Are bullets with sprue bumps showing on gas checks accurate

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Last Post 24 September 2008

CBA Member

billwnr posted this 22 June 2008

I've noticed that some bullets with sprue bumps have the bump showing on the bottom of the gas check after seating and sizing.

That left me wondering how does the accuracy of ones showing sprue bumps compare to the same type of bullet with no sprue bump showing.

Do they shoot into the same group as ones with no sprue bumps or do they open up the group? I would think they would open the group up some but haven't done any shooting to prove/disprove this.

I know bullets with sprue bumps can have the bumps flattened out if one is using a taper die or a bump die. Even the gas check flattens out when the bullet is run thru a bump die.

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Standard Member

CB posted this 22 June 2008

I have often pondered the same question and I did a test to see if this where the fact.

Ultimately I found it is a matter of confidence, given that I was taught that the base of the bullet is the most important part of the bullet, I prefer to have mine as close to perfect as possible.

I have shot them with the bump from the sprue in the base, without the bump, tapered, bumped and just about everything inbetween without a real noticeable increase or decrease in accuracy. If the bullets are all the same, then they will go into the same group.

I did discover during this what causes the bump from the sprue. It is the manner that the sprue is broken with. Many have some sort of mallet and give it a quick sharp smack to break the sprue.

When you do this and it results in a bump in the base of the bullet, it is because you are hitting the sprue on a downwards angle resulting in the sprue bouncing upwards from it hitting the top of the mold instead of being broken parallel to the top surface of the mold. This will also cause the top surface of the mold to gall, especially in the aluminum molds such as LBT or Lee. This can also bend the sprue and result in bases that are cockeyed.

I break all of my sprues by hand pressure where I can. This is mostly 2 cavity molds that are made of Aluminum, but I do the same with some of my RCBS, Lyman and SAECO molds.

This is the reason that Lee has the sprue handle on their molds and is also the reason why Veral designed his sprue plates with the angle on the sprue plate.

Now I have a minute if any bump on my bullets.

In addition I will say that I have altered the ejection pin in my lubersizer die. The stock die ejection pin has a dish in the top of the pin, I had mine ground perfectly flat after I noticed that the bottom of the gas checks had a convex instead of flat base.

Now I cant say to what degree all of this will affect accuracy, but it does wonders for my confidence knowing I have done all I can to make the bullet base as close to perfectly flat as I can.

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CBA Member

billwnr posted this 22 June 2008

Jeff Bowles wrote:

I've noticed this too. The RCBS sizer dies have a flat topped pin and the Lyman ones have the dipped pins.

I'm not sure of the purpose of the Lyman pins but it probably has to do with excess lube.

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CBA Member

R. Dupraz posted this 22 June 2008

The Saeco ejector pin is cupped on the top and flat on the bottom. According to the designer, the cup is to help prevent lube buildup under the bullet. To aid in flattening bullet bases when sizing with the Saeco, I just flipped the pin end for end.

Also, one can eliminate the little sprue stub by adjusting the sprue plate zactly right after the mould is hot and working.

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Standard Member

hunterspistol posted this 26 July 2008

:coffee This is a subject that Wayne Blackwell did the initial tests on, using 44 magnum handguns,38 Special, and 30-30 WCF. The difference in accuracy was effected by drilling holes or cutting notches in the cast bullets.

The basic difference- a perfect base with a 1/4” hole drilled off center will throw the group from 1.75” to 3-7" The thought was to introduce a defect into an otherwise perfect bullet. With the nose drilled, or the side of the nose drilled, gives you the 3” group. Drilling the base gives you the 7” group-this is a drastic example.

The original experiments included offcenter sprues to see how much it effected accuracy: from a weighed, inspected lot with an initial group of 1&9/16"-the offcenter sprues showed a group of 2&3/8", when the bases were drilled, the group spread out to 6&15/16"!

Sorry it took a while to get to the answer but, I'm looking it up and reading while I'm typing. True enough, the group with the base defects is the largest. These were done at a distance of 25 yards for the handgun loads and 100 yards for the 30-30 WCF.

The Art of Bullet Casting-from Handloader & Rifle magazines 1966-1981

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CBA Member

billwnr posted this 26 July 2008

bullets with drilled bases are different from bullets with sprue bumps.

Drilled bases are the equivalent of shooting bullets with voids.

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Standard Member

jhalcott posted this 26 July 2008

I have never worried about the little sprue bumps on 44 caliber HANDGUN bullets as I normally do not shoot them very far. 50 yards would be a long way for me with the 44 and cast in a revolver..

bil, maybe YOU could test this out for all of us. Shoot a few 5 shot groups with “BUMPED ” bullets and a some with “bumpless” bullets. Comparing those results and posting them here would possibly answer the question.:}

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Standard Member

CB posted this 27 July 2008

Back in the 1980s I used to worry about this so I deliberately started positioned the sprue plate to one edge of the bullet. Then I had off center sprues but very little or no bumps.

I won a lot of postal matches against people whom I assume had centered sprues so I thought this extra work was worthwhile.

However, when I wasn't carefull I often cast bullets with sprue bumps centered and non centered that went into a reject box.

I hate to let things go to waste so I shot a lot of five shot groups alternating between groups of bullets with nice smooth bases and groups of bullets with awful bumps in the gas checks. Guess what. Just like with Jeff's tests, there was no difference in accuracy.

As far as I can tell the worst thing about base bumps is that they make the bullets wobble around annoyingly on the bench.

John

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Standard Member

cityboy posted this 27 July 2008

R. Dupraz wrote: RDwhat is yor definition of “zactly right"?Jim.

Also, one can eliminate the little sprue stub by adjusting the sprue plate zactly right after the mould is hot and working.

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Standard Member

cityboy posted this 27 July 2008

Jeff Bowles wrote: I break all of my sprues by hand pressure where I can. This is mostly 2 cavity molds that are made of Aluminum, but I do the same with some of my RCBS, Lyman and SAECO molds.

JefHow do you manage to break the sprue when the metal has frozed? I have tried this a few times and it was a no go. I guess you must be a heck of a lot stronger than I am. I can do this on the single cavity molds if I follow Lee's instructions. I have never tried Lee's method on 2 cavity iron molds.Jim

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Standard Member

CB posted this 28 July 2008

I think what Rich is saying is to adjust the sprue plate tension after the mold is hot.

Breaking the sprue by hand... Well that is something not everyone can do, but if you get a good pair of heavy leather gloves to help with the heat and pressure in the palm of your hand and if you pop the sprue right after the lead solidifies you can do it by hand on a 2 cavity mold. Also the direction the spue piviots must be away from the handles. You grasp the mold in your hand sort of like if you hold your hand flat and place the mold in your palm, wrapping your fingers on the side and your thumb/palm on the opposite side and squeeze, you can do it.

Needless to say if you let it cool to long or for larger than 2 cavity molds it will be a tough if not impossible task.

When you hit the sprue with a mallet, most times you will hit it with a downward strike and this will cause the sprue to bounce up leaving the bump. If you hit it with an upward strike the same will happen. This also is what causes some of the galling underneath the sprue. I have heard of some guys putting an extension or modifying the sprue to make it easier.

Another trick is to modify your sizer die (if you use a Lyman or RCBS sizer) so the ejector pin is flat on the end of the pin and not concave. Lymans are almost always concave. Depending on the hardness of your alloy, this can help minimize the bump.

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CBA Member

R. Dupraz posted this 28 July 2008

cityboy:

What I meant by zactly right was , adjusting the sprue plate when the mould is up to temp so that there is no perceptible up and down play between it and the mould while still allowing the sprue plate to swing closed with just the slightest resistance by it's own weight. I have found that by doing this and then using the flat end of the push pin in my Saeco sizer to size and seat the GC's has pretty much eliminated the uneven bases. However the bottom of the sprue plate and top of the blocks must be true and flat to begin with.

Regards

GUSA #4

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CBA Board

Wally Enga posted this 28 July 2008

Try putting a good sprue plate lube on the top of the mold and on the sprue plate --- you will be able to cut it easily with your hand as soon as the sprue solidifies and still not get lead smears --- and a smooth flat base.

The best I have seen is Bull Plate Sprue Plate Lube.

http://bullshop.gunloads.com/prices.html>http://bullshop.gunloads.com/prices.html

Use VERY sparingly and with the cavities filled. If you get even a hint of it into the cavities, you are all done casting until you scub the mold cavities out again.

Wally

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CBA Member

R. Dupraz posted this 28 July 2008

Another lube that works really well, is cheap and readily available anywhere is ''Lock-Ease", a graphited lock fluid. It has a high temp carrier so it can be put on a hot mould and after this fluid evaporates, a film is left that lasts. Used very very sparingly it works great for the handle hinges, alignment pins and under the sprue plate to prevent leading. For about three bucks a can you get a life time supply. But just as Wally said, if it gets into a cavity, ya get to start all over.

GUSA #4

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Standard Member

cityboy posted this 29 July 2008

I use NEI mold prep to lube the top of the mould blocks and the alignment pins. I think it is graphite suspended in isopropyl alcohol. It should be easy to make up a batch of home mix.

City boy

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Standard Member

deltaenterprizes posted this 20 September 2008

There must be something to the sprue effecting accuracy because Hoch and others make nose pour molds for extreme accuracy. The base is the steering wheel of the bullet that is why jacketed match bullets are hollow points.

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Standard Member

CB posted this 20 September 2008

deltaenterprizes wrote: There must be something to the sprue effecting accuracy because Hoch and others make nose pour molds for extreme accuracy. The base is the steering wheel of the bullet that is why jacketed match bullets are hollow points. But are nose pour molds the only one's winning?

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CBA Member

billwnr posted this 20 September 2008

jhalcott wrote: I have never worried about the little sprue bumps on 44 caliber HANDGUN bullets as I normally do not shoot them very far. 50 yards would be a long way for me with the 44 and cast in a revolver..

I'm not interested in proving it is important to those who believe it isn't. I tend to let those believe in the things they want to.

I do know that with bumped bullets and flat bases I have shot 5 shot groups of .303 and .313. (I have also shot larger but choose not to make that too public:shock:)

I haven't shot groups that small with bullets that have sprue bumps...but there are a lot of other variables in play that can't be taken out of the testing equation.

If I could find a way of bumping a bullet but not flattening the base I could get my answer. I can shoot bumped bullets and test against unbumped bullets but that won't let me rule out the bumping variable.

I think the way to test the variable of perfect bases vs. variable sprue bumps is to convert a mould to nose pour and then shoot it against an bottom pour mould. That would be close enough to ruling out other variables.

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Standard Member

cityboy posted this 20 September 2008

I have come up with a way to get rid of the base bump, and limited testing suggest that it may work. I use a small Vee block and a flat file to get rid of the bump. The Vee -block keeps the base square. I wrote this up and sent it to Glenn Latham, and he said he planned to use it in CBA journal.

Cityboy

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CBA Member

billwnr posted this 20 September 2008

This is an interesting way of dealing with bumps. Sounds like it would work real good.

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Standard Member

bfrshooter posted this 21 September 2008

Well, I either get a bump or a little tear out depending on the mold or alloy. Been that way for 53 years too! :fire I have never determined that either done any harm to accuracy but I don't shoot benchrest, just revolvers. I get little wobblers on the bench when the bump shows through the gas check and my boolits are too hard to flatten them in a size die.

I don't normally size anyway, either using an oversize Lee or just a tube to remove excess lube.

The only thing I have ever been concerned with are the edges of the base but then filled out drive bands are important too. I never weigh boolits. My first boolit out of the mold will be perfect and I usually empty the pot without a reject.

Some guys get real anal over a boolit but I quit worrying about the small stuff. To put it in perspective though, I am an absolute nut about accuracy from my revolvers and over 1” groups at 50 yd's will bother me and I get too many 1” groups at 100 yd's. I recently shot 3 shots at 200 yd's with my .44 SBH to measure drop and they were in 1-5/16". Yes, bumpy bases too! :coffee

I know some don't believe me so here is a picture of 3 groups shot at 50 yd's with my BFR 45-70, a cast boolit from my home made mold and the left group from a boolit sent for me to test, it is only 3 shots. This was done to prove boolit jump to the forcing cone has no effect on accuracy and it is an old wives tail that jump must be short with the boolit right at the end of the cylinder. That only limits the boolits that will fit the gun.

Are bullets with sprue bumps showing on gas checks accurate

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