .084 grain. Looking through my records, I find standard deviations below .1 grain happened 31 times in 245 lots of bullets, about once in eight times. I suspect that the Heavy cavity variation is a fluke, an outlier if you will, and that the next time you cast the variation from both cavities will be similar. If not, if the pattern persists, maybe there's something to be learned from the mold or casting procedure.

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fa38 posted this 03 July 2014

The blocks are recut RCBS blocks.  I usually cast with two moulds, letting one cool while I open and then fill the other mould.  About halfway thru my casting session I stopped casting with the other mould.

I then cooled the sprue of the Doc Brewer mould by laying sprue plate on a wet rag for a few seconds.  I do this because I do not like it when cutting the sprue off you leave a hole pulled out of the base of the bullet and I don't want to wait until it cools normally.  Cooling the sprue gives you a nice clean base and speeds up the casting. 

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onondaga posted this 03 July 2014

http://www.castbulletassoc.org/view_user.php?id=562>fa38

Casting with 2 molds denies a possibility that may be increasing the weight span of your castings. This affects the instant when you can cut sprew in your thermal cycle.

I changed one of my casting habits and caused a very significant improvement in the weight accuracy department. It is just a simple thing I copied from a member here that posted about it.

Wearing a glove and using a timing that allows pushing the sprue cutter gate down and opening by hand prevents sliding up or hopping up of the cutter plate during cutting function. The amount of sliding up or hopping up varies by striking force, vector and alloy hardness. Those variations disappear with a timing change for cutting sprue  that allows gloved hand cutter opening while pushing the cutter gate down in contact with the mold top. Timing is critical and it is a new routine, but it works. Monitor hardening crystallization of the sprew  puddle and as soon as it spreads, push the plate down and slide it open with a gloved hand. This works cleanly and well with practice on the new routine.

This habit change reduced the weight variance of my bullets very significantly for me as it reduced nub height of the sprue at the cutoff point to zero and I get no heavy bullets from the cutter gate sliding up or hopping up from cutting hard alloy and the necessary higher force needed to tap the cutter plate to cut harder more cooled alloy.

Gary

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jhalcott posted this 03 July 2014

Your talking about a HALF grain variation in bullet weight here. DOES that variation make that much difference in GROUP size or terminal performance when hunting? I used to be VERY anal about bullet weight till a good friend teased me about it. I just HAD to prove him wrong! It didn't work out that way. Statistically, groups of weight sorted against non sorted were equal. Sub 2 MOA groups with both convinced me not to worry about weighing bullets. BUT, IF YOU think it helps you, it probably does!

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fa38 posted this 03 July 2014

onondaga thanks for your comments

I have been using a gloved hand for all 1,2 and 3 cavity moulds to do just what you have suggested for about 15 years.  All the bases on my bullets are level so any weight variation is not due to base bumps. 

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fa38 posted this 03 July 2014

jhalcott I mark the bullets that are real light, some of them 3 or 4 grains light, on the nose with a red marker.  I shoot mainly offhand matches and have shot the red nose bullets to compare my score using them versus the good bullets and have had very few shots that have been way off call. 

I also shoot the bad bullets to condition the barrel for the first shots of the day and after its been setting for the 10 or 15 minutes between relays.

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John Alexander posted this 03 July 2014

jhalcott wrote: Your talking about a HALF grain variation in bullet weight here. DOES that variation make that much difference in GROUP size or terminal performance when hunting? I used to be VERY anal about bullet weight till a good friend teased me about it. I just HAD to prove him wrong! It didn't work out that way. Statistically, groups of weight sorted against non sorted were equal. Sub 2 MOA groups with both convinced me not to worry about weighing bullets. BUT, IF YOU think it helps you, it probably does! Very good question.  But one we apparently don't want to ask and certainly don't want to test for.  It just seems more virtuous to sort bullets by weight.  I have never seen a well designed study that says it is worthwhile.

I have gone one step further than your test.  After not being able to see an accuracy difference between weight sorted bullets and unsorted.  I also shot comparison group pairs -- one group with bullets all the same weight compared of a group of the both LIGHT and HEAVY “culls".  Still no accuracy difference at the 1MOA accuracy level.

I also agree with you that if sorting bullets into those little boxes improves confidence then keep at it.  It may also help to keep some cast bullet shooters occupied and out of the bars in the afternoon.

John

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onondaga posted this 03 July 2014

http://www.castbulletassoc.org/view_user.php?id=562>fa38

There is a variety of other factors that effect bullet weight consistency. Here are some factors  that I have considered and worked on, but I am sure there are more:

Pot temp variation caused by dropping cut sprues into pot. A thermometer will verify this effect.

Thermal cyclic rate of pot can be modified and narrowed with a PID temperature controller.

Mold temperature is effected by casting cadence, I don't stop to check bullet quality.

Casting cadence maintained by any cooling of the mold, for example wet sponge contact, does cause wide thermal variation in mold temperature very quickly with little control. Adjusting wait time with mold closed after cutting sprue causes a much slower and less abrupt mold temperature change.

Mold temperature changes have a much greater effect on mold fill out and thus bullet weight change than pot temperature effect. Casting cadence constancy that controls mold temperature should be monitored to avoid disturbances.

A mold guide on the pot can be used to control the length of the flow and avoid eyeballing flow length that effects flow temperature and mold fill out.

After adding metal and fluxing I watch the readout of my PID till stable before resuming casting.

Actually use a clock with a second hand to monitor cadence.

These methods and controls generally produce +- 1/2 of 1% weight variation or less for me. Some things can really throw me off. An example of that is when I am casting rifle bullets and then change to another pre-warmed mold like a 1 oz rifled slug or a 12 ga round ball mold or some other mold that definitely requires a different wait time after cutting sprue until opening the mold. I have some notes about that and pot temperatures  on the boxes my molds are in, but Murphey's Law won't be denied.

Gary

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joeb33050 posted this 05 July 2014

fa38 wrote: Since it was raining today I decided to cast some .25 cal bullets for the Summer ASSRA match at Beeson's range near Etna Green Indiana which is shot Thursday thru Sunday the weekend after the 4th of July.

The mould is a Doc Brewer two cavity with a different shape bullet in each cavity. I threw 4 or five bullets back into the pot as I was casting and wound up with 259 from the heavier cavity and 256 from the light cavity.

Light cavity                          Heavy cavity 105.5- 4                                108.5- 20 105.6- 7                                108.6- 17 105.7- 12                              108.7- 65 105.8- 101                            108.8-71 105.9- 126                            109.9- 35 106.0- 7                                109.0- 34                                          

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badgeredd posted this 06 July 2014

If I am understanding correctly, you can identify bullets as coming from one cavity or the other; is that correct?

If it is, I suspect different comments may be following. Also, if I am understanding correctly, note that the light cavity is producing bullets of less than 1/2% variation. The heavy cavity is producing bullets in the same range but slightly heavier. I'd guess you are missing something in your casting cadence or procedures.

What I am noticing is the 3% approximately variation between the two cavities. I could be that one cavity is cut differently than the other, which seems unlikely, but is definitely possible, given my recent experience with a mold from a commercial, well known mold maker.

Are the bullets details the same as in fully filled out and of consistent appearance? No rounded edges anywhere? No apparent voids? Consistent surface appearance?

Edd

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OU812 posted this 06 July 2014

I have always separated bullets by cavity. A two cavity mould makes seperating easier.

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fa38 posted this 06 July 2014

Two different bullets out of the recut RCBS blocks by Doc Brewer. The smaller bullet has been run thru a sizer and lubed.  The base band was very lightly touched by the sizer.  Not a real good photo.

http://s1297.photobucket.com/user/MartinStenback/media/IMG_0005_zpsd2c25778.jpg.html> />

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Coydog posted this 06 July 2014

The only time that I worry about weight is when I will use the rounds for long shooting because of I did a test and fund that one weight shoots better then another.Also what I do is when I do go with the weight I put them in groups of like if you have 180 is the weight then you have some that are 180.6 or 179.8 I would put them in the 180 weight but I weight them after I put GC and lube on them not before because after the bullet is ready to load that is what you have to work with. So I go with +-.5 to the whole number like you go with 180 +-.5 and then you have the weight that will work for the load that you tested with you will need to change the load if it is a smaller weight. Also bigger weight also.I found out when I did some testen in one of my rifles to make my own data.But for short yards I do not weight each bullet.

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fa38 posted this 08 July 2014

   I cast another batch of bullets out of my .25 cal. Doc Brewer mould with somewhat similar results with a slightly different alloy. The heavier bullet had more at the heavy end of the weights.

    I threw 5 or 6 back into the pot as I was casting. I used only one mould and cooled the sprue occasionally with a wet rag when there was evidence of metal tearing out by not letting the sprue cool enough. I did not throw any back for this reason

    I don't know what the allow consists of, just that it gives a heavier bullet. In my single shots if it breech seats so hard that it is about to wreck my seating tool I remelt the batch of lead and throw in some soft stuff until it seats easily.

Light cavity        Heavy cavity

105.6- 1            108.4- 3 105.7- 0            108.5- 6 105.8- 1            108.6- 3 105.9- 4             108.7- 41 106.0- 16           108.8- 62 106.1- 25           108.9- 40 106.2- 144         109.0- 81 106.3- 147         109.1- 84 106.4- 9             109.2- 28

totals 347                     348

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joeb33050 posted this 08 July 2014

The table showing number cast, avg weight and standard deviation is attached-Computergod does not allow us to post tables here. 
First, if you can cast 500-700 bullets in a DC mold in one sitting, you're way ahead of me. My best shot is maybe-MAYBE, 300 or so total from a dc mold. After which I need to walk around and drink to loosen up. Second, I don't think the difference between any two values, avg or stdev, is significant. Third, on the last session the heavy bullet distribution is bimodal, got two humps. Something happened to make the two humps. Again, not significant. But interesting.  That's all I can extract from the data, but It would be interesting to see the results from a session where you turned the mold around 180 degrees, where the top bullet became the bottom bullet or front became rear-however you say it. 

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