Bit of barrel play, normal or dangerous?

[Can-down]

Yes, a metal spacer ring is what I thought of to fix this also. I had tightened the mag extension to the point it got stuck, but the play was still there. Probably too little threading on the extensions as one poster suggested.

What kind of mag extension do you have?

If it's the kind that separates the tube and the nut then you should try as was suggested to put the nut on without the tube and tighten it up then when she's snug thread the tube in and clamp it.

If your tube is threaded to far into the nut it will stop the nut from threading in far enough to engage the lug on the barrel.