Yes - belts are said to correct a headspacing problem with small case shoulders - everyone knows what is claimed.

There are guys here who shoot .400 Whelens without any headspacing trouble and they have just about as much shoulder as the .400/.375 H&H. Is care in loading this tyye of round required - absolutely, one must know what he is doing. As far as gun writers saying similar things, there have been some. Most jumped on the .35 Whelen headspace trouble, then Remington commercialized the round and those writers who followed 'the old wives tales in writing this, because they were paid-by-word, were left looking mighty silly, when new handloaders who bought .35 Whelens had no difficulty loading for it. Friend of mine was one. I helped him. I could name a few of those writers, but some might have passed on now and that would somewhat distasteful.

I have a counter Challenge - name one modern belted factory ctg.(Post 1906) that REQUIRES the belt for proper headspace.

Neither the .300H&H's 8 1/2 degree nor the .375's 15 degree shoulder angles NEED the belt that H&H put on those cases, due to the gross difference between the shoulder diameter and the neck diameter.

I dare anyone to create excessive headspace by firing a .300 or .375 - one with belt turned off, with the case then properly headspaced on the shoulder. (have to neck it up then down for crush fit, then fireformed. Only neck size part of the neck, not touching the shoulder and fire it with a factory equivalent loads - 3 or 4 times or more and see if it produces excessive headspace. Will NOT! The belt was not needed.

Factory equivelent loads of 1912 will suffice. 2,600fps for .375 and/or, .300 H&H with 180gr. at 2,750fps - Barnes COTW, cordite loading. Belts turned off will present no problem at the pressure levels that will present - as noted, headspace must be produced on the shoulder, properly. If you do not understand this, do not attempt it.
According to Lee- 60.7gr. IMR4350 will do the trick in the .300 and in the .375 H&H, about 76.0gr. AA2700 should do it.

Belts were something new in the early 1900's, needed, perhaps for the ill-designed 1905 .400/.375H&H which was almost no shoulder and the possibility of pushing back the shoulder with the bolt, a distinct possibility. The belt was most certainly not needed for the two H&H rounds listed above, nor for any other modern belted magnums that I am familiar with.

Challenge to you - name some. Name one! Name a current belted magnum that NEEDS the belt - a ctg. that does not have enough shoulder for proper headspacing.

Yeah- that was overly long. Reduced to about 1/3rd.