ok so the early M-16s used 1:12 twist and 52/55 grain bullets. Apparently this light bullet has better ballistics out to a certain yardage and the higher velocity caused better fragmentation.  Barrel life would also be better.

If this is so, why the the US Gov. switch to 1:7 twist later and heavier bullets?

Which route is best with 20" barrel if you plan on stockpiling whatever ammo you gun needs?

What are the dis/advantages to each set-up (wound produced, accuracy at both short and long range, etc)?

I have heard a lot of misinformation on the history, design and capabilities of this round. Id like to share a little history with you just to clear things up a bit.

Back in the early 70s there was a project to develop a SAW or Squad Automatic Weapon. Over 1000 different configurations were considered from 5.56mm to 7.62mm and a new 6.0x45mm cartridge was selected. This round would fire a 105 grain projectile at 2450 fps. This round was selected by the Army and Rock Island Arsenal because of the larger diameter to produce a tracer that would meet the over 800 meter daylight burn required by the spec for the SAW program. Also this would allow the 800m helmet penetration requirement for the ball ammo. THESE WERE THE ONLY TWO REQUIREMENTS that mattered to the miltary for the ammo. The Army never designed a round to fragment or cause massive trauma. In the case of the M193 this was a fluke, an accidental bonus that was discovered later. There were no terminal performace requirements for the SAW ammo.

Multiple weapons were considerd for the SAW program including the HK21 belt fed, the FN Minimi and a heavy barreled M16. Of these only the Minimi was still cosidered after testing. The problem was there were 3 seperate experimental SAWs in 6.0x45mm and the 62 grain ball ammo of the Minimi would not work in these or in the M16A1s of the time. Becasue they did not want to introduce a third caliber for NATO rifle ammo and have a seperate ammo for the SAW and becuse the Minimi would allow the lightest SAW when loaded with 200 rounds of ammo the Minimi was selected and the 6.0x45mm cartridge was dropped.

The original FN SAW had a 1 in 9 twist barrel to stabilize its experimental XM777 62 grain steel tipped round. This also stabilized the tracer round. However this tracer came nowhere near reaching an over 800m daytime burn. So they began the process of increasing the length of the round to stuff more phosphorus in it because they no longer had the diameter of the 6.0 to work with. The experimental tracer round, XM778 was much improved but still could not get past 800m so reluctantly the Army reduced it criteria to at least 800m not over 800m and the XM778 met the new specs.

New problem. The XM778 was now so long the 1/9 twist would not stabilize the round so the twist was increased to 1/7. The Army knew it needed to have only one type of 5.56mm ammo to be the NATO standard so when the design of the M16A2 came up it was required to fire the SAW ammo and needed a 1/7 twist to accomplish this. This is where the 1/7 twist comes from... the burn length for the SAW ammo. The XM777 and XM778 were submitted to the NATO trials, were adopted and are our current M855 and M856 ammunition.

Consider this... M855 was never designed to be an anti-personel round it was designed to have increased penetration in a machinegun. This round was a compromise from the start. We selected it to get a lighter SAW and have a universal ammo standard.

In recent years the military has realised the shortcomings of this compromise and has begun testing ammo for use in the M4 carbines because of limited fragmentation caused by lower muzzle velocities. With the advent of scientific terminal ballistics studies the military is now searching for increased fragmenting range for the rifles because the SAW ammo is a marginal performer since it was not designed with terminal ballistics in mind. I can only hope the military does not make the same mistake twice and pushes for a third 5.56x45mm round to be adopted by NATO once they find an ideal performer. Only time will tell if they fix their short sighted mistake or just put their heads back into the sand.

The military is now using heavier bullets which fragment more violently at close range and fragment further.  These are the 75 and 77 grain rounds we discuss so often.  These require the 1/7 twist to stabilize them in short barrels.  It would take far too much time to discuss the merits of these rounds and why they are so much superior than the M193 55 grain ammo so I willleave that for another post and you can use the search engine for that too.  Remeber though twist has nothing to do with terminal effectiveness it only has an effect on what rounds will be stabilized.

For the ability to use the better and more expensive 75 and 77 grian ammo and get excellent accuracy get a 1/7 twist.  For a bulk ammo and practice ammo use M193.

Just briefly, the 1:7 twist barrels and their corresponding ability to stabilize heavier bullets gives better knock-down power and better penetration.

You're smart to be considering a 20" barrel as that barrel length will assure excellent velocity (with the proper ammunition, etc.) and excellent performance.  (Actually an 18" barrel is perfectly adequate but you would have to go to a custom barrel.)

Excellent, DevL!  I remember seeing pix of one of the M16 variants proposed, a belt-fed (Norrel or Ceiner design?).  I think I remember reading that it was supposed to also take USGI mags, so that, if out of belted ammo, it could be fed from other squad members' mags.  I have pictures somewhere, perhaps an old issue of SWAT?  What ever became of that?

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that site cleared up alot for me

AWESOME LINK
thanks guys =)