The bolt carrier I’ve been waiting for forever finally showed up this week. Unfortunately the gas key is catching on the gas tube so the action won’t completely close. The new bolt carrier assembly seems to function in my M&P 15 upper and the bolt carrier assembly from the M&P 15 seems to work fine in the new upper. If I loosen the screws in the gas key so that it will wiggle a little it cycles OK.

Any suggestions on what I need to work on here? Bolt carrier assembly? Gas tube? Both?

Thanks

Tag for out come..

Do NOT loosen the gas key screws.  You will now need to re tighten and stake the key.  Assuming the Young's gas key isn't bent or otherwise damaged (compare it to your existing carrier), you would bend the end of the gas tube slightly until the carrier gas key slides over it with no interference.  You test this by removing the bolt and cam pin from the carrier.  Turn the upper upside down and elevate the upper receiver about 30 degrees.  Allow the carrier to slide down and it should engage the gas tube properly and end up against the breech.

Guess I didn’t go into enough detail on this part. The Young gas key was not staked to begin with and I just loosened it to see if I could get it to cycle with some play in it and it does. I had no intention of running it with the key loose.

My confusion with the whole process is that the Young bolt carrier seems to fit in my Smith and the bolt carrier from the Smith seems to work in the new upper. This made me unsure if I should focus on the gas tube, which works with the Smith BCA, or focus on the gas key, which works in the Smith upper.

Sounds like I need to work on the gas tube?

It sounds like you need to follow Swatter's advice and restake the bolt carrier key. It should have been properly staked first place before you fired the weapon. If that key falls out while you are firing you can very easily damage your upper receiver, charging handle, BCG or all of the above.  That wiggle is only temporary, yes it will allow for an interface with the gas tube, albeit a banging home rather than sliding. At some point, (very soon) it will just fall off and then the whole project is total FAIL. First things first, once that's done you can function test the weapon and if it still doesn't work at least it's not the carrier key. Don't forget when you torque it that there is a profound difference between inch pound and foot pound, or in proper English, pounds feet.

He never fired the weapon, he was just fitting it.