The Centurion Arms C4 Rail Handguard does not come with a barrel nut.
The manufacturer doesn't specify if the Centurion Arms C4 Rail Handguard will fit DPMS Pattern AR10's. I recommend contacting the manufacturer for that information. Here are some options for handguards for a DPMS Pattern rifle.
https://www.opticsplanet.com/handguards-and-forends.html?_iv_gridSize=120&_iv_parent-gun-make=dpms
Yes, the Centurion Arms C4 Rail Handguard will work on an M16 A4 clone.
These are the updated versions of the Centurion Arms C4 Rail Handguard, so there are no white markings.
Yes this will fit POF puritan if it is AR-15 style. Make sure to measure the gas tube length to get a proper hand guard. If anything can contact us at techsales@opticsplanet.com and message us with the measurements.
It will work with the piston gas block, but may not work with the piston itself. We recommend checking directly with Adams Arms as this handguard is not on their list of compatible handguards (but that is not an exhaustive list). You can reach them here: https://www.adamsarms.net/contact-8