Seating, Sizing and Melting

[Phantom30]

I decided to write this thread to frame a few questions and see if I can get some helpful answers.

Chamber nomenclature: (this is not definitive, just what I call them as reference)
Throat the cylindrical part of the bullet chamber where the front section of the bullet which sticks out of the casing, rests when loaded.
Taper the tapered part of the bullet chamber where you neck down from throat to bore diameter. This area is also called Jump or Leade. This is also the place where the lands and grooves begin to form. Sometimes people refer to the combination of throat and taper as leade. Freebore is also used by some, but not here, just call it taper.
Bore, the cylindrical section of the barrel that contains the Lands and Groves at full height or depth. The bore rider boolit nose will rest in here when loaded.

1. In a caliber with a standard cylindrical throat chamber, such as 308W, where do you safely seat the boolit for optimum accuracy performance, at the end of the throat or backed off from the start of the taper? Why?
2. What diameter do you size the boolit drive bands with respect to the throat diameter, larger, equal, smaller? Why?

Figure below displays a modern bore rider boolit to chamber configuration

Consider the same questions for the 300WM, it has NO definable throat, just a taper. Do you seat it so the drive bands contact the taper? Or offset back from the taper contact point and risk allowing the hot gases to escape around the boolit and potentially blowtorch the boolit as it begins to move? Why do you make these choices? The 300WM no throat configuration is displayed here.

The 458WM has a taper almost twice as long and the amount of lead sticking out of the case at max COAL. This really goes to the question of melting, since the movement of the boolit can go a long way, depending on seating, before contacting the taper. This allows opportunity for high pressure hot gases to escape around the boolit and blow torch it. Therefore, some have expressed an opinion that the 458WM is cast boolit unfriendly.
Then there is the 45-70 with no throat and nearly no taper. So the round gets stuffed into the bore’s lands and grooves before firing as shown. How does this safely work without increasing the peak pressure significantly?

1. Then the last question, will PC for example, say with HF red, actually provide the protection necessary to stop the blow by or blow torch effect in these various chamber shapes? Jacketed bullets don’t appear to be effected.

[Soundguy]

hmm... I just cast up some 500 with gas checks... will see if they blow torch or not.

[runfiverun]

I watched that discussion develop last night.

that top boolit picture is not a very good throat fitment

[Phantom30]

Yes, but what is correct, that's what the 308W enlargement in the third picture suggests, How much off the taper, how close to the filling the throat and why. What's the rule of thumb etc.
First there must be a rule because of reloading tolerances that gives you a safety margin so you don't generate overpressure because of seating placement, especially with PC added on
Second there must be a rule for how you size the drive bands, say your gun's throat is 310, do you want to size 311 and jam it, 310 and make it snug, or 309 and leave loading tolerance, and why does it make a difference.
Three, some of these rules are for safety and some for increased performance, but what are they

[runfiverun]

well we are dealing with lowered pressure so the safety is covered.
powder coat lowers the pressures even further.

so what we are left with is how the alloy flows under pressure as it is moving forward.
the mechanical fitment [filling every nook and cranny with alloy] starts out pretty fair but quickly the static fitment and flow of the alloy takes over.

so you need just enough diameter to touch the steel but there has to be allowances made for the boolit moving forward and moving the metal coming behind.

the 458WM discussion the other night focused too much on the taper in the throat, and not how to fix the problem from .0020 away.
look at the Ranch Dog designs thy were made to deal with a big ol' volumous ball seat area that if you tried filling that you'd never be able to chamber the round.
the 5.56 AR throats are the same way, so you need to shape and strengthen the first part of the design that is going to touch steel.

I'm trying to recall who it was having the issue [capt redd? maybe] but he was only looking at the problem and not the solution.
take your picture above.
move the nose back, round it off and taper it to guide the rest of the boolit into position.
give it a slightly tapered front drive band to use that taper as a bump in point.
the drive bands following will ride low in the rifle but the taper and the nose will center things when they get to the barrels rifling.
the bump in point will hold the gas back and the drive bands being strengthened will get them into position before being accelerated down the barrel.
you can also help yourself by slowing down the powder speed.
the engraving pressure will give it a little kick and get it burning.

the same things are happening as normal your just delaying them from happening until you can get the boolit centered in the barrel.

[Phantom30]

Thanks for the help. Since this is a bore rider design don't we start out partially centered before movement? OK my transition ramp between the bore rider and the drive band is .003 over 0.030 inches are you suggesting I feather it out even further? If the transition on the boolit approaches or matches the slope in the taper don't you run the risk of inducing a drag spike in the acceleration because of all the metal contact at once, rather then just the leading edge of the drive band? I so have feather it further, a number in mind?

your are saying enough diameter to touch steel, is that in the throat, so I would size to 310 for a 310 throat? But back off the taper with the drive band for 0.005 say?

Boolit shape, I have the RN RTB design over on Bama's 350 yards accuracy thread, is that more like you mean? These are all bore rider desgins for bolt guns because we are beyond max COAL in many cases.

You stated PC lowers the pressure even more. Does this mean that PC has a higher Obturation point than the lead so it doesn't block the gas flow as quickly thereby allowing some of the gas to pass it and thus doesn't increase the pressure?