Is it same as a ar15 barrel? Is there minor differences, I would like to replace the m4 barrel with a lighter barrel and put the put the gas block and qd mount on from my oem barrel.

No the barrels are not the same.  Gas port is smaller on the ACR you would need to do some tweasking to make it work.   ADCO or Marvin Pitts can turn down your barrel.

That's what I thought, I'm wondering if I can get a barrel that isn't been drilled for the gas port. I'm probably going to turned down and get the oem barrel fluted.

Also, the new 14.5" and 10.5" barrel kits are a thinner straight profile without the M203 notch. The new production 16" barrels are still the M4 profile with the M203 notch, though they're now made with a more normal 1/7 twist. As said above, while it's an M4 profile, it's not the same.

A gunsmith with ACR experience can convert most AR barrels with a carbine or midlegth gas system for use on the ACR with your piston (although the piston will need to be modified for a carbine gas system). It requires redrilling the gas port, turning a notch for the pistol guide, and drilling pin holes for the gas block, so it's not something most people can do themselves. 

Member bigdog200399 does a lot of work on the ACR and can convert barrels. 

Marvin Pitts is another gunsmith that does ACR barrel conversions, but I don't know what his backlog is these days. 

There's a science to it.  They don't usually use factory AR15 barrels without mods but once in a while you get lucky.  Out of every barrel I've done, I've only had one 300blk that was almost good out of the box. All of the other ones required significant work and/or tuning