cast bullet truthiness

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mtngun
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cast bullet truthiness

Postby mtngun » Thu Feb 14, 2008 7:58 pm

from the old forum:

As my friend "Bass Ackwards" likes to remind me, there are no absolutes when it comes to cast bullets. That said, some things are truer than others.

1) Cast bullets like bearing length. Bearing length helps to form a good gas seal. Bearing length helps keep the bullet from getting cockeyed. IMHO, people tend to worry too much about things like BC and powder space and not enough about the bearing length.

2) If I could only have one alloy, it would be wheelweight. Tape-on wheelweight is soft enough for black powder, air-cooled wheelweight is about right for low velocity plinking loads, and heat treated wheelweight is great for high performance loads. You can do it all with a bucket of wheelweight.

3) The sprue hole size must match the alloy. Give wheelweight a big sprue hole and a matching pour stream and you'd be surprised how well it yields, even at low temperatures. Black powder alloys also fill out better with a big sprue hole, however, the soft bases will deform from the force required to cut a big sprue. A big sprue is not necessary or desirable for hard alloys. Mass produced molds usually have a sprue hole that is about right for hard alloys or for black powder alloys but woefully inadequate for wheelweight. The problem is exacerbated by the pitiful trickle from some bottom pour pots.

Casters frequently ask "what is the right diameter?\ It is a good question, but the answer is not cut and dry. I'll give it one more try, focusing on the things that are apt to go wrong with bullet diameter.

Revolvers
1) Revolver bullet is not big enough to seal the throat. Poor accuracy & leading, especially with plain base bullets. Use throat diameter bullets, if they will chamber, or use soft bullets that obturate.

2) Throat diameter bullets make case mouth OD too big to chamber. Use a gas check and live with undersize bullets until you can buy a better gun, or use soft bullets that obturate.

3) Revolver throat is smaller than barrel. Bullets diameter is reduced as it passes through throat, poor accuracy and leading. Send cylinder to "throat doctor" to be reamed to groove diameter or slightly larger.

4) Revolver forcing cone is smaller than barrel. Bullet diameter is reduced as it passes through forcing cone, poor accuracy & leading. Lap forcing cone.

5) Throat diameter bullet won't chamber easily because front band is snug fit in throat. Use a bullet with a shorter front band, or a reduced diameter front band.

6) Crimp die squishes cast bullet. A known problem with the Lee Factory Crimp die and perhaps others. Use ordinary roll crimp die.

Smokeless Rifles
1) Bullet too small to form gas seal in throat. Use throat diameter bullet.

2) Bullet too big to enter throat (30-06 throats are typically only 0.309" - 0.310", some cartridges like the 348 Browning and some 45-70s have absolutely no throat :shock: ). Use smaller diameter or a 2-diameter bullet, or invest in a throating reamer.

3) Bullet too small to create snug fit in neck. Use bullet diameter that results in reasonable (0.003" for hunting rifles) neck clearance. Example, typical 30-06 chamber requires 0.314" - 0.316" diameter to get rid of slop in neck.

4) Cartridge won't chamber because case mouth OD is too big with desired bullet diameter. This is a common problem with the 38-55 and a few other other old black powder cartridges. Turn case necks or re-cut chamber. Sometimes a soft, undersize bullet will obturate to fit, but this may not be reliable with smokeless powder.

5) Bullet has to be seated into the powder space. So what? I have shot thousands of deep seated bullets without incident.

Bore riding bullets
1) Undersize nose wobbles around in barrel. Lap mold, or "bump" nose in lubrisizer, or use an alloy that increases the diameter, or switch to Loverin design.

2) Bullet difficult to chamber because nose is accidentally bumped up in lubrisizer. Size bullets in push-thru sizer before lubing.

3) Bullet is difficult to chamber because nose casts too big. Size nose in lubrisizer, or switch to an alloy that casts smaller, or switch to Loverin design.

Falling block rifles
Falling block action does not have camming power to engrave the nose, either seat deeper or else select a bullet design that does not engrave hard.

Lever actions
Because they require a certain cartridge overall length (COL) to feed correctly, selecting a cast bullet for a lever action can be especially tricky. Usually a 2-diameter bullet is required, but it just depends on the cartridge. There is no easy answer for lever actions. Measure carefully and experiment with dummy rounds.

BPCR
Not my area of experience, but they tell me that the bullet has to be undersize to allow chambering despite fouling.

Muzzleloaders
1) if the diameter is too big, it will be difficult to start. Use smaller diameter, or use undersize body with only the top band engraving.

Sizing
1) Sizing does not hurt accuracy, but bad sizing dies can hurt accuracy, and there are a lot of bad sizing dies out there. Learn to lap your dies so that they have a tapered entrance with no sharp edges.

2) I now do almost all my sizing with homemade push-thru dies. Even a push-thru die still needs a smooth, tapered entrance, though. My RCBS lubrisizer "only" gets used for light duty sizing, nose sizing, nose bumping, and lubing bullets that have already been sized.

General
1) Changing the diameter to solve one problem may lead to another problem. Yes, cast bullets can be tricky. That's why they are so interesting.

2) The only way to know for sure which diameter is most accurate is to experiment. There are no absolute rules. Cast bullets can be finicky about these things.

3) Usually, but not always, it doesn't hurt for a cast bullet to be oversize as long as it chambers. There are reports from very reliable people that oversize bullets may hurt accuracy in certain guns, but it has not happened to me -- yet.

4) A gas check cures a lot of ills.

5) Long bearing length cures a lot of ills.

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mtngun
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Re: cast bullet truthiness

Postby mtngun » Thu Feb 14, 2008 8:18 pm

Or here's another way of looking at the same thing -- there are at least four critical diameters that may affect cast bullets:

1) the bore diameter -- usually a bore riding nose should be bore diameter or slightly larger, depending on whether or not the action has the camming power to engrave the nose. Note that the bore diameter is often bigger near the breech than at the muzzle, due to erosion.

2) the groove diameter -- usually we want the bottom half of the bullet to be a very tight fit in the grooves.

3) the throat diameter -- usually we want the bullet to be a snug fit in the throat, but that is not always feasible.

4) the diameter clearances at the neck -- usually we want about 0.002" - 0.003" clearance in the chamber neck area, enough to allow the cartridge to easily chamber and easily release the bullet, but not too sloppy. If the chamber neck is too tight, you can recut the chamber or turn case necks. If the chamber neck is too sloppy, an oversize bullet may shoot better.


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