this is my first carbine build,   I like the 1-7twist chrome lined barrels 16".   I reload and shoot alot of 69gr and 77gr loads.  my question is;   will the short gas system give me problems with the heavier bullets.  I know the heavier loads use a slower burning powder and will keep the pressure higher as the bullet passes the port and heard of some problems that can be caused  I.E. case rims ripped off the brass or hammering(or maybe it's called bounce?) of the bolt group.  For that reason I am looking into the midlength 16" barrels too,  so far all I know of is RRA   with their SS 1-8 twist barrel.   does anybody make a midlength  chrome lined  1-7 or 1-8 chrome moly barrel?  Also would a 14" barrel w/perm flash hider  work better with this?

At a minimum, I'd go with a full 16" barrel, like the one giffman is offering in the group buy - a chrome lined, midlength, 16" barrel with a 1/7 twist.  18", in the SPR profile or 20" in the SAM-R profile would likely be better for accuracy.  I would not go with a 14.5" barrel unless you knew that all of your shooting would be relatively short range.  Since you load 69gr & 77gr SMKs, I'd go with the SPR profile barrel, maybe a White Oak in 1/7.  I have that setup and regularly shoot 1/2" groups at 100yds and 1.5" at 200 yds.  The 18" barrel isn't much more than a carbine and I'd guess you won't regret it.  If not that, I'd go with a 16" barrel if you are worried about function, but I have a 14.5" setup that runs like a champ with 77gr reloads, so maybe you could go with that.

IMO 1/8 = 1/7.  There isn't much difference unless you are trying to shoot the .mil tracers, which is the real reason that the 1/7 is needed, to stabalize the longer 1/7 tracer rounds.