Here is an excellent post by Dr. Gary Roberts. This is quoted from another forum so I hope he doesn't mind...[;)]
Whenever you acquire 5.56 mm carbines, there are a couple of things you must decide. First, what barrel twist are you going to use—preferrably, go with 1/7. Next, what barrel length are you going to choose—for a GP carbine, ideally go with 14.5 to 16”. Finally, what are your expected engagement scenarios? Entry, perimeter, sniper support, long range, CQB, vehicles, etc…
For general purpose use with 1/7 twist barrels from 0 to 600 yards, I would choose one of the combat proven 5.56 mm (ie. 5.56 mm NATO pressure loads, not .223 SAAMI pressure loads which run about 200 f/s slower) heavy match OTM loadings: either the Hornady 75 gr TAP using the OTM bullet w/cannelure or the equally good 77 gr Nosler OTM w/cannelure loaded by Black Hills, followed by the 77 gr Sierra Match King OTM—which, while exceedingly accurate, is slightly less reliable in semi-auto and full-auto guns, as it unfortunately does not have a cannelure, as well offering slightly reduced terminal effects. If your expected engagement scenario is at more typical LE distances, say out to 200 yards, then the .223 SAAMI pressure loads are fine. The experimental BH loaded 100 gr exhibits impressive fragmentation, even at relatively low velocities, however, their trajectory is like a rainbow—definitely for use under 150 - 200 yards.
NOTE: For general purpose use, if you are stuck with 1/9 twist barrels, you cannot use the heavy match OTM loads; you need to pick the 69 gr SMK OTM, the 68 gr Hornady OTM, the Winchester 64 gr JSP (RA223R2), and the new Federal 64 gr TRU (223L) JSP loading. You are screwed with 1/12 twists, I would probably choose the 55 gr Federal bonded JSP load (Tactical--LE223T1 or identical Premium Rifle--P223T2) in order to ensure adequate penetration.
If you are going to engage vehicles, then the only .223 loads which effectively penetrate automobiles are the 62 gr Federal bonded JSP Tactical (LE223T3) and the similar 55 gr Federal bonded JSP load (Tactical--LE223T1 or identical Premium Rifle--P223T2). The new Hornady 60 gr barrier penetration JSP bullet is not as effective as the Trophy Bonded Bear Claws. None of the OTM bullets, even the heavy 75 – 100 gr loads, offer good performance through automobile glass. FWIW, contrary to what many believe, 62 gr M855 FMJ also is not very good against glass.
If you must have a short barreled 5.56 mm weapon, such as the Colt Commando, Crane CQB-R, HK 53, HK G36C, etc… and you have a 1/7 twist short barrel, you will find that the 75 gr Horn, 77 gr Nosler, 77 gr SMK, and 100 gr BH loadings offer acceptable performance; with a 1/9 twist, stick with the Fed 62 gr Tac JSP (LE223T3). Remember, your effective engagement distance is significantly reduced compared to the longer barreled carbines.
Bottom line, I feel the versatile 75 gr Hornady TAP OTM and 77 gr Nosler OTM loaded by BH are the best available general purpose loads, as they offer good performance out to 600 yards or so and also are effective in short barrel carbines; the Federal Tactical loads using the Trophy Bonded Bearclaw's are the clear choice when intermediate barriers are present.
All this may be somewhat moot within the next 6-12 months, as there is a new military developed caliber, soon to be commercially available, that is dramatically superior to all existing 5.56 mm loads.
More good stuff...
The original AR15 had a SLOW twist of 1/14; as the M16A1 developed, this was changed to a slightly FASTER twist of 1/12.
The change in twist rate to the TIGHTER/FASTER 1/7 was made with the M16A2 in order to stabilize the long M856 tracer round and had NOTHING to do with trying to, “make the 5.56mm perform "like" the 7.62mm round”. Remember SS109/M855, as well as M856 was originally designed as machine gun ammunition for engaging enemy troops wearing body armor during conventional infantry combat at distances of several hundred meters when fired from the Minimi/M249 SAW. Development of the Minimi and its ammo began some 10 years prior to the M16A2.
As noted previously, the terminal performance problems sometimes noted when M855 in fired from M4 Carbines has little to do with 5.56 mm CARTRIDGE performance and a lot to do with specific BULLET performance—in this case the use of machine gun ammunition in carbines; in its intended role out of the M249, M855 performs adequately.
There is NO controversy as to which twist rate is best, which bullets work the best in each twist rate, and what are minimum barrel lengths—all of these parameters are easily measured and quantified through appropriate testing.
You state, “Personally, I feel that a 16" barrel is the absolute minimum length to consider in the 5.56mm cartridge, and anything under 20" demands the use of a 1-9" twist and light to middle-weight (55 to 64-gr.) bullets.” The military disagrees: M16’s use 20” barrels, Mk12’s have 18” barrels, the SPR-D/Recce a 16” barrel, M4’s 14.5” barrels, and the CQB-R a 10.5” barrel. Contrary to your assertion, they all demonstrate their best terminal performance using 62 to 77 gr bullets and all have 1/7 twists. The 1/7 is clearly the best twist rate available, as it allows use of the widest variety of bullets, including the combat proven heavy OTM’s, as well as the bonded JSP’s. Likewise, numerous LE agencies use 1/7 twist rifles and heavier bullets with excellent success. Also, remember that nearly every .223 load, regardless of bullet weight, has LESS "overpenetration" worries than any other commonly used service cartridges, including the various duty handgun calibers.
Contrary to your statement, switching from 62 gr bullets to the more effective 77 gr bullets has resulted in GREATER accuracy at ALL ranges, as discussed by Buxton ( http://www.dtic.mil/ndia/2003smallarms/bux.ppt ).
7.62 x 39 mm is a good cartridge in carbines like an AK or Robinson RAV02 ( http://www.robarm.com/rav02.htm ); however, in general, it has not been very reliable in AR’s as a result of less than ideal magazines and gas tube fouling from dirty third world ammo.