M855A1 Performance Review

[spyderco monkey]

To give a sense of how radical an improvement M855A1 is over M855 (green tip) heres a look at a ballistics chart for a M4, firing M855. M855 has a G7 BC of .151, whereas M855A1 is reportedly slightly better at .152.

M855= 2700-2500fps minimum impact velocity for fragmentation
M855A1 = 1988fps proven fragmentation, likely fragments down to 1700fps

What we see is from the M4, the M855 is reaching its frag threshold within 100-120yd. By comparison, with a 1988fps threshold, M855A1 is fragmenting at 330yd, a 3x increase in effective frag range. With a 1700fps frag range (likely given the instant frag we see at 1988) that extends frag range to 4x, 440yd.

[TiroFijo]

To give a sense of how radical an improvement M855A1 is over M855 (green tip) heres a look at a ballistics chart for a M4, firing M855. M855 has a G7 BC of .151, whereas M855A1 is reportedly slightly better at .152.

M855= 2700-2500fps minimum impact velocity for fragmentation
M855A1 = 1988fps proven fragmentation, likely fragments down to 1700fps

What we see is from the M4, the M855 is reaching its frag threshold within 100-120yd. By comparison, with a 1988fps threshold, M855A1 is fragmenting at 330yd, a 3x increase in effective frag range. With a 1700fps frag range (likely given the instant frag we see at 1988) that extends frag range to 4x, 440yd.

Excellent information and analysis, thanks.

One thing, the gel test you referenced is with a reduced velocity load to approximate the impact velocity at X range. Unfortunately, in real life the gyroscopic stability factor at X range is not the same since rotational velocity decay is much less than the linear one. How much this affects fragmentation is anyone's guess (for this type of bullet mine is: not much).

[TGS]

Excellent information and analysis, thanks.

One thing, the gel test you referenced is with a reduced velocity load to approximate the impact velocity at X range. Unfortunately, in real life the gyroscopic stability factor at X range is not the same since rotational velocity decay is much less than the linear one. How much this affects fragmentation is anyone's guess (mine is: not much).

An excellent point.

A Marine Gunner (a chief warrant officer rank in the USMC who serves as a SME to the commander on infantry weapons systems) once told me that M855 (or maybe M995, can't remember) actually had greater barrier penetration at distance than at 10 feet due to this. He used the example of a football, and after it's thrown it yaws significantly, becomes more stable, and then destabilizes and starts yawing again. Again, can't remember the specifics, I want to say it was 100 or 200 yards.

@rcbusmc24 incase he remembers anything

[TiroFijo]

An excellent point.

A Marine Gunner (a chief warrant officer rank in the USMC who serves as a SME to the commander on infantry weapons systems) once told me that M855 (or maybe M995, can't remember) actually had greater barrier penetration at distance than at 10 feet due to this. He used the example of how a football, and after it's thrown it yaws significantly, becomes more stable, and then destabilizes and starts yawning again. Again, can't remember the specifics, I want to say it was 100 or 200 yards.

@rcbusmc24 incase he remembers anything

IIRC the swiss and chilean found the same when testing the then new M855/SS109 bullet with slower twist barrels than the US/NATO standard 1:7"

[spyderco monkey]

An excellent point.

A Marine Gunner (a chief warrant officer rank in the USMC who serves as a SME to the commander on infantry weapons systems) once told me that M855 (or maybe M995, can't remember) actually had greater barrier penetration at distance than at 10 feet due to this. He used the example of a football, and after it's thrown it yaws significantly, becomes more stable, and then destabilizes and starts yawing again. Again, can't remember the specifics, I want to say it was 100 or 200 yards.

@rcbusmc24 incase he remembers anything

This is known as the 'fleet yaw effect'. Basically from 0-50m, the bullet is flopping up and down pretty violently, before it settles down to a more straightline, tip forward flight path.

You can see that flop here in this slow mo AK74 footage:



https://www.thefirearmblog.com/blog/...effectiveness/

The first article explores this background, and the challenges the investigators had to face in very satisfying detail. What resulted from their investigation was a landmark discovery in terminal effectiveness science: Bullets – all bullets, not just .22 caliber ones – experience a period of very violent yaw and turbulence when they exit the muzzle, causing their angle of attack relative to their flight path – that is how “straight” the bullet is in flight – to vary wildly. Within 50m, they found, two bullets fired from the same gun, at essentially the same time, might impact a target at two completely different angles. A bullet impacting head on into gelatin would stay stable for much longer than one impacting at a high angle, and would deposit its energy much later. This explained the problems some users – but not others – were having with their weapons. In some instances, the FMJ projectiles would hit the target at a desirable high angle of attack, tumble and fragment within a short distance, and reliably stop the target, while in others, the same type of projectile would hit at a flat angle, and might not yaw for many inches.



Perhaps even more important than this discovery – in terms of the solutions it informed – was the fact that all projectiles tested, including 5.56mm, 5.45mm, 6.8mm, and 7.62mm projectiles of a wide variety of weights and types of construction, exhibited this flaw. While changing calibers to, for example, the 6.8mm Remington, might increase the energy per shot of service carbines, doing so would not solve the fleet yaw problem on a fundamental level. Further, it was determined that, while no such design currently existed on the commercial market, a projectile in the current 5.56mm chambering could be designed to overcome this effect, one which could yaw and fragment at all angles of attack, ensuring reliable terminal effects. These conclusions led directly to the four current in-service yaw-independent projectiles, M855A1, M80A1, Mk. 318, and Mk. 319.

[spyderco monkey]

Excellent information and analysis, thanks.

One thing, the gel test you referenced is with a reduced velocity load to approximate the impact velocity at X range. Unfortunately, in real life the gyroscopic stability factor at X range is not the same since rotational velocity decay is much less than the linear one. How much this affects fragmentation is anyone's guess (for this type of bullet mine is: not much).

An actual shot on gel at 300m would have been optimal for sure. Alas theres very few long range gel tests; reduced velocity is likely the most we can ask from free youtube channels.

That said, the M855A1 was designed to be 'yaw independent' to work at pretty much any impact angle, so should be minimally effected by yaw impact.

[Hambo]

Lots of real data and info....

The main point of your post is that one shouldn't waste time with clickbait YouTube videos.

[Shawn Dodson]

This is known as the 'fleet yaw effect'. Basically from 0-50m, the bullet is flopping up and down pretty violently, before it settles down to a more straightline, tip forward flight path.

You can see that flop here in this slow mo AK74 footage:



https://www.thefirearmblog.com/blog/...effectiveness/

The first article explores this background, and the challenges the investigators had to face in very satisfying detail. What resulted from their investigation was a landmark discovery in terminal effectiveness science: Bullets – all bullets, not just .22 caliber ones – experience a period of very violent yaw and turbulence when they exit the muzzle, causing their angle of attack relative to their flight path – that is how “straight” the bullet is in flight – to vary wildly. Within 50m, they found, two bullets fired from the same gun, at essentially the same time, might impact a target at two completely different angles. A bullet impacting head on into gelatin would stay stable for much longer than one impacting at a high angle, and would deposit its energy much later. This explained the problems some users – but not others – were having with their weapons. In some instances, the FMJ projectiles would hit the target at a desirable high angle of attack, tumble and fragment within a short distance, and reliably stop the target, while in others, the same type of projectile would hit at a flat angle, and might not yaw for many inches.



Perhaps even more important than this discovery – in terms of the solutions it informed – was the fact that all projectiles tested, including 5.56mm, 5.45mm, 6.8mm, and 7.62mm projectiles of a wide variety of weights and types of construction, exhibited this flaw. While changing calibers to, for example, the 6.8mm Remington, might increase the energy per shot of service carbines, doing so would not solve the fleet yaw problem on a fundamental level. Further, it was determined that, while no such design currently existed on the commercial market, a projectile in the current 5.56mm chambering could be designed to overcome this effect, one which could yaw and fragment at all angles of attack, ensuring reliable terminal effects. These conclusions led directly to the four current in-service yaw-independent projectiles, M855A1, M80A1, Mk. 318, and Mk. 319.

The term "Fleet Yaw" refers to the variation in bullet stability among the "fleet" of guns. See slide #7 - Time for a Change: U.S. Military Small Arms Ammunition Failures and Solutions

Whereas "flow stabilization" is what causes a bullet's stability to improve in flight.