Article 2: How I Make Pound Casts

Ian

Well-Known Member
#1
Disclaimer: The following information is the product of my own meandering experience and includes a few original ideas and concepts of my own and some that others have been kind to share with me, and is always subject to change as I learn more. It is in no way intended to be the last word, or even the first, just something that may help. Feel free to share like or contradictory experiences and otherwise contribute to the knowledge fund.

The Basement Articles #2, Part one:

Following the statement "Fit is King" and considering first the static part of fit, we must learn exactly what is the dimensional goal we wish to achieve with our fixed ammunition. The most logical method I've found to do this is to make a sturdy impression of the rifle's chamber and throat from which detailed measurements can be taken.

While several common methods exist to cast impressions of chambers, I personally prefer the impact or "pound casting" method, and will limit my own discussion here to this type. (everyone else feel free to add descriptions of other methods that work for you) This method was invented, I believe, by some clever cast-bullet shooter friends of mine who were seeking an inexpensive and simple method of getting the rifle's critical internal measurements without the hassle associated with pouring molten Cerrosafe, sulphur, or other things directly into the chamber.

Essentially, a "pound cast" is made by installing a lead-filled cartridge case and malleable slug into the chamber and throat, locking the breech, and expanding the whole thing to fit the chamber and throat area completely via hydraulic force applied by a rod inserted through the muzzle. Here's a step-by-step general idea of how I do it, which is by no means intended to be comprehensive or absolute.

1. Clean the rifle's chamber, throat, breech face, breech locking mechanism, and bore with brushes and solvent, dry and put a thin film of oil the chamber and throat. Use a HEAVY oil, such as gear lube or case lube to prevent sticking and make it easier to extract the slug after forming. Apply grease or oil to the locking mechanism per normal lubrication practices.

2. Obtain a roll of vinyl electrician's tape, a brass or steel rod longer than the barrel but that will insert into the barrel when wrapped in one layer the tape, a medium hammer, a gas check of the caliber of the gun being casted, a sacrificial cartridge case that has been fired in the gun and still has the primer intact, a propane or other hand-held torch, large spoon, a heavy-for-caliber bullet mould or slug mould made with a bore-diameter hole drilled in a piece of hardwood, enough pure lead to cast several heavy (long) bullets or slugs, enough wheel weight or similar-hardness alloy to fill the cartridge case, and some pliers.

3. Melt the wheel-weight (or similar) metal in the spoon with the torch, heat the top half of the case enough to anneal it (dull glow ok, and pour the molten metal into the case while it's still hot. Fill to about halfway up the neck with the alloy and allow it to cool. Grasping the case with pliers and gently lifting and tapping the case head on a solid surface and re-applying heat to keep the alloy melted for a few more seconds will help dislodge air pockets and settle the alloy.

4. Using the same spoon, cast some heavy bullets using pure lead, or make a slug mould that will cast a cylinder of bore diameter that is about 1/2" longer than the longest "standard" bullet the gun is intended to fire. This is a general idea, you just want enough metal to fill what's left in the case neck, the whole throat, and the origin of the rifling.

5. Clean and oil the alloy-filled case, oil and install the dead-soft bullet or slug into the mouth by hand, and insert it into the chamber. Force the action closed behind it and place the gun muzzle up with the buttplate on a firm, non-marring surface (like a clean work mat or cardboard).

6. If a gas check is available, start it into the muzzle squarely with a punch, concave side up. Wrap the metal rod with a closely spiraled, single layer of electrician's tape to protect the bore, and build bore-diameter "bushings" for a reasonably snug, centering fit on the ends and middle of the rod if one layer of tape isn't sufficient to support the rod in the center of the bore. Use the rod to push the check all the way to contact the slug, making sure the tip of the rod is captured inside the cup of the check and that the check hasn't gotten turned sideways. Sometimes a fired .22 rimfire case or pistol cartridge case can be used instead of a gas check depending on the caliber of the rifle. The object is to keep the tip of the rod from sinking and sticking into the soft lead. The check provides a sort of piston and separation point.

7. Hold the rifle and support the rod at the muzzle securely with one hand, and firmly tap the end of the rod with the hammer to compress the alloy and annealed case within the chamber. You should be able to feel the rod sink as the metal swages to the form of the throat and chamber, and suddenly the blows should begin to feel very solid. Once it feels like the rod isn't going to go any deeper, invert the gun while holding the rod in place, place the tip of the rod on a solid surface (floor) and open the breech while leaning on the buttstock to apply pressure to the rod. Pushing on the rod while opening the breech will aid in extraction and help prevent broken extractor mechanisms.

8. After removal, inspect the pound cast for complete fillout. It doesn't have to be pretty, just filled enough to measure accurately in the critical places such as both ends of the neck, total chamber length, throat entrance, and the entire throat up through the ball seat and into the full land height.

9. Now you have something to measure and keep around that won't shrink, dry out, or change with time, and will have a much better idea of the fit parameters of the particular rifle.

Here's a photograph of a pound cast I made for a wildcat experiment, together with some ammunition built to closely fit the chamber and a bullet designed to fit the throat throat based upon pound cast measurements. Should be close enough to get the idea.

 
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Ian

Well-Known Member
#2
Part II:

Now that you have a chamber cast, what's next?

I'll try to outline some of the things I do in a very general way, but much of it is subjective and dependent on the individual gun.

The first order of business is to determine how large of a bullet the rifle will chamber and fire without scraping the bullet on the throat entrance. As I mentioned in the "Fit" article*, I've found the best accuracy usually occurs in rifles that have their bullets fitted very closely to the throat entrance dimension. This is not a fast rule for every rifle, but a general approach only. To obtain this measurement, simply measure the step in front of the end of the chamber, right against the base of the step, and PLEASE use a quality, name-brand 0-1" MICROMETER, not your Chinese calipers, ok?


Next, I like to measure both ends and the middle of the neck to determine taper, if any, and the essential chamber neck dimension. Again, as mentioned in the "Fit" thread, the loaded case neck-to-chamber-neck clearance needs to be minimal, on the order of half a thousandth or so, and certainly less than one thousandth, for maximum bullet support during firing. Excessive clearance here is almost chronic with production rifles, and is probably the single most accuracy-destroying clearance of all of them.

To establish how much loaded chamber neck clearance you will have, subtract two case-neck-thickness diameters and the throat entrance diameter from the average chamber neck diameter. For example, a .30-'06 chamber neck might measure .3435" in the middle, and brass is typically .0135" thick, or .027" total. The throat entrance is .3105". Subtract .027" and .3105 from .3435" and you will end up with .006". SIX thousandths total neck clearance is unfortunately common since it promotes safe chambering and firing of jacketed bullets and the jackets don't seem to mind being launched crooked or having a lot of gas blow around them before engraving, but cast bullets suffer mightily through this, ESPECIALLY at high velocity. I believe that this excessive chamber neck clearance is the principle reason why there seems to be such a low "accurate velocity" limit when shooting cast bullets in rifles, and why they are traditionally launched with low pressure charges of fast-burning powder. In the case prep thread I will explore methods of achieving smaller case neck clearance, and maybe in the #10 article I will discuss some techniques that sometimes improve accuracy when excessive neck clearances cannot be remedied.

The next point of interest is the throat shape. Matching the bullet's nose profile as closely as possible to the throat dimensions and shape will greatly increase accuracy in most instances vs. a bullet that simply has a bore-riding nose and parallel driving bands sized to the "common knowledge" dimension of .001" larger than barrel groove diameter. This doesn't mean that two-dimensional bullets fitted well to the bore and throat entrance can't shoot well out of a gun with a long taper to the throat, for indeed they do sometimes, but generally it's more difficult to achieve or maintain accuracy as velocity is increased, or get excellent accuracy in the first place, if the nose isn't shaped like and fully supported by the throat when chambered and during firing.

Notice that I didn't "slug" the entire bore and measure the groove dimension? That's because I base bullet size on throat entrance diameter, which is 99.5% of the time (H&R .38-55 being an exception) larger than groove and the chamber necks are almost always large enough to accept a cartridge loaded with a throat-entrance-diameter bullet.

Those are the basics of bullet fit based upon pound cast dimensions, I'll describe more about fitting the cartridge case itself to the chamber in the case prep article.**

*"Fit Is King" is a separate reference. http://www.artfulbullet.com/index.php?threads/article-1-fit-is-king.132/
**"Case Prep for Accuracy" is an additional article, link HERE: http://www.artfulbullet.com/index.php?threads/article-3-case-prep-for-accuracy.133/#post-1956
 
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JWFilips

Well-Known Member
#3
Ian Wrote: "In the case prep thread I will explore methods of achieving smaller case neck clearance, and maybe in the #10 article I will discuss some techniques that sometimes improve accuracy when excessive neck clearances cannot be remedied."

Ian,
I'm not sure when #10 articles is coming out but I'm sure I will be glued to it.
My Ruger .243 77V has that sloppy neck issue. I has shot some fine jacketed bullet groups in it's early days of the 1980. When I recently brought it out of mothball and started shooting it again with cast boolits I was actually surprised to see that it can shoot lead alloy so well especially since I'm just putting together my kit to load cast for it.
Granted most of my shooting has been at the 50 yard range.

I recently formed some 243 brass from 308 brass & I know you commented on it that (in my .243 brass thread) "I shouldn't turn the necks too much". Well you were right! In reality I should not have turned the necks at all ( for my chamber) I basicly measured my old 243 brass and brought the necks of the new reformed 308 brass to the same spec. Even when loaded with a fat .246" boolit and leaving the case neck flair in place I'm still easily .004" shy of chamber neck dimension. Those "thick necks" of the reformed .308 brass were my friend & I didn't realize it! Learned a hard lesson!
Jim
 

Ian

Well-Known Member
#4
Anything between half and two thousandths TOTAL loaded case neck-to-chamber-neck clearance, measured in the middle of the neck, is pretty much ideal for cast bullets. You really notice the difference at elevated velocities and particularly at longer ranges, meaning at least 100 yards and on out to 2-300.

I tried to keep the in-article references accurate, but my plans changed for later articles and I may not have corrected all of them. Article #10 addressed case fillers and buffers, and I can't remember if my case prep article included some of the methods for dealing with excessive chamber neck clearance. Forming cases from other calibers that make thicker necks is the easiest way, re-barreling with a tight-necked chamber is another, and if all else fails you can electroplate the outsides with pure copper (I've done this to excellent effect with .30-'06 cases but to say that doing so is a royal pain in the ass would be a magnificent understatement). Leaving a little unsized neck base and a little mouth flare can help in some instances, as can a single layer of Scotch tape on the outside of the necks.
 

JWFilips

Well-Known Member
#5
Ian,
You wrote: "Leaving a little unsized neck base and a little mouth flare can help in some instances"
Well I guess I must be channeling something here; but that is what I started doing! But The scotch tape thing is worth a try.

I would like to shoot out farther & that is where I'm seeing issues.

It has not been a
mathematical
.........1/2 inch at 50 yds = 1" at 100 yds....I'm starting to see the departure here and it is troubling me. (It also could be that my velocities are under 2000 fps)
 
#6
To determine the correct bullet diameter for a rifle, the groove diameter of the barrel is NOT the determinant.

INSTEAD you want to measure the throat, or the unrifled portion of the barrel forcing cone or "ball seat" ahead of the case mouth, before the rifling starts. The best way to do this is from a chamber cast or upset throat slug.

Most accurate for measurement purposes and easiest is to upset a throat slug, or as some people call it a "pound cast."
Start with a sized case with DEAD primer in its pocket. The way I do this is to heat the lead pot, then fill the sized case with DEAD primer plugging the flash hole, and generously overflowing the case.

After the lead cools, clean all spilled lead off the case exterior, then file the exposed lead FLUSH to the case mouth.

Take a piece of PURE lead buckshot or short chunk of pure lead wire and drop it into the EMPTY chamber, letting it fall into the throat of its own weight. (With very long throats you can use a longer piece of wire or a SOFT bullet with long bore-riding nose and not a long grooved section).

Insert your lead-filled dummy case and GENTLY tap it into the chamber using a piece of brass rod until you can close the breech. You are using the lead filled dummy case to force the lead slug into the ORIGIN of rifling. In short throated barrels it helps to drive the slug first into the origin of rifling, far enough to chamber the lead dummy behind it, then close the bolt and upset the slug against the lead dummy using a Brownell Squibb Rod threaded onto the end of your cleaning rod.

You don't need to use a hammer, just let the weight of the rod make many light taps of the squibb rod against the slug until you get a clear "ringing" sound. It need go no farther!

What you want to measure is the diameter of the UNRIFLED portion of the chamber forward of the case neck BEFORE the rifling starts! Extract the dummy and GENTLY tap the lead slug out and measure it. THAT is the diameter you want to size your bullets to!

Using Cerrosafe, etc. is more trouble and you then need to compensate for shrinkage, etc.

The upset pure, dead-lead slug is exact and straight forward!

If you forget EVERYTHING you ever read about slugging barrels and simply cast chambers from now on, and get bullets to FIT THE THROAT you will be far happier in the long run.

The limiting factor in safe bullet diameter is neck clearance. You MUST measure the neck diameter of the chamber on the cast. Most chambers have enough clearance ahead of a fired case mouth that a properly upset throat slug will get you a portion of the case mouth and its transition angle to the throat or ball seat, so that you can measure neck diameter at the mouth and throat diameter of the ball seat.

The loaded cartridge neck diameter must not be larger than 0.0015" SMALLER than the chamber cast at that point, to ensure safe expansion for bullet release. This is absolutely essential for custom target barrels which often have tight-necked chambers which require neck-turned cases. As a general rule the largest diameter of cast bullet which chambers and extracts freely, without resistance, will shoot best.

For instance in a .308 Winchester target rifle with .339" tight-necked chamber and using case necks turned to 0.012," maximum bullet diameter is determined by"

[neck (.339")-2(neck wall thickness 0.012)] - 0.0015 = 0.3135" for a "fitted neck" in which fired cases do not require sizing, but bullets will be held by case springback only. For necked sized fixed ammo, subtract another 0.0015" or .312" IF the chamber ball seat is that large. In a new barrel chambered for jacketed bullets, probably not. Min. SAAMI throat as on the pressure test barrel is 0.3105".

Unless you know your throat is smaller, try .310". If the barrel has been fired more than 1500 rounds with full power jacketed loads .311" will be better. If you shot a couple seasons season of highpower with it, .312" will fit just fine.

John Ardito set all of his CBA benchrest records shooting .312" bullets in his .308 Win. and wildcat .30 cal. rifles.

This is seldom a problem in the Russian M91/30s and Chinese copies chambers are notoriously sloppy!

In a typical Finnish M39 7.62x54 chamber the chamber neck is 0.340". Typical case mouth wall thickness of Norma or Sako commercial brass is 0.013," so .340" minus twice neck thickness (0.026") = .314", minus 0.0015 for safe expansion = .3125" max. bullet for a typical Finn chamber in an M24, M27, M28, M28/30 or M39.

It is not unusual for WW2-era Russian and later Chicom rifles to have throats as large as .316" and groove diameters of .314". If you expect anything resembling normal accuracy you MUST cast your chamber, measure it, and then buy a mold which fits your THROAT, not the groove diameter of the barrel.

As a general rule the largest diameter of cast bullet which chambers and extracts freely, without resistance, will shoot best. For most Finnish rifles this is .311-.312" and for Russian and Chicom rifles .313-.314".

Use .30 cal. gaschecks, pressing them on by hand and then pushing the base of the bullet against a table edge until the gascheck is bottomed against the bullet shank. Only then size the bullet. Otherwise the GC will not be seated squarely on the base of the bullet and any hope for accuracy goes out the window.

The Lee C312-155-2R was designed for the 7.62x39 and also gives good results in most 7.62x54 rifles when cast 12 BHN or harder, sized to THROAT diameter, and loaded with 16 grs. of #2400 for plinking, or 30 grs. of 4895, 4064, RL15 or Varget, if you want a heavier hunting load.