Barrel blocks are custom made items.
You can design the blocks and have a machine shop make them up out of aluminum, brass, or a hard "engineering grade" plastic.
OR, you can make your own from hardwood blocks made of oak with the interior matched to the barrel by using epoxy bedding compound to make a close fit to the barrel.
These are made by routing a groove for the barrel, coating the barrel with release agent or wax and bedding them one side at a time with a good tough epoxy.
When both blocks are on the barrel, you want a slight gap between the blocks for good clamping power.
After the epoxy has fully hardened, use a Dremel to open up key areas so the blocks will fit all barrels of that size and type.
You usually need the clearance work because barrels do vary slightly in size due to polishing variations, and some variation in barrels made during different years.
With standard modern S&W barrels you don't need full length blocks, just short sections about 1 1/2" long is usually enough.
These blocks are used in a good bench vise. I had an old used Wilton that I had the jaws modified to hold barrel blocks.
For round barreled guns you need a real gunsmith's hydraulic barrel vise as used by rifle builders, only it can be a smaller size.
The round barrels are much harder to get a firm grip on so the hydraulic model is needed to insure a barrel won't slip in the blocks and strip the finish.
Treat the barrel blocks VERY carefully.
After molding and clearancing, clean the blocks with hot soapy water or paint thinner to remove all release agent and clean them TOTALLY clean.
Any slightest bit of grit will embed into the blocks (even the aluminum or brass types) and will scratch the gun's finish.
Mark the blocks clearly as to the make and model of barrel they were made for, and mark them with arrows so you get them on the barrel in the same orientation each time.
Store the blocks in individual plastic bags and store in a clean safe place where they won't get tossed round.
Before using a block, wipe it clean with a cloth, and clean the barrel itself of all old lube or dirt. Don't lay it around on the bench where it can pick up grit. It comes out of the bag and immediately on the barrel.
Immediately you're finished with it, clean it again and put it back in the bag.
