Just put a new barrel on my 3gun ar. went from a carbine length system to a mid length. reused the gas block with the mid length gas tube. gun wont cycle the second round. first shot will eject but wont pick up the next round. im using the factory buffer and spring that came with my stock DPMS rifle. buffer weights 2.8 oz. do i need a lighter spring or buffer?

i pulled out a heavy buffer that i had in my AR pistol and no joy. still wont cycle.

pulled it down again and there is carbon marking around the gas hole on the barrel. no blockage in the tube as i can blow thru it. i think its lined up correctly. brass ejects at 3 o'clock.

What size gas port?  What gas block?  Is the gas block getting a good seal?  Is the new gas tube fitting into the gas key properly?   Will it lock back on an empty mag?  Is the new gas tube leaking a lot where it enters the gas block?

tore it all down today and measured and took pix. the gas hole is .75 on the new barrel. same as the old barrel. Gas block is a low profile with set screws. it has been used on a couple barrels.

I did notice a bit of wiggle between the gas tube and block. took it out and you see pix below. this is only about 10 rounds fired. i think im seeing some carbon around the tube where it leaves the block.

I think my next step will be a new gas block.

I've been witness to 2 gas blocks in as many years that developed hairline cracks in them. Very hard to see unless I sprayed them with brake cleaner. Anything would work but the BC evaporates fast on the outside but stays "wet" in the crack. It exhibited the same symptoms as yours.

Did ya beat that thing with a hammer, or what?

thanks. i ordered a new gas block. hope that fixes it

UPDATE:

New gas block installed and problem persists. i know everything is lined up correctly due to some carbon marking around the barrel port.

i threw on the old worn out barrel and carbine gas system and it ran like a champ. i give up. im just gonna buy a new carbine length barrel and sell this mid.

Need more info.

You mention its a mid-length, but what is the barrel length? Also, are you 100% the barrel's gas port diameter is 0.075"? If so, your gas port may be undersized which is causing your weapon to short-stroke.

if you are game to try one last possibility, check the end of the gas tube where it enters the gas key. if it is out of spec because of wear it will leak just enough gas to cause function problems.