Chambers / Throats / et al.

[Dannix]

I was looking for 6.8 SPC trajectory numbers the other day and came across this page and a "various chamber designs" page linked there, quoted below.

So what sort of chamber is more bullet friendly than others? Is one of the below chambers a lead friendly chamber, or would another need to be speced for an ideal boolit 6.8SPC chamber? Would an ideal boolit chamber compromise j-word performance somehow?

Also, barrels. What's the rational for choosing between one groove option over another?

Original SAAMI Chamber showing the angle error. On the drawing above, based on the dimensions specified in the drawing the angle works out to 70° 55':.


Original second version SAAMI Chamber ("SPC II") showing more generous specs
but continuing the angle error. On the drawing above the based on the dimensions specified in the drawing the angle works out to 71° 30'

* I have recently been informed that the SPC II reamers currently (2/09) being cut now actually have a 45 deg angle, which would mean that the horizontal measurements would be .105 and .0905", giving .01495"


DMR (Designated Marksman Rifle) chamber showing improved angle. If you do the
math you get an angle of 35° 1'. Note the tighter neck (.305") and throat (.277") dimension. There have been several variations of this drawing circulated with the angle varying from 25 to 37 degrees. and all associated with barrels using the 5R rifling form.

[Dannix]

Looking at the forcing cone vs throat routes for .357 Maximum, I get what he's saying about the forcing cone (are forcing cones always bad?), but his drawing makes it look like it's a perpendicular, hard, 90 degree sharp edge from chamber to throat. Surely this cannot be, as boolits could get shaved, yeah?

"bad", SAAMI .357 Remington Maximum chamber.


"good", no taper at all from chamber to throat?

source

Accuracy in fixed barrelled guns is absolutely superb, BUT not from the ill-conceived forcing cone chambers the industry is forced to adopt in order to march lock step with SAAMI, insurance companies, and our sue happy American legal system.

What would prompt SAAMI et al to go for a forcing cone at all? Possible slight misalignment in wheel guns?


I'm trying to wrap my head around chambers, throats, forcing cones, rifling, boolit optimal, j-word optimal, et al.

[S.R.Custom]

...Accuracy in fixed barrelled guns is absolutely superb, BUT not from the ill-conceived forcing cone chambers the industry is forced to adopt in order to march lock step with SAAMI, insurance companies, and our sue happy American legal system.

That's not why factory barrels have big fat forcing cones. They have those forcing cones to accommodate a wide variety of MFR ammunition variations, everything from varying case length and neck thicknesses to different bullet diameters, OALs, ogives, bearing lengths... and to a certain degree, even ham-fisted attempts at handloading cast bullets. Add to that the possibility (inevitability?) of concentric mis-alignment in the manufacturing of either ammo or rifle (or both) and you can see the need for helping the bullet hit the hole in a not so subtle manner.

No, the factories don't do a big ugly forcing cone because the man's keeping them down, but because a factory rifle has to choke down the widest variety of ammunition possible without puking. If a rifle pukes, the owner gets pissed. And if the owner gets pissed, he takes his gun back to the store and talks bad about the MFR on internet chat forums. And that costs money.

There's more to it than that, of course, like the role of SAAMI and how they came to be the arbiter of industry standards, but that's the underlying motive.

Looking at the forcing cone vs throat routes for .357 Maximum, I get what he's saying about the forcing cone (are forcing cones always bad?), but his drawing makes it look like it's a perpendicular, hard, 90 degree sharp edge from chamber to throat. Surely this cannot be, as boolits could get shaved, yeah?

Actually, that's a pretty crappy drawing. What it doesn't show is the thickness of the brass. But looking at this drawing, one has to make the assumption that the brass is at least as thick as where the outline of the bullet meets the case. Which is inside the line of the throat. So no, shaving would not be a problem.

Just about all chambers have a forcing cone to some extent. I'm currently building a customer's rifle in .308 Palma. That's pretty much a .308 Winchester with a minimum throat. It's a hardcore competition chambering, but even still there's a freebore component and a leade angle. (In the case of this particular Palma chambering, it's a .060" FB and a 1.5° angle. If you do the math, those two will give you the opening diameter of the throat, but I don't care what that is; I'm going to seat my bullet out far enough that the ogive tangent point is jammed up against the end point of the freebore at .060".)

Also, barrels. What's the rational for choosing between one groove option over another?

Cost of manufacturing, and how much the MFR is willing to compromise cheapness for accuracy.