No, they don't have to custom at all, the M700 for example has a 375 length mag. I have been using a 300 Sendero for years & it
is not a problem in any way. I just asked a friend about the mag. length of his new Vanguard MOA & it has a long Magnum mag. as others do too!
The 264 bullets are long, but the gripping area is fine, but it was a good thing for writers to worry about at the time. The 7mag is the same way, but some have decided it is not a problem, a pet cartridge for them I guess. In the past I owned 3 7RM & loaded a bunch of those heavy & long 7 mag bullets & neck length was not an issue, not once. The 223 actually has a less than 1 cal. neck & the 75 OTM now used by the Mil. & the 77MK that alot of us use are fairly long bullets, now consider the battering that goes on with mil. ammo, clips, etc., yet
not a problem. The 223 has power issues for the mil., but the short neck part of the equation is a non issue.
That is why the short neck is not brought up by writters the way that it used to be, they have stood the test of time. The numbers tell us that.
7RM is the most popular mag & 300WM is number 2 & I don't see any changes in the near future.
I am looking for a Sendero in 264 now. I know of some already sold & they are extremely accurate, but the owners do not know of the old articles, I won't tell some so they will continue to get those small groups.