I have a question regarding the 1:7 twist; I've really got my eye on the Colt M6400 M4 clone, and I'm aware the twist is 1:7.  All of my other AR's are 1:9 and I was wondering if it did any damage to the 1:7 barrel to shoot 55gr ammo, as I have an ample supply of it.  I realize accuracy will vary, but I don't want to damage the barrel on such a high dollar gun.
Thanks a lot in advance, as you can tell I'm new here.

I shoot 55gr in 1:7 barrels all the time.  It is nothing to worry about other than your accuracy at longer distances will not be as good with the faster twist and lighter bullet.

Ditto what Raptor22 said!  The 1/7 will work just fine with the 55 gr, I use a lot of XM193 on my 1/7 barrel and it works great!  The biggest advantage of the 1/7 will be the ability to use the heavier good stuff like the 77gr. Nosler OTM bullets.

I actually like the 1/7 so much that from now on I'll probably only get the 1/7 as my standard barrel.  The only other twist I will consider is the 1/8 for a super accurate SPR clone...[;)]