Depends on what you want the gun for. I have some good action design books by people who make them from scratch, but it's also not that super hard to just buy one.
Gattling bun barrels are made to shoot patterns, not groups - you'd do better to just buy an accurate barrel.
I have a pdf somewhere from NTIS where they mounted accelerometers on barrels to measure the "Whip" during a shot, so that they could optimize the thinness of the barrel to be deliberately inaccurate. They want a shotgun-like effect for those guns.
An old M2 barrel might be ok, but...if it's old, it's likely worn or rusted.
No point shooting something that costs over a buck a round (even if you do the loading) if you're not going to hit what you're aiming at, after all. 50's aren't that much fun to shoot, even with the Barrett muzzle brake, and mine is a trip to the doctor unless you just lash it to something and then use a lanyard to pull the trigger.
I'm looking for that paper now, but it seems to have "walked away" from my pryo stuff. I have lots of saved home and public stuff from prior computers, but for some reason the documents directories are all suddenly empty. Hack? Luckily I still have the old machines, not turned on for years. I'll have to fire one up.