The barrels, which are mentioned among the Shipley goods, may have been used for the simple process of cleaning armour by rolling it in a barrel with sand and bran, as is still the practice in the East. Such an article is found in the inventory of Dover Castle, 1344 : " 1 barelle pro armaturis rollandis;" and in that of Hengrave Hall, as late as 1603: " 1 barrel to make clean the shirts of maile and gorgetts." — See Arch. Jour., xi. 382, and note, p. 386. In the Monasticon, vi. 625, some land is held " by the service of rolling a coat of mail once a year."Sussex Archaeological Collections relating to the History and Antiquities of the County. Published by the Sussex Archaeological Society, 1857 p. 255
I have experimented with rolling mail to clean it, using play sand, corn cob pet bedding or litter, and a 50/50 mixture of the two. I used a small 1.25 cubic foot cement mixer from Harbour Freight. You can buy tumbling barrels designed for the purpose, but one big enough for a mail shirt is a lot more expensive than the small mixer.
Any of the above will eventually clean off rust after several hours of rolling. Play sand leaves a clean but somewhat dull matt finish. Corn cob leaves a significantly brighter polish. A 50/50 mixture is unsurprisingly somewhere in between.
Walnut shells are also sold as pet litter. Both corn cob and walnut shells are used as polishing media by ammunition reloaders, and their suppliers also sell those media, more carefully graded for particle size and more expensive than the pet litter. There seems to some debate among reloaders over the relative merits of corn cob versus walnut shell, and whether the specialized media marketed to reloaders works any better than less expensive pet litter.
Play sand is probably not ideal. References to tumbling barrels in late 19th and early 20th century texts often refer to “sharp sand” as a medium. Also, a lot of modern play sand isn’t silica, because of concerns about the risk of silicosis, and may not be as effective as an abrasive.
Some of the texts mentioned above suggest that you should use about as much medium by volume as the metal you are trying to polish, and that you want the barrel full enough that the work will tumble rather than slide.
The same texts describe a variety of polishing media. Emery, pumice or sharp sand is relatively aggressive. Leather scraps or shavings are often used for a final polish. Hardwood sawdust is often described as an intermediate medium, sometimes with a modest amount of emery or rouge added. If bran and sand were still used to clean mail in the 19th century East, perhaps the mixture used mostly bran with a small amount of sharp sand.
Update: Bertus Brokamp found a reference in the accounts of the count of Holland, 1360-61, to bran purchased for, among other things, "to scour the armor with"
Another update "Paid, the xv daye of Julye, at the campe at Dunglasse, by th’andes of George Ynglyshe, for tow urynalles and one skeyn of threed, vjd.; for canvaus to make a bagg to scowre my Lordes shyrt of meale in, xiiijd.; and for brane to the same, ijd.;" 1549
Great Britain, Charles Manners Rutland, John James Robert Manners Rutland, H. C. Maxwell Lyte, Richard Ward, Robert Campbell, and John Horace Round. 1888. The manuscripts of His Grace the Duke of Rutland ... preserved at Belvoir castle. London: Printed for H.M. Statioery Off., by Eyre & Spottiswoode.