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A Unified Theory of Cast Bullets? - Mountain Molds

A Unified Theory of Cast Bullets?

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mtngun
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A Unified Theory of Cast Bullets?

Postby mtngun » Wed Jul 26, 2017 9:53 pm

I've started a table for "proven cast benchrest loads." It does not include plain base loads, pistol loads, production loads, or military class loads. Generally my criteria for inclusion is a 100 yard 10-shot average of 1.1" or better -- in other words, ten shot MOA. This table does not pretend to include every load ever shot in CBA competition, rather it attempts to include a representative sampling of loads that have performed well at Nationals. I'm sure I left out many fine competitors, but only because I wanted to keep the table at a manageable size and besides, most competitors are shooting very similar loads. If you can think of any particularly noteworthy or unusual loads that I left out, give me a holler.

The idea is to look for any quantifiable trends (while acknowledging there will always be important factors that cannot be quantified).

Sometimes data was missing, like the seating depth and the bullet length, so when that happened I made reasonable guesses. Pressures and filling ratios were estimated by Quickload. The results should be viewed as merely a rough estimate.
Image

Things that I suspect are important:
-- the average pressure is ~35,000 psi. More pressure = more bullet deformation. No one is winning benchrest matches at 60,000 psi.
-- the average RPM is ~121,000. More RPM = more dispersion due to the inevitable unbalanced bullet. No one is winning benchrest matches at 200,000 RPM.
-- the average powder filling ratio is ~90%. A powder that fills the case generally burns better than a powder that leaves a lot of air space.
-- the average acceleration is ~91,000 G. More acceleration = more bullet deformation.
-- the average velocity is ~2100 fps (as of this writing). Pushing a given bullet faster reduces wind drift, but it also increases pressure, acceleration, and RPM, which tends to hurt accuracy.
-- clearly the 30BR is the most popular cartridge. In theory switching to a 308 or a 30-06 would allow the same velocity at lower pressure and lower acceleration, but you'd run into either poor powder fill ratio and/or poor powder burn. No one is winning the heavy varmint or unlimited class matches with a 30-06 stuffed with 4831.
-- the 6PPC with its typical 14" twist can be very accurate with cast bullets, but its bullet weight is limited to 75 - 80 grains, putting it at a disadvantage for wind drift.

Things that I suspect may be non-optimal:
-- the average Miller stability factor is ~1.8, but that may be "overstablized." By comparison, jacketed benchrest loads typically run a 1.30 stability factor, the bare minimum to reliably stabilize out to 200 yards. The faster you spin the bullet, the greater the dispersion due to bullet imbalance. When buying a new barrel, I recommend selecting a twist that will provide a SF between 1.30 and 1.50. Admittedly, the theoretical difference in accuracy due to twist is tiny and difficult to prove.

A partial list of things that may be important but can't be easily quantified:
-- the quality of the barrel
-- the type of throat and the quality of the throat
-- the bullet design
-- the bullet alloy and BHN.
-- the lube
-- how cleanly the powder burns (carbon fouling may hurt accuracy).
-- how well the powder burns in a given load. At the modest pressures typically used with cast bullets, many powders will burn dirty.

A partial list of things that may not matter:
-- nose pour vs. base pour molds. If one were superior then you would expect it to dominate benchrest competition, but in fact they are both well represented in CBA competition.
-- lathe bored vs. cherry cut molds. Again, if one were superior then you would expect it to dominate benchrest competition, but in fact they are both well represented in CBA competition.
-- "bumped" vs. not bumped. "Bumping" is just another word for "swaging" so I prefer to stick with the common English language and call it swaging. I believe the practice started as a way to make undersize noses fit properly, by "bumping" the nose up to a fatter diameter. In theory this swaging operation may generate enough pressure to collapse internal voids and square up the base, but the main thing seems to be ensuring a good fit in the throat, which can also be accomplished by casting slightly oversize and then sizing down in a nose die, as I do. As far as I am concerned the "bump" question could be dropped from TFS.
-- nose diameter & base diameter. What matters is fit, and your barrel and throat may have different dimensions than my barrel and throat. Many competitors are shooting tapered bullets so the TFS info on nose diameter and base diameter no longer seem relevant and as far as I am concerned could be dropped.

Other Interesting Observations:
-- H.L. Yarborough's 2007 load at 2750 fps is average for pressure and RPM despite its high velocity. Its acceleration (150,000 G) is unusually high, though. Ditto George Morrison's nearly identical load.
-- I admire Dan Hudson's 2005 load using a 16" twist barrel at 2200 fps, resulting in below average pressure, below average RPM, and below average acceleration. His 160 gr. bullet would drift more in the wind, but on the whole it's an appealing combination.
-- some competitors are doing very well with the heavier (215 gr.) bullets even though their pressures and RPM are a little on the high side. Presumably the heavier bullet reduces wind drift. They could drop their RPMs by switching to a 13" twist barrel.

In general you want to push a heavy, streamlined bullet 2100 fps at 35,000 psi, 120,000 RPM, 90,000 G's, and SF 1.3. It seems like any attempt to break out of that range runs into a trade off with at least one of those parameters. For example I'm currently attempting to find a CBA-worthy load at 2300 fps instead of the usual 2100 fps, but the added velocity tends to increase pressure and acceleration. Slow powders reduce pressure and acceleration but tend to burn dirty at 35,000 psi. Fast powders burn better but increase pressure and acceleration. No matter which way you turn, there's a trade off. :?

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Re: A Unified Theory of Cast Bullets?

Postby mtngun » Mon Jul 31, 2017 12:16 pm

One of the challenges of loading cast bullets is finding a powder that will burn well at reduced pressures. Here's how the three most popular powders in my table stack up:

N135
-- used in 26% of loads in my table
-- Low 25,000 psi (the lowest pressure included in table)
-- Average 34,600 psi

IMR8208
-- used in 14% of loads in my table
-- Low 26,000 psi
-- Average 40,000 psi

Varget
-- used in 17% of loads in my table
-- Low 25,000 psi
-- Average 34,900 psi

I'm sure they'll all burn well at higher pressures, the question is will they burn at 35,000 psi? The data seems to suggest that N135 and Varget do tolerate 35,000 psi, but IMR8208 may prefer 40,000 psi. However the sample size is small so I can't draw any firm conclusions.

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Re: A Unified Theory of Cast Bullets?

Postby mtngun » Mon Jul 31, 2017 6:32 pm

Just some of the obstacles a cast bullet load must deal with.
Image

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Re: A Unified Theory of Cast Bullets?

Postby mtngun » Fri Dec 22, 2017 9:14 am

Table shows RPMs for a given velocity and twist.

CYAN = safe RPM's
YELLOW = challenging RPM'S
ORANGE = hold your mouth just right
RED = lotsa luck
.
RPM = 720 x Velocity / Twist

Image

Bangbangman
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Re: A Unified Theory of Cast Bullets?

Postby Bangbangman » Mon Dec 25, 2017 12:20 am

Great analytical approach. Keep us informed.

Merry Christmas!

PS. Took another deer and two fox with one of your bullets yesterday. No hunting today- family Christmas ;)

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mtngun
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Re: A Unified Theory of Cast Bullets?

Postby mtngun » Mon Dec 25, 2017 8:04 am



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