Personally, I feel the 338 Lapua necked down to 7mm and improved is about as much case capacity as you can practically use with the current powders we have right now. My 7mm AM, same as discribed above will provide around 90% load densities with the 200 gr ULD RBBT and US869. With WC872 you can get up to the 95% load density range but if you add another 15 grains of powder capacity to that you will not be able to use it until Richard gets his 250 gr ULD RBBTs out for us to play with.
Even then however, I suspect the 338-378 necked down to 7mm would still have to much capacity to provide consistant loads.
When I designed my 7mm AM I looked at the 30-378 Wby case really hard and quickly realized that not only was the case capacity more then I wanted, the cases themselves were just to darn soft for good case life at top end pressures. There is very little if anything that a 7mm on the big Wby case can do that the 7mm on an improved 338 Lapua can not equal because of a better capacity balance and dramatically stronger case to work with higher chamber pressures and still offer long case life.
I fully agree with Barney about the 375-408 being superior to the 338 version but realize a couple things, the LRB bullets are not known for consistant accuracy over a wide range of different rifles. On top of that, they are Extremely expensive, far to expensive in my opinion. If they cut the price in half they would still be the most expensive LR bullet out there for the most part.
Barney says they have been performing very well on game at long range and that may well be. I believe every word Barney says and I have talked to him about this as well but in my opinion, that is probably due more to shear bullet frontal area instead of any bullet expansion at all at extreme range. I could be wrong but that is my opinion.
As far as the Sierra bullet options, they are working on a 350 gr and 375 gr version to the best of my knowledge and as of right now, those bullets will only be made for Chey Tac and not available to the public at this time so if you want them you will have to buy them through Chey Tac when they become available.
Again, ballistically, with the right bullet, the 375-408 is superior to the 338 version. Only problem is that the list of bullets available is extremely limited.
For the 338 version there is the great 300 gr SMK as well as the 280, 300, 325, 350 and new 375 gr ULD RBBT from Wildcat Bullets now that the 338 cal dies seem to be working well for the time being anyway. Keep your fingers crossed.
Basically you need to decide what you want to do with this rifle and if you want to be limited to a very select few bullets if you go with the 375-408 wildcat. Also remember you jump in rifle cost and reloading tooling cost dramatically as well as increased rifle weight and overall size and length with the larger 408 based rounds. Certainly not a down point in my opinion but may effect your decision depending on what you want to do with the rifle.
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