Finding a source for a 450B barrel to install on a bolt receiver is not as easy as finding sources for a blank for a.308 Win. Here's a summary of some advice I've given to a couple of posters who have asked about the barrel on my bolt rifle, with some further information.
(I purchased my barrel from Numrich about 21 years ago. They don't have any more of these barrels. I don't know who made it for Numrich.)
Lothar-Walther is one possibility. Email and phone contact info is on the home page at lothar-walther.com. I cannot find a .451" or .452" barrel on their web site. However, according to my gunsmith they stock .451" barrels in normal steel at 1" dia by 23" length, with a 16" twist. They may do a custom twist without a lot of trouble or expense.
My gunsmith has installed a fair number of the L-W barrels, and he recommends spending the extra money to have them turn it to a contour you like, rather than having a smith do the contouring. Their finish is so good on such jobs that he's had to do nothing more than a final polish, which saves the customer $$. His guess is that a barrel will cost about $220. L-W makes good barrels; I've seen several, and their internal finish is very nice.
Another possiblity is McGowen rifle barrels: mcgowenbarrel.com . I've never seen one of their barrels, but some reviews say they're pretty good. They are one of the few manufacturers that list a .451 barrel with either a 24" or 16" twist. Length can be about as long as you'd care to carry. Their web site is flaky on my browser. They might produce a contoured barrel a bit cheaper than the L-W.
Brownell's lists a .451 blank by Shilen, 28" long with a 1-16" twist. As far as I can tell, Shilen doesn't list this barrel on their own web site. Shilen barrels are generally well regarded.
Brownell's also list a .451 blank by Douglas, 27.5" in length, 1-16" twist. Douglas has been making good barrels for a very long time.
There are undoubtedly other sources for .451-.452 rifle barrel blanks. (I suspect Wildcatter knows of some.)
It will be helpful to the 450B community to post sources in addition to the above, and to describe successes and problems in procuring barrels.
--Bob