Please affirm or dispute my logic.

I have developed an 8mm Wildcat. It is based on a shortened and blown out 300 (or 338) RUM cartridge and holds 105.0 gr of water full to the top. The 8mm Rem Mag holds about 98 gr, and the 340 Wby Mag, about 103 gr. Logic and experience indicates that my max loads should fall between the two, i.e. greater than the 8mm Rem Mag, but less than the 340 Wby for all bullet weights and powders (in fact given the practice of Wby loads being on the high side of safe pressure and their usual use of a long throat, I would be wise to stay well beneath it).

I intend to load using 8mm Rem Mag max loads plus about 3% (i.e. about 2 grs) and stay at least 4% below 340 Weatherby max loads (i.e. about 3 grs). I can't trust the classic high pressure "signs" (primer flattening, stiff bolt lift, etc.), and will work up an accurate load anyways, but as with any wildcat, am left to my own devices to determine max loads.

Any thoughts, advice, experiences would be greatly appreciated.

Well, I like the big case, lower pressure idea, so if it were mine, I'd just run at about RemMag speeds (or maybe a wee lower, say 200g/2950-3000) and take pride in the fact that it is so gentle on the gun. The Rem does waaaay plenty, and if you need to go .3g more, work up to it safely, and tell all your friends it beats the Remington. Keep in mind you have more boltthrust than the Remington, even if you're running at "only" 60k, and if it's design was in part intended for the k98, you want it to perform within the realm of reason. Put it in a bigger action and you somewhat dull the point of its inception, n'est-ce pas?

Bwana-be,

Wise words that I will put in practice. I will hunt with this gun, and with plenty of power at hand, I want an accurate load without excessive flinch-inducing recoil. For interest's sake though (it is a new cartridge), I'd still like to explore its potential.

As for bolt thrust, I have assumed that the 8mm ARP would have less bolt thrust than the 8mm Rem Mag. The 8mm Rem Mag is longer, but the 8mm ARP has more surface area, and less taper (about half that of the 8mm Rem Mag). Did I miss something?

Well, if bolt thrust is determined by the size of the rear of the case and the max pressure, then the fatter case at a given pressure will have more bolt thrust. True, it would be mitigated by the amount of taper and therefore coefficient of the brass against the chamber on the way back to the boltface, but I don't know by how much. The Rem already has a more-or-less standard taper, fairly small compared to the old days, so unless you went to something like .06:1 then you probably arent changing much, in my uneducated opinion. Maybe one of the engineers around here will help out with the math.
Truth is, a good Mauser, especially one reheat treated or otherwise check-out for suitability, could probably do the job. I just know there are a number of smiths who don't like the idea. Most Mausers with that size (fat) case in them will likely be Jefferys running at 50k or whatever. Not a lot of data, if you know what I mean. I'm sure there's a number of Dakota chambers in Mauser actions - which is essentially what you have, an 8mm Dakota of sorts - and they probably are doing just fine at 62k.
Be safe.

I found a guy who knows what he's talking about.

Good points. The bolt thrust at any given pressure is indeed higher in the 8mm ARP than the 8mm Rem Mag, an estimated 12% by comparing casehead surface areas. What I have assumed, is that the 8mm ARP, with taper of 6.4 thou/inch compared to 10.8 thou/inch for the 8mm Rem Mag, makes up for that. My assumption was not wholly scientific, but based on logic (how a wedge works), and anecdotal (legendary?) accounts of Ackley observing almost no back thrust on minimum taper cases in dry (i.e. free of oil) chambers. I do employ dry cases and chamber, but I confess that I am working on guesswork.

All I can say for sure is that bolt thrust is no greater than 12% more than for the 8mm Rem Mag. It will be less than 12% due to decreased taper giving less of a "wedge" effect, and the greater surface area (~6% more in the 8mm ARP) giving more "grip" on the chamber wall as the pressure forces the cartridge surface against it. It could conceivably be less than for the 8mm Rem Mag, but that is a guess - the point of your post.

There must be some egg-head who has already done the math.

In homine mens regalis est - but we can still screw up

Andy, I have a very similar wildcat I built on a 98 action that wasn't heat treated.The parent case was the .404.Bwana-b has it right about bolt thrust effect. Even with reduced loads now I get hard bolt handle lift. Set back is real and more so with fat cases.The bolt thrust increase for the size differential alone is about 17%. When you start going to 60,000 psi verses 50,000 psi (Which most wildcaters do) you have an additional increase of 25%. A total increase of over 40% might need some thought.

Well you have my attention. To be clear, I want to see how it can perform at sane pressure levels, i.e. its potential, and should be done in about another 20 firings. From there, I intend to load this with less than max 8mm Rem Mag loads, which will operate at a lower pressure than the 8mm Rem Mag by virtue of the larger case and hence compensate to some degree for the greater bolt thrust.

You experienced lug set-back and consequential hard bolt lift - I will watch for that.

Please comment on the effect of less taper on bolt thrust. This is a widely accepted principle, and of course there is the well-known experiment conducted by Ackley. Nonsense? Exaggerated?

quote:

Originally posted by Andy_P:
Please comment on the effect of less taper on bolt thrust. This is a widely accepted principle, and of course there is the well-known experiment conducted by Ackley. Nonsense? Exaggerated?

I can not offer anything usefull regarding tapper vs. bolt thrust.To say anything would be conjecture at best. Never played with it. roger

I have "researched" this and reflected on it and am unable to conclude anything more than I must assume that: compared to other cartidges (regardless of taper and surface area), I will see an increase in bolt thrust proportional to the increase in casehead area (and any pressure increase). Consequently - "beware".

Strong cases have been made elsewhere that cartridges grip a (dry) chamber so well that little is needed to keep them in place during firing (all refer to the single and famous ackley test with an "improved" cartridge), hence little bolt thrust. Others contend that is a moot point, because it is not cartridge movement backwards in the chamber that exerts force against the bolthead, rather it is stretching of the immobile cartridge in all directions. Casehead expansion is a symptom of this. Some things continue to be unexplained, for example, one of Newton's laws indicate that the thousands of ft-lbs force of the exiting bullet will have an equal force in the opposite direction, i.e. towards the boltface. If the cartridge is gripped fully by the chamber, then the only force against the boltface will be of the stretching cartridge, the other forces being borne by the chamber and those parts attached to it (i.e. receiver, stock and the shooter).

It would be a great research project to calculate various forces created during firing: their sources, direction and magnitude. Until then, we can only guess (and discuss).

quote:

Originally posted by Andy_P:It would be a great research project to calculate various forces created during firing: their sources, direction and magnitude. Until then, we can only guess (and discuss).

I totally agree that this would be interesting, however, there is so much else to play with and have fun doing and so little time left to do it.I kind of like to invest my time into stuuff that I can eventually touch, feel and play with.

Testing powders, bullets and rifles takes up a lot of my time.I've designed a thousand wild cats on paper,and that's fun, but Have only created 8 that I can shoot.

on your quest roger