There are a few old-school smiths who still build centerfire dangerous game rifles in calibers larger than .950 (Memphian Ken Owen, for one) and the Barrett XM109 is chambered in 25mm. So despite the incessant fawning over the JDJ .950, it is not and never was the largest centerfire rifle made .
Ken Owen's *Mastodon* 4-Bore double rifle is not only as suitable for the purpose for which the cartridge was developed, but equals, at least, the craftsmanship and beauty of the offerings of the British and European double-gun builders of the previous Century.
My own entry into those circles had to do with a single-barrel 2-bore, and, eventually, development of a long-range 20mm x139b-based wildcat cartridge for use in the RH-Alan Ručni Top 20 action, [normally found in a 20x110mm chambering] with the neck opened up for a somewhat larger diameter projectile. Somewhat!
But no, you're quite correct that the .950 JDJ, is not and never was the largest centerfire rifle made. Not hardly!