Bearing surface on a bullet design. How to estimate / calculate?

[Kjeksen87]

Hello there fellow casters.

I am trying to design a 147grs bullet that will chamber in CZ pistol chambers or other "short throated / no freebore chambers"

Now, to do this I need a reasonable amount of bearing surface on the bullet. Too long will force me to set the
bullet very deep in the case to make it chamber smoothly. The result of that is that there will be very little spare room for powder.
Too little bearing surface, and the bullet will in theory not be as accurate.

All bullets will be cast with hardball alloy BHN 15-16


How does one calculate or estimate how much bearing surface is actually needed? Reckon that bullet length comes into play ofcourse..
Is there a set of ground rules? Mathmatics? Witchcraft? Or is it a "much as possible" kind of deal. What if as much as possible is too little?

Anyone With some good input regarding this matter?

[Yodogsandman]

You need the measurement of your chamber/throat/lands/bore/groove diameter. Any boolit nose measurement at or over your bore size is your bearing surface. The more bearing surface, the more accurate your boolits should shoot. Parallel sided noses on boolits are less accurate than those with a slight taper that helps the boolit "self center" in your bore. A strong action, like a bolt action will help to crush fit when chambering the boolit. If it's going to be used in another action, plan on making the boolit shorter. Use the taper to figure out the total boolit length down to your cartridges neck junction. It really helps a lot to make a sketch of your chamber and bore with dimensions and then work your boolit dimensions from there.

There's plenty of guys here with more experience than me, though. Hopefully they can give you more info than I can.

[35remington]

The first order of business is in deciding if a bore riding design is wanted, or a mostly body type similar to a Loverin. Case neck length and throat length will help you make that decision along with desired bullet weight.

A bore riding design has more guiding surface than it appears......if it fits correctly.

On edit....if you're talking about a pistol, compare to existing designs. There are probably no short bearing surface 147's for 9mm. If this is what you are discussing, that is known as a clue.

[Kjeksen87]

Just to clarify, it is a 147grs 9mm Round nose. im designing. If I compare to existing designs, probably only a couple would actually work properly in a short throated barrel.

[Ballistics in Scotland]

You can use the bullet design utility on www.mountainmolds.com (http://www.mountainmolds.com) , even if you don't buy the mould. They can't do bullets with a true round nose, but a design with their minimum meplat can give you a very good idea of the total weight your bullet will have. Or you could design a full wad-cutter and find what weight is in the cylindrical section.

[Kjeksen87]

I already have software for calculating and designing bullets. What I am uncertain of, is the amount of bearing surface needed to make the bullet precise enough. To make it chamber with reasonable case capacity left I need to comprimize the amount of bearing surface on the bullet, as bearing surface is usually inside
the casing. Some chambers with generous throats or alot of freebore will allow some amount of bearing surface to be outside of
the casing, but those are not my worries.

[Blackwater]

Instead of worrying about surface area, I think I'd tackle it from a different angle. I'd see how deep a sample bullet that you think would be good sticks down in the case, and use that for the shank length, and then just design the nose so that it'd fit the short throats. Just an idea, anyway.

If you want surface area, just multiply the shank length you want to play with X the diameter X pi. That'll give you the area without any fuss.

And BTW, I have an old Saeco 240 gr. SWC mold in .44, and a buddy had the Lyman Kieth. My Saeco sits down in the case deeper and the Lyman has the longer nose. For once, that worked out and my bullet shot best in my gun and his in his gun. So many things go into accuracy and what bullet an individual gun likes, that I'd just take the tactic I listed above and just try it.

Predicting accuracy can be a very tricky thing, and the only way to really know what any given gun will shoot best is simply to try it and see on the range, varying powders and charge wt. to find the sweet spot for each powder with that particular bullet. I have many cans of partially empty cans of powder from having done this through the years. That way, I KNOW what my gun likes and will shoot best, and yes, it can make a difference.

The powder we WANT to use might not be the same as the one the gun likes best. I've never borrowed a cup of sugar from a neighbor, but HAVE borrowed some small amounts of powders I didn't have from friends, and they from me, just to see what a given gun wanted to shoot. Most poo poo this, but it's enabled me to have some very accurate loads for each gun I've had, and kept. It's fun and enlightening too, to try at least some different loads when I get to shoot, so it's fulfilling, edifying, and an efficient way to wring our a gun, too. Just the way I do it, and there's really no "wrong" answer here. Just different ways of looking at the problem, and of dealing with it. FWIW?

[Yodogsandman]

Suggest getting your CZ barrel throated and shooting off the shelf boolits. But, that would be no fun!

[Kjeksen87]

Suggest getting your CZ barrel throated and shooting off the shelf boolits. But, that would be no fun!

You`re indeed right. No fun in that!

[Sgtonory]

Suggest getting your CZ barrel throated and shooting off the shelf boolits. But, that would be no fun!

Add to this. I ordered the tool from Brownell for $50 a lot cheaper than a new mold and get to use it on any guns in the future.

[reddog81]

No 147 9mm bullet is going to have "too little" of a bearing surface. Given your short throat constraint the biggest concern will be how much of the bullet is inside the case. Size of lube groove, base type, profile of nose, diameter are all things I would consider and will basically set whatever the bearing surface ends up being

[475AR]

I already have software for calculating and designing bullets. What I am uncertain of, is the amount of bearing surface needed to make the bullet precise enough. To make it chamber with reasonable case capacity left I need to comprimize the amount of bearing surface on the bullet, as bearing surface is usually inside
the casing. Some chambers with generous throats or alot of freebore will allow some amount of bearing surface to be outside of
the casing, but those are not my worries.

Which software are you using to design your bullets?

[runfiverun]

your only gonna get so much in the case.
cut a couple open and see how the case gets thicker.
you either need to mimic that taper or stop shoving lead in there at that point.
I shoot cast in my CZ-75b retro without issue but my boolit only weighs about 135grs. [and sized to 358]
it has full diameter drive bands but the nose is a rnfp type and has a quite short bore riding section in front of the drive band.
the rest of it angles off just enough to miss the square rifling.
I seat the front band flush to the case mouth so it starts it's journey into the rifling directly from the case.
anyway.
we went through this when drawing up the HM-2 mold [which I can't seem to find a drawing of anymore]
and the newer pistols is why we designed it the way we did.
you should be able to get by with a 147 designed for the 357 sig with it's truncated cone nose and do pretty well with a size of 358 and an alloy just harder than ww's.

[Kjeksen87]

Which software are you using to design your bullets?

I use the precision cast bullet software from Mr. Tom Myers

http://tmtpages.com/draw/draw.htm

I will have to tinker alot more with design and length. I would want a short bullet as possible so a large radius at the nose
is better I think. Anyway, the design software is really nice. Recomended.