Sizing as it relates to BHN (.001 vs .002 over)

[zubrato]

Conceptual question
Will sizing cast bullets .002" larger than the .001" over bore standard give a softer alloy a little more "bite" into the lands and grooves and possibly prevent leading?

Every so often someone will post sizing .002" over bore diameter improves accuracy, so maybe there are more benefits to sizing to over more than one thousandth.

Let's say an alloy is ever so slightly softer than the ideal BHN, all other parameters are ideal with high quality lube and sized .001 over.

Would sizing bullets just a thousandth larger reduce traces of leading, say especially in a barrel with shallow rifling?

[Outpost75]

Bullets should be sized to CHAMBER THROAT diameter. Ignore barrel groove diameter. That is MISINFORMATION. Do a "pound cast" and measure the diameter ahead of the case mouth before the rifling starts. THAT is the size your bullet should be.

[DougGuy]

Pistol, revolver or rifle?

[turtlezx]

75 your saying use the largest boollit that will chamber easily ???
could be .005" over size would be ok ??

[DougGuy]

I always was under the impression that the cast boolit should be .001" to .002" greater than groove diameter.

In reality it only needs to BE groove diameter but that doesn't leave enough lead to be forced into the barrel which makes a good seal in the rifling.

[zubrato]

Let's assume for an autoloader, rather than a revolver.
I have had excellent success sizing .001 over bore, and all my S&w revolvers have .357 throats with .357 barrels with no leading issues with absolute screamers.

Assuming all other variables are constant, including throat diameter that will allow for both 001 and .002 over to chamber and extract reliably, will the slightly softer lead have better bite into the barrels grooves as its being swaged to bore diameter?

If so, would that only improve accuracy or also allow for ~50fps or more leeway given a slightly less than ideal hardness?

[Chris C]

I shoot low pressure loads for target work. (keep my velocities on rifles under 1300 fps mostly) I size .0015" to .002" over the bore size because I'm concerned obturation from the initial blast won't be sufficient to seal and keep the fire from going around the bullet. I do that, across the board on all my firearms.......pistol or rifle. My barrels don't lead up.

[runfiverun]

yes, no, maybe, it depends.
lets think about this for a second.
what happens to your boolit when you squish it down? [it gets longer]
what happens to it when you accelerate it quickly? [it gets shorter]

so how it depends.
the powder you use, the gap of the cylinder, or the taper of the leade.
the alloy's make up has an influence also.
harder is not always better.
mis-match the alloy's "flowability" under pressure to the design and things do not work.
it really doesn't work when you mis-match them with the acceleration of the powder you use.

match up a pre-slumped design with a softer alloy [low tin and low antimony] and accelerate it gently and you can achieve some very impressive velocity's.
[these boolits are in a fairly pre slumped shape they rely on straightness and alignment and a static fitment under pressure and use all the available case neck/ball seat area for the full drive band and the front drive band usually mimics the ball seat to the rifling angle]
they do well over a wide range of velocity's but you need to match their launch [acceleration] by slowing things down a bit until the boolit is fully engraved into the rifling then speeding it along for best results when near jacketed velocity's.

now take a design that needs the rear of the nose to align with the barrel so everything can rattle around for a bit to find it's own center and a medium balanced tin/antimony alloy is going to work better to resist riveting.
[the nose mimics the angle of the end of the leade and usually has strong drive bands with plenty of room for the displaced alloy to be pushed back into the open grooves] these usually work best in AR type rifles.

now a design that matches the throats angles but is going to resist deformation because of a straight sided design that doesn't allow the alloy to move well and doesn't allow for alloy displacement. [just about maximum amount of drive band and engraving capability]
this design needs a harder high antimony alloy to hang in there and resist deformation [this type of boolit design is based on mechanical fitment]
the boolits shape remains pretty much the same the whole time it's main downfall is it's highly susceptible to trailing edge failure affecting accuracy at some point along the velocity trail.
it can work exceptionally good or exceptionally bad.

[Outpost75]

75 your saying use the largest boollit that will chamber easily ???
could be .005" over size would be ok ??

Yes!

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.

Safe bullet release requires that the loaded cartridge neck diameter be larger than 0.0015" SMALLER than the chamber cast at that point. This is seldom a problem in milsurps or ordinary sporting chambers but is absolutely essential to be aware of in loading for custom target barrels which may 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 in which seated bullets are held by case springback only.

For necked-sized, fixed ammo, subtract another 0.0015" or .312" IF the chamber ball seat is that large. A barrel which has been shot over 5000 full charge loads will be. A new barrel with factory chamber probably not. The Min. SAAMI throat as on the .308 Winchester standard industrial pressure test barrel is 0.3105".

Unless you know that your throat is smaller, in most .30 cal. rifles with new barrels try .310". If your barrel has fired more than 1500 rounds with full power jacketed loads .311" will be better. If you shot a couple seasons season of NRA highpower with it, including rapid-fire strings, then .312" will usually fit just fine for cast.

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

Worrying about oversized bullets is seldom a problem in the 7.62x54R Russian M91/30s and Chinese copies because their chambers are notoriously sloppy!

The typical Finnish M39 7.62x54 chamber has a hamber neck of 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 7.62x54R and wartime British .303 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 US M1903A3 Springfields and M1917 US Enfields which actually saw service, as well as Finnish 7.62x54R rifles this is .311-.312" and for Russian 91/30 and Chicom copies .313-.314". Typical WW2 production .303 Brits require .315"!

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.

[OS OK]

What do you do about the shrink back of the cast material. Is that a standard number unique to the particular manufacturer? Have never cast before just slugged.

[Cowboy_Dan]

The cerrosafe, etc. chamber casting kits have instructions which give the shrink rates for the alloy. However, there is a sticky called something like "this is how I do a pound cast" which gives detailed instructions on how to do an impact cast with lead which will retain its size forever.

[OS OK]

That's what I've been doing.