Fiftydriver, eddybo, Dave,
I'm glad to see you guys posting on the subject, and this thread is a great read. It appears many of us are on the same quest right now, as Dave knows. We (GS Custom Bullets) were just recently asked to design an ultra long range .375 bullet for two guys doing this very thing. One is using a 375 Snipe-Tac and the other a special 375/50 conversion. As you all know, the powder is the limiting factor.
In the case of the 50 conversion, we ran into two configurations that worked well, so far, and the guys in Australia can confirm this. One is limited around 150 grains real case capacity and was designed by the SMc crew, Superior Ballistics Inc. Those guys know what they are doing when it comes to powder burn efficiency. Thing is, their case design actually promotes a faster burn, which is not really what we are after here. They did however reach 3100fps with that case using H50BMG and ADI 2218, with the prior being 102% case fill! The other discovery they made is that if you use the US869 at higher pressures, around 70,000psi+, it burns better (and in a shorter barrel), then either of the two prior powders. It was actually the best performer for our specific use, but we declined because of the pressures...yes, we like to be safe, and the spent brass to extract...
They are however offering the case design to the market, if anyone cares to buy it from them. No real agreement was reached, so it's fair game.
The other design was very basic, much like your work, with it being a 416Barrett necked down and improved, while setting the shoulder back a little to get a .8"+ neck. This proved to be the ticket, so far. It's 177 grain useable capacity was just about right (I believe that was actually higher, but they might have rated that based off real powder charge cap, not sure). And the longer case neck worked great for this longer bullet and it looks like (statistical date reported from SBI) the longer case neck will greatly improve throat life, as well as the GS Custom bullet design does, so this is a great combination for the long haul (pun intended).
The dark side of this story is that it is being loaded by a private company that uses custom powders I have no access to. The reported maximum is said to be 3,400fps with the design bullet, 415 grains, and a 34" barrel. And, to thicken the plot, I have heard VV is coming out with N570...
The bullet is finished and the first small order is on it's way here for me to see this thing work first hand. The gentleman with this 375/50 should have it all lined up for us to load up and shoot to 3000m...to see the true potential of this bullet first hand. Interesting thing is it's basically the same design as the 338 295 grain bullet that has been shipped all over the world since 2004. It's stable through transonic and accurate at these ranges, so I am dying to see how this turnes out. On Quickload at least, it shows the 415SP outrunning the 350LRBT (Jamison) bullet supersonic by 500 meters, worst case. So I cannot wait.
In my small and realistic opinion, I think what Dave is doing with the .688 case is a fantastic idea. It might end up being the best option for a so-called standard sized action (non-BMG). The case is also the base for the 577 T-Rex from A-Square. They are making their own cases, and I am hoping they have improved their quality in this area to make the case feasible for use at 65,000psi., and consistent. Expensive to say the least though.
I must add though that I have magnum-itus as much as any guy here does. And I cannot get enough of the stuff we are doing here. Eddybo, rock on man! Just let me know if I can help with anything as it is my passion as well, aside from work. I've been toying with this for over 2 years now.
Forward ignition is the right idea, I believe. You would be best off making a small reamer to open up the primer pocket and allow a SS or brass tube with a T-flare. The tubes can be rather simple and cheap, and VariFlame might be able to help you out with that (spoke to them a few years ago). Looks like they will outlast your brass many times over.
Fifty, If you are ever interested in trying the GSC 458 566SP bullet against the Lehigh/Predator bullets, give me a call. It has been extensively tested from a 460 Steyr with great results! GS CUSTOM USA - Contact