A few comments:

M855 specifications:

NATO specifications for SS109 (U.S. M855) Ball call for a 61.7 grain bullet (+/-.75 gr) with a hardened steel penetrator at a velocity of 3,025 fps (+/- 20 fps) from a 20 inch barrel 25 meters from the muzzle. Typical velocity 15 feet from the M16A2's muzzle is around 3,100 fps.

The accuracy requirement from a test barrel specifies a mean radius for two 30 shot groups that equates to a maximum of approximately four MOA from 100 to 600 yards. However, typical accuracy of average lots in an M16A2 (20", 1-7 twist barrel) is just over 2 MOA. This is about twice as good as the requirement. It's also nearly identical to the M193's 2" mean radius requirement at 200 yards - giving effectively the same 2 MOA accuracy. If there's a difference, it's that M855 ammo doesn't have to meet a 2 MOA requirement - it can shoot as badly as 4 MO and still be accepted - but the potential is there for 2 MOA accuracy in the better lots.

The original goal for the M855 projectile was penetration of a steel helmet at a range of 500 meters, which was a modest increase over M193 which as originally designed was supposed to penetrate a steel pot at 500 yards (but didn't quite work out that way).

M855 actually did quite a bit better and the penetration requirement in the acceptance criteria for M855 is penetration of a 10 gauge SAE 1010 or 1020 steel test plate at a range of at least 570 meters (623 yards). On average, the M855 and M80 ball rounds will accomplish this out to a bit more than 700 yards. In comparison, the 55 gr M193 round using the shorter Type A 55 grain bullet will penetrate a 109 gauge steel plate reliably at 400 yards, and will penetrate about half the time at 500 yards.

Trajectory differences

In a 20" barrel, M193 has a velocity that is about 100 fps faster, but it has a lower BC. In practice the trajectories of M193 and M855 vary by less than an inch out of 400 meters and at 100 meters the difference is only .1".

The rub is that the trajectory is separate from what is needed to zero the rifle on the same point of aim with each round. As noted the M855 round is 100 fps slower, but has a slightly higher recoil impulse, with the potential for the muzzle to rise very slightly more before the bullet exits, which changes the angle of departure. That's not all bad as the higher angle of departure helps compensate for the slower muzzle velocity.

In practice, you should see both rounds impacting on the same 4" bullseye at 100 yards, but beyond that it will depend on the specifics of the rifle or carbine you are using.

In short, you'll have to try it and see if the results are close enough for your purposes.

Rifling twist and accuracy

Contrary to popular belief, the optimum twist for the 62 gr SS109/M855 projectile is 1-9". The faster 1-7" twist was adopted due to a rather stupid requirement that the M16A2 also stabilize the much longer M856 tracer round adequately to achieve the same penetration standard at 500 meters. That was done basically to ensure complete ammo compatibility with the M249.

What that means is that the 62 gr M855 round is actually slightly over stabilized in a 1-7" twist barrel, and that 1-9" is a better choice. The 1-9" twist will also accommodate 55 grain bullets well - particularly, longer boat tailed FMJs or boat tailed ballistic tip bullets, allowing for very good accuracy with well made bullets, despite the faster than optimum twist.

Not all 55 gr FMJs are created equal

The original bullet designed by Eugene Stoner in conjunction with Sierra for the XM193 round, had a longer 7 caliber ogive and a 9 degree boat tail that gave it a better ballistic coefficient than the later Remington designed bullet used in XM193 production. (In later testing this was later called the "Type B" bullet by the US Army.)

Remington redesigned the bullet as they encountered problems with inadequate stability in the 1-14" barrel originally specified for the M16 and they elected to use a shorter bullet - but didn't bother to notify the US Army of that change. (This bullet was later referred to as the "Type A" bullet by the US Army.)

That change in projectiles in XM193 ammunition was unfortunate, as during arctic testing with the M16, the XM193 round was found to be unstable in extremely cold temperatures and the US Army increased the rate of twist to 1-12" anyway.

The original longer Type B bullet had a significantly better BC than Remington's Type A bullet, which meant the Type B bullet in the original Stoner cartridge could penetrate a steel pot (10 gauge steel plate for test purposes) at the required 500 yard range. However, Remington's Type A bullet with it's shorter length, and lower BC could not meet the same requirement - penetrating the steel plate only about half the time at 500 yards.

In later testing the Army determined that at 300 yards for example, when fired in the same M193 cartridge, the Type A bullet lost 156 fps compared to the originally specified Type B bullet.

The need to launch the Type A bullet at significantly higher velocities to get the desired plate penetration is what led to the whole fiasco of increasing chamber pressures and searching for a different powder. All of which could have been avoided if Remington had just stuck with the original bullet when it was asked to produce ammunition and just told the US Army it needed to use a 1-12" rifling twist - which the US Army adopted anyway.

This is the long way of pointing out that not all 55 gr FMJs are created equal. Personally, I like the Hornady 55 gr FMJ as they are well made as FMJs go and have a very good (by 55 gr FMJ standards) BC of .247. I've found that bulk Hornady 55 gr FMJs are more consistent and more accurate than the bulk Winchester and Remington M193 projectiles, and like the Type B projectile, retain velocity better.

I normally get 1.25" MOA accuracy in my 20" (1-12" twist) Colt SP1 and M-16A1 clone (using a police take off M16A1 upper half) with Hornady bullets, and around 2 MOA accuracy with the Remington and Winchester projectiles.

In my varmint AR (20", 1-9 twist, bull barrel) I get 1 MOA accuracy with the Hornady bullets, but I'm lucky to get 3 MOA with the Winchester and Remington rounds. In this case, the faster twist exacerbates the inconsistency of these bullets, where the center of form and center of gravity are not always on the same axis.

In my various 1-9 twist 16" carbines, the differences are less pronounced, with all the rounds producing more or less the same 2.5-3 MOA accuracy. This is due in part to less accuracy in the barrels hindering the better made bullets, and less velocity (and thus lower spin rates) reducing the disadvantage of the less well made bullets.