I am looking for an M4 upper for a preban lower and I am curious as to what twist rate I should get.  I know where I can find an upper with a 1 in 7 twist and a 1 in 9 twist but I am not sure which one to get.  
Most likely I will be shooting surplus 55 grain ammo so I am leaning towards the 1 in 7. I have also heard arguments for the 1 in 9 and now I am confused.  Can someone explain which barrel I shoudl get and why?  

Thanks.

1 in 7" was meant for the M855 (SS109) 62 gr. bullet. Some feel it is too fast for even the M855. I believe the twist was choose because of the length of the M856 tracer.

The M193 55 gr. bullet originally used 1 in 12" in the M16A1 ( 1st AR's had 1 in 14") I would the 1 in 9" because it will handle both cartridges well. Also, I've heard that 1 in 7" barrels wear out faster.

I sure someone will give a better technical more correct answer shortly :)

The longer your bullet is the faster your rate of twist needs to be in order to stabilize it.  The ideal twist for a 55 grain bullet is 1/12"

1/7 was chosen as stated previously to stabilize the VERY long tracer round.  It is also used to fire the new 77 grain ammo the military is using.  If you ever want to be able to shoot the new, very heavy, long range match rounds get a 1/7 and if not the 1/9 will last longer and shoot more accurately with light weight ammo.

Im my experience with the .223 Rem in different brand rifles, i have found the following.

1-12" barrels, like on the remington varminters, shoot bullets from 40Gr. to 60Gr. Flat base bullets the best. My Remington 700PSS in .233 will shoot under 1/2" at 200 yards with my 40Gr. Nosler Ballistic tip reloads if i do my part.
It WILL NOT SHOOT THE 68GR. HPBT bullets.
If i zero it with a 55Gr. SP @ 200 Yards,
the 69Gr, loads will shoot 4Ft. LOW and 8 Ft.
TO THE RIGHT at 200 YARDS. I actually missed a  4' by 4' target at 100 yards, 5 of 5 shots with the 68Gr. load once...Pretty wild...try it if you like.

1-9" barrels will shoot the 52Gr. to 69Gr.HPBT the best. Savage and the newer Remigton PSS guns are 1-9" twist.  You may be able to shoot a 70Gr. Flat-base bullet i it also. They don't shoot the lighter bullets as well as a 1-12" gun will, but they are acceptable for short range work with the 1-9" barrels.

1-7" barrels are meant for the heavier bullets from 60Gr. to 80Gr. The fast twist is needed to stabilize the longer, heavier bullets. The lighter bullets (40Gr.) will not shoot well in this twist gun. Sometimes the 55Gr. will not even shoot well in them, depending if they are a Boat tail of not. 60Gr. to 80Gr. bullets shoot best i this twist.

IMHO, if i wanted to shoot your "average weight"
bullet, I think i would stick with a 1-9" twist Barrel.  They will excell with the 60-70 Gr. bullet range. while still shooting the 55Gr. stuff well enough. The 1-7" barrels are great for the heavier stuff, but will wear out faster also. Stay with the 1-9" Barrel and you won't go wrong.

KyARGuy