On average, a 338 Edge will get you around 2850 fps with a 300 gr SMK in a 30" class barrel. This is on average. Some slower barrels will be in the 2800 fps range, some fast barrels will get you 2900 fps and yes there are a very few fast barrels that when loaded to very top end pressures may get you around 2950 fps. In all honesty, I have seen this with one rifle in the several dozen rifles I have chambered in this caliber.
I have also built many custom 375 RUM rifles with 28 to 30" barrels and yes, the 375 with the same weight bullet will get you higher muzzle velocity then the 338 caliber shooting same weight bullets, BUT, not dramatically more. Where a 338 Edge will get you 2850 fps average velocity, the 375 may get you another 100 fps tops with same weight bullet.
Now if you figure you get a very fast barrel and your pushing the load to top practical pressures around 65,000 psi for RUM brass, its totally possible to see a 375 RUM get you 3050 fps with a 30" barrel.
Now improving the shoulder to a 50 degree angle will get you VERY LITTLE measureable powder capacity increase on the RUM case simply because the RUM parent case is pretty darn square already with min body taper, a simple shoulder anlge increase will likely increase powder capacity less then 2 grains. That amount on a cartridge that has a +100 gr powder capacity is very minimum and will result in very little velocity increase when loaded to the same appropriate chamber pressures.
Certainly we should not be attacking anyone for any reason, it is certainly their choice to do what they want with their rifles but my concern is when someone posts velocity results on a public forum with some members or guests that may not have alot of experience and they think that if a 375 Tejas can get 3200 fps with a 300 gr bullet, They should easily be able to get 3100 fps with a standard 375 RUM with same bullet. This is simply not wise to atempt.
My main concern is that everything does not always go perfectly. There is always a risk of getting a piece of brass that may have a flaw or is weaker then they should be, put in a 70K pressure load and bad things can happen quickly. Anyone that has had a case fail in a modern high pressure rifle learns very quickly that there are no advantages to pushing a case harder then its design limits.
Lately I can not believe the amount of talk about smaller chamberings coming very close to matching the performance of 408 CT class wildcats in 338 and 375 caliber. From someone that has alot of experience with all of these large class chamberings, I worry that many are running the ragged edge. My concern is more a safety issue then anything else.
Hell, I have customers that are breaking 3450 fps with my 338 AM with 300 gr bullet weights when the load I recommend them to use is right at 3300 fps. Its just human nature I guess but it is not what I recommend and I use these things every day of the year.
Nothing more or less, I just know from testing MANY, MANY rifles in this class that these velocity levels are WAY over even what the exceptionally fast rifles in these chamberings are producing and that should trigger red flag warnings all around.
If your only getting one or two firings on your brass cases, your WAY over pressure and you better hope and pray that you do not run across a weak case or you to will quickly realize how unpleasant this combo can be. It is really no fun at all.