I just received a new custom contender carbine barrel. I put it on a new G2 frame, that has never been used.
The hammer will not stay back when I cock it.
That is the case with the barrel on or off.

Suggestions.

I do have a call in to the barrel maker.

I contacted the barrel company, match Grade Machine.
They apparently put in bad lugs in the barrel
So I waited about fifty days for a barrel that was supposed to be done in about 30 days.
Now it does not work and I get to spend $20 to return it and wait some more.
I leave in 48 hours on a PD trip which is why I bought this barrel.

Don't ask me how happy I am with their customer service.

Next time around, give Bullberry a ring.

I'm surprised.
MGM has been awesome for me.

Long story. Sent the barrel back to MGM. It worked fine for them. I sent my action and they got it to work. Not sure exactly what they did.
Their communication and speed of resolving this issue was great and they returned it via priority mail so I got to shoot it today. MGM did all they could and as fast as they could to resolve the issue.
I fire formed some 20 Ackley hornet loads and the barrel is very accurate.

I am glad your issue was resolved. I have four MGM Contender barrels and all shoot great.
Bullberry has been excellent as well.

I had a similar problem but mine was with the factory barrels. Sent it back to tc and they actually swapped the frame out for a new one. Not sure what was wrong but apparently it was way out of spec.

I have had the hammer decline to stay cocked when I screwed in the grip/buttstock bolt in too far. This may be totally irrelevant to the OP situation, but its the first thing I check now when that happens.

dale06, most likely situation is the latches are not engaging fully to activated the (for lack of a actual part description) lock out so the action cannot be fired unless a barrel is present,

MGM mentioned is can be an issue. But it was not in my case.