That is a good concern. Too short and you are just shooting yourself in the foot though.... Reduced powder capacity and possibly velocity. I have done nothing with a 7mm Rem Mag before, so this is new to me. Most of my custom builds have been 6.5 and under and defiantly not magnums.
Throating a chamber short has it's advantages but it also has its disadvantages to.
If you build a rifle around a particular style and weight of bullet it will sometimes come back
to haunt you because the barrel may not like the bullet and your stuck with rechambering
or at least re throating the chamber.
If you are lucky and the barrel likes that bullet Great. But if it doesent hang on.
Most of us like velocity and more free bore/throat is one way to get it without excessive
pressure.
This will always be a debate and there are lots of opinions as to which way to go. I prefer
building a rifle that will shoot most any bullet well, rather than build one for one bullet and
risk getting one that wont shoot anything with out re doing it.
Free bore is not a problem if the rifle is built correctly (Just like the belted cases) and in many
cases it is a plus.
I recomend a standard chamber to all but the most experanced who can trouble shoot a problem
and work it out.
There are many reasons to use a special throat dimension but none that can't be overcome
with proper planing In My Opinion.
So be prepared for issues if you short throat your mag.
J E CUSTOM