Just a suggestion for easily placing things in the oven:
I made a "T" bar form 5/16 rod (available at any home improvement or hardware store) with the "T" being about 8 inches long and the leg extending completely the full width of the inside of the oven. The purpose of using a "T" is top prevent the rod from slipping out angularly and dropping the part.
I just take the racks out and use this to suspend parts inside the oven. Shotgun barrels, receivers, frames, etc, are just put over the rod, other parts suspended by wire from it. If you're going to use it for barrels make it from appropriately sized brass rod or suspend the barrel from it using bare copper wire through the bore (don't want to shove a rough steel rod or wire through the barrel). You can hook everything up to the gimmick before painting and then transfer it to the oven easily enough, eliminating the potential of messing up the finish in the process.
Use weld, braze, silver solder or any solder that is of a higher melting temp that you will be running the oven at to assemble the two pieces of rod to form the "T". This is one of the simplest and handiest gadgets I've made lately, and takes no almost thought or skill to make.
Of course, most finishes will warn against using the kitchen cooking oven for curing, so I never do this. Just made the suspension rod for the fun of it.
My comments reflect my opinions generated from my experience. Take 'em for what they're worth. If your experience is different that's fine with me. Hopefully, you feel the same.