Cast bullets create higher pressures?

[dgang]

Just perusing 2005 Alliant Reloading Guide for .357 Mag. cast bullets and found some data I don't understand.(not an uncommon state) They give 7.8 gr. of Unique for 158 gr. JSP @ 1280 FPS, developing 33,200 PSI. The same data states 6.8 gr. of Unique for a 158 gr. cast LSWC @ 1295 FPS but with a higher pressure of 33,900 PSI. I can understand the difference in the amount of powder and velocities, but fail to understand why the cast bullet with less powder has higher pressure. Any help out there?
Thanks in advance and good shooting' to ya.

 

[mehavey]

Cast bullets immediately deform to seal the bore.
Takes less powder to reach pressure.

 

[USSR]

One of the determinates of pressure is how much of the bullet is inside the case. Seat a bullet deeper in the case and pressure rises. Since we have no idea what bullets are being used and how much of each bullet is inside the case, kind of hard to nail it down definitively. However, it is extremely common to use less powder with lead bullets to obtain the same velocity as jacketed bullets. Hope that helps.

Don

 

[44 AMP]

Another factor can be the amount of bearing surface. Lead is softer than copper, and therefore easier for the rifling to engage but it is possible the bullet has a different amount of bearing surface which will affect the result.

Was there any other information in the data, such as cases, primers, test gun, etc?? Lots of things can cause measurable but not significant differences.

 

[dgang]

Measured both cast and jacketed bullets, cannelure is the same distance from the base of the bullet so case capacity is the same. The idea of the cast bullet deforming quicker and obstructing the bore makes sense. Nothing about the cases but same primers Thanks to all of you.

 

[USSR]

Measured both cast and jacketed bullets, cannelure is the same distance from the base of the bullet so case capacity is the same.

I'm confused??? I went to the 2005 Alliant reloading manual and the only information it gives about the bullets are 158 JSP and 158 LSWC, with no indication of who made the bullets, their length, and cannelure and crimp groove location. So, how can you measure bullets when you don't know what bullets to measure?

Don

 

[Hal]

Lead is rather typically over sized also. It can take more initial pressure to swage the bullet down to bore size.

In .44 caliber, I run .430 " diameter lead and .429 " diameter jacketed.
In .357/.38 I run .358" diameter lead and .357" diameter jacketed.

 

[J.G. Terry]

Sometimes, Alliant data is hard to understand or figure out. One year's manual gave maximum pressures only. Not that we are dumb but have trouble with the way the data is presented. As pointed out there are many factors that can cause the figures to look funny or change around.

My favorite was a new Alliant powder was suggested for 338 Magnum. There was no Alliant data for that powder for 338 Magnum. The Alliant rep was helpful finding some data elsewhere. When something like this crops up that data is compared with other data.

Added: A phone call got me the the current 2017 edition of the Alliant guide. Check out the website for data.

 

[Unclenick]

All Alliant's "recipes" have always been maximum loads. They expect you to know for yourself that you have to knock 10% off to start. I don't recall ever seeing a pressure number from them, though Lyman has pressures where they use Alliant powders.

As to lead pressures, despite the wider diameter, lead bullets take much less pressure to swage into a barrel. If you don't believe it, try using a brass rod and a mallet to drive one through a bore. Then try the same thing with a jacketed bullet. The cast bullet might be BHN 15, but the copper jacket BHN is often around 60, so it requires a lot more engraving force.

Lead bullets typically produce lower pressures except for magnum pressures in revolvers. Skeeter Skelton commented long ago that in magnum loads he had more pressure signs from lead in his revolver loads. This is likely due to the softer metal blowing out to pretty much fill the forcing cone and sealing off the chamber of the revolver before finding its way into the barrel. If you look at a reprint of F.W. Mann's 1907 book, The Bullet's Flight from Powder to Target, he has lots of pictures of the result of gradually cutting a barrel down until the bullet was starting out at the muzzle (pretty much what a revolver chamber is) and the lead bullets all have the profiles of Christmas trees after firing with the peak pressure right behind them.

 

[dgang]

In speaking to others and the answers here, it seems that a cast bullet will quickly obturate in the barrel, quicker and more completely that a jacketed bullet, and cause over pressure by sealing off the gases before  the bullet leaves the barrel. Seems sensible to me. As for measuring the cast and jacketed bullet at the cannalure to the base, I have several different samples here. All measure within a few 100's of an inch. Don't think that would make a difference in the amount of powder being used. Thanks for your interest.

 

[Paul B.]

I'm incline to call reconstituted bull fodder. Bravo Sierra if you prefer. Consider this; two 158 gr. bullets, one cast lead, one factory jacketed. Odd are their length would probably for all practical purpose just about equal. Seated to the same depth I believe the lead bullet would show equal or less pressure for the following reason. Lead is soft while the jacket of a bullet is much harder to engrave into the rifling. I can't remember when I last shot a jacketed bullet in my .357 Mag. but I do recall shooting some jacketed factory vs my cast bullets in a .44 magnum. I was using Elmer Keith's load of 22.0 gr. of 2400 powder for both loads. Brass from the cast loads literally fell out of the chambers while the jacketed loads needed a somewhat stiff push ejector rod. This was using an S&W M629 and a Ruger Super Blackhawk and firing both type loads in each. FWIW, the factory bullet was a 240 gr. Speer and my cast bullet which weigh 250 gr. in my alloy. Whether that proves anything or not I guess is irrelevant bull Alliant and Speer are owned by ATK IIRC and neither one sells cast bullets.
Paul B.

 

[USSR]

In speaking to others and the answers here, it seems that a cast bullet will quickly obturate in the barrel, quicker and more completely that a jacketed bullet, and cause over pressure by sealing off the gases before the bullet leaves the barrel. Seems sensible to me.

May seem sensible, but it just ain't so. As previously stated by Unclenick, the greater lubricity of lead in relation to copper jackets, with all other things being equal, will result in less pressure for a cast or swaged bullet load.

Don

 

[Unclenick]

As I mentioned earlier, I've only ever heard of this happening in revolvers. I think Dgang has hold of a valid principle except for where in the gun it is happening. I think it's in the throat. Dr. Lloyd Brownell found gas bypass around a rifle bullet explained why a jacketed bullet loaded to jam the lands (starting with the throat almost obturated and preventing significant gas bypass) created a roughly 20% rise in pressure over what occurred when the bullet was seated with some jump. In the revolver, a jacketed bullet also sees gas bypass that goes around the bullet and out of the barrel/cylinder gap even when the leading edge of the bullet bearing surface has first obturated barrel. That escaping gas limits how fast pressure can rise, just as Brownell found it did in the rifle. But when a lead bullet hits a revolver's forcing cone, pressure swells the bullet so it not only obturates the bore but the back end obturates the throat as well, stopping gas bypass out of the barrel/cylinder gap until after the bullet passes the gap and is in the bore. That gives pressure rise timing a head start, letting it get to a higher peak.

Note that hard alloy cast bullets have to get to a higher pressure than softer bullets do for the above to happen as they won't upset until a higher pressure is reached. You can expect a threshold at the pressure that upsets the bullet adequately, above which there is a pressure increase. This could be tested in one of the SAAMI velocity and pressure barrels that have a gap to simulate the revolver barrel/cylinder gap.