I am most certanly not an expert either, but I'd like to see some modern testing on backthrust coparrisons of various ctgs. although it probably isn't necessary.

P.O.Ackley's testing years ago is all I've seen & studied and it was quite dramatic in it's findings. Handloading Books #1 and #2 by P.O.Ackley.

All things being equal, from chamber pressure, chamber condition (smoothness), brass condition and chamber lubrication, the case with the most taper produces the most back thrust.

Since the .300H&H and the .458 have identical CIP standards of 62,000PSI, I'd definitely believe that even though slighlty longer overall in case length, the more tapered case would have the most back thrust.

Whereas the .300 H&H has the longer case length, it's wall section (body + neck length) is virtually the same as the .458 Winchester. This makes the chamber wall 'bearing' length of the cases about the same at around 2.4" to 2.5". The sloping neck of the .300H&H is of no use in helping grip the chamber walls, so therefore is actually a detriment, not a help. Thus due to it's taper, the .300 H&H will produce more back thrust - how much more & is this important in this scenario - who knows?

Condition of the barrels is very important in this regards, a dry chamber producing/allowing more holding power at peak pressure and therefore less back thrust than an oiled chamber which allows the ctg. to slide backward more easily. Therefore, if one is dry and the other 'damp' with oil, the roles can easily be reversed.

In this instance, Curl's .300 Win Mag will actually produce less bolt or rearward thrust at the same pressure level than a .300 H&H due to the reduced taper.

When dealing with pressures this high in a DR for use in hot climates, I feel the use of powders that are very little effected by heat is also of vital importance. This is why I always suggest the Hodgdon "Extreme" powders made by ADI in Australia. From hot to cold temps, they hold their pressure and speeds very well in comparrison to the other available powders - even today.

Pressure excursions from heat will increase the potential for a back thrust problem many fold. It isn't just cordite that does this. There are so many variables that effect back thrust, we must 'stack' the deck if possible & an understanding of the forces involved will help us in that regard.