I am bringing up this slightly older thread. I have been working up my M-70 in 416 Rem for my first cape buffalo hunt this Aug and came across this thread.
My observation is this. It's not sharp shoulders that cause sticky extraction. I personally run a lot of 30-40 degree shouldered cases, as do our employees and customers. Sharper shoulders can usually be run hotter than shallower shoulders before showing pressure. I know that is counterintuitive, but let me explain.
When a cartridge fires, several things happen. The primer slides back, the case expands, the neck expands off the bullet, and the bullet is launched. Yeah, we all know that, it's been in the Hornady manual for decades.
However, most of us don't consider that when the case expands it stretches the action. The bolt gets pushed back, then it slams back with a vengeance.
How do we know that? Decades of actions makers and rifle builders being concerned about bolt thrust. The most extreme example is probably the poorly heat treated late M-98s when the bolt lugs would actually set back into the receiver. Actions built with modern steels aren't nearly as bad, but they do stretch a little. A large, tight fitting BR action probably stretches the least, and may explain why BR guys can get away with hotter loads, and a small factory action (like a Tikka) probably stretches the most.
With that in mind, all of a sudden we see how a shallow tapered case with a shallow shoulder case will be worse for handling pressure than a sharp shouldered case. The more tapered case will wedge in more tightly than a sharp shouldered case when the bolts slams forward. The sharp shoulder provides a stop for the that last action of the bolt slamming back forward and is less likely to wedge in tightly.
So if anything, a tapered case with a shallow shoulder would be worse for pressure. But what about when someone has a rifle with a tapered case that handles pressure than one with a sharp shoulder?
This is where working in the industry in a place that builds rifles and chambering my own rifles on my own lathe at home provide insight.
The groove diameter of the particular barrel as well as chamber dimensions seem to be much bigger factors than action stretch in terms on a rifle handling pressure.
Groove diameter varies along the length of a barrel bore. It's eye opening when you start chambering barrels and are checking to see which pilot bushing to use. I have seen an easy half thousandth variation in diameter. Of course that is bore diameter on the top of the lands, but it does give an idea of the consistency of the barrel.
Generally, the tighter the bore the faster it shoots, within limits. That is counterintuitive as well, but I know a barrel maker who has done such testing. There is a catch. If the groove diameter is tight immediately in front of the throat, the rifle will show pressure sooner. We have fixed those with a bore-rider throat and sometimes with fire-lapping, though I hate fire lapping.
A perfectly dimensioned barrel would be at spec at the throat and then slightly tighter at the muzzle. That is what all custom barrel makers strive for when lapping a barrel. These variations in bore/groove dimension are a likely factor in why that probably 1-2 of 100 barrels are "fast", and 1-2 of a 100 barrels are "slow"--give or take.
The next big factor affecting pressure handling is chamber dimensions. We have seen that we generally need a chamber that .0045 to .005" larger at .200 line than new brass to avoid "clickers". While clickers can be a sign of high pressure, when they happen with mild loads, clickers are more of an indicator of a chamber/sizing die mismatch. Too tight a chamber will give clickers, and an excessively oversized chamber can have clickers as well--both with mild loads. A sizing die needs to reduce the diameter of the case at the .200 line, IIRC, around .0015 to .003--depending on the cartridge, to avoid low pressure clickers.
With a custom built rifle and a lathe operator that knows what he is doing; the chamber will be straight and min SAAMI/CIP spec or very close to it. You really don't want it any tighter.
Now with a factory rifle, chamber dimensions are all over the place. We have an employee that formerly worked for a very well-known rifle company. That company would order finish reamers at max SAAMI spec and then replace them when they wore down to min SAAMMI spec. I would suspect that all the big OEMs do that, and it explains what we have seen with chamber sizes on factory rifles. If they use a reamer too many times, the chamber will be too small and you will have all kinds of pressure issues. Too large isn't as much of a problem unless it is way oversize.
This isn't a slam on OEMs. My shop can maybe do 150-200 rifles a year right now, while the OEM can do what, thousands?
Now some here will reject everything I have posted and because their experience has been the opposite. I am okay with that. We know a lot more about WHAT works than WHY. The WHY is theory and speculation, the WHAT is actually results. Most of these theories and ideas work maybe 70-80% of the time, meaning there are always lots of exceptions.
