Loose A-5 Flatline

Well I would just send it in to Tippmann, Im sure theyll replace it free of charge.

Give them a call on the phone and tell them about it and most likely theyll tell you to send it in.

You need a new barrel adapter.  Same thing happened to me. Just order the part and screw it in like you would normally and you should be fine.  

If the Adapter is worn down you'l need to return it to the company you bought it from... If you got no warrenty you're out of luck my friend. Tippmann will only replace the gun, I don't think they will replace your Flatline. If it's the reciever that are worn they "might" replace them.

I tried than but it just growled and bit me

I have had the same problem but it was my fault.  When I polished the internals on my A-5 I did such a GREAT job that I polished the receiver down baby smooth.  Well too smooth.  So what I did to fix the problem was to lightly spray just the receiver part down with Duracoat paint (black) and let dry.  I will have to do this from time to time but it works.  Just be careful not to overspray all over the internal of the receiver halves.  Only spray the part of the receiver where the barrel locks into.

I had a similar problem with the Flatline being a bit loose.  I suppose Tippmann might fix it if I sent my gear to them, but I found that a quick wrap of teflon tape made the problem go away.