I am in the market for a new M4 upper and see the various twist rates and wonder which is best, for what purposes and why.  Thanks.

1/9 is for 55-69gr bullets, 1/7 is for heavier.

heavier bullets are expensive, but more effective on flesh.

Gotya, doubtful I would ever go above 62 grain... if I want to shoot heavier bullets it would be for distance and accuracy and that would be through my 20 inch HBAR I think.

Interesting comment.  I have always been of the mindset that "speed kills" when talking 5.56mm and wounding effects.  Can you point me toward your source for the heavier bullets being "more effective on flesh" comment?  I'd like to learn more about it.

TIA,

Bud

Your effective frag range with M4 14.5" barrel

55gr M193/Q3131A... ~100 yards
75/77gr OTM SAMMI/NATO... ~200 yards

HiramRanger,

I recommend the 1/7 for a "Fighting" rifle. A 1/7 will shoot 55-100 gr which is IMHO more versatile unless you think you'll only want to shoot lightweight varmint ammo.

The 1/7 gives you the ability to step up to really good stuff while still allowing you to shoot the cheap 55 gr stuff too.  In my case I don't see myself ever going below a 55 gr bullet with my new upper so why go with a 1/9?  

Besides, I always have my old 1/9 upper if I ever need it![;)]

I went through this same thing when I was selecting my upper and I'm glad I went with a 1/7.

As far as accuracy goes when I took my 16" 1/7 and zeroed it with XM193, I was able to get 5 shot 3/4 MOA groups out of it!  Of course that was off sandbags but have no doubt this rifle shoots better than I can!

Good luck and keep us posted!

Bud,

Go to the Ammunition forum and read the tacked 75/77 Grain testing thread.  Also check out the Ammo FAQ.

i've heard people say 1/7 is too fast for a sub-60gr bullet, could possibly spin them apart or some such. Am i imagining things or can anyone comment on this?

See this thread.

[URL]http://www.ar15.com/forums/topic.html?b=3&f=16&t=163070[/URL]

This one too..

[url]http://www.ar15.com/forums/topic.html?b=3&f=12&t=163071[/url]

Velocity is paramount with all 5.56 rounds.  Heavy bullets do not need as much velocity in order to yaw, and fragment, this is there advantage.  You need to read the ammunition forum more often.  I would also recomend you read the ammo oricle.

The Azalin

A 1-7 is good for 62-80 grain bullets.  [b]But you say 62 grain[/b]

62 grain gets its lethality from 2 things fragmentation at about 100meters and less--- and after 100meters from bullet instability.

The 1/7 overstabilizes rounds to the point that they can pierce soft (living) targets without yawing much at all.

I would think the 1/9 is better since it is not overstabilized and will probably more rapidly destabilize in tissue past 100meters.


[b]when the bullet is no longer fragmenting instability in tissue is a very good thing[/b]


Otherwise the 1/7 will probably heat marginally faster (more resistance) and will definitely get dirtier as 1/7's take a lot of jacket material into the lands that 1/9's do not.

1/9 Stable in air with 62 grain

1/7 Stable in air with 62 grain


(One of these is more stabilized [spinning faster] 1/7)

This means that the 1/7 past 100meters will probably have a greater tendency to remain stable for longer before begining to yaw.

bullets that do not fragment are more deadly when they yaw.  So I am assuming the slower 1/9in twist will impart slower spin on the bullet and leave it more ready to yaw on impact. This greater yaw on impact should translate to greater stopping power at ranges beyond fragmentation distances.

Green0,

You're really trying to squeeze this thread for all it's got. [;)]

Once the bullet makes contact to flesh the extra few revolutions makes no difference in what happens after contact and penetration.

See [url]ammo-oracle.com[/url]

77 gr ammo has a thin jacket, and the longer round breaks apart at lower velocities. 100 gr is better, but ultra super duper extra special classified.

This ammo (mk 262 mod 0/1) has been used in SPRs to kill at over 600m in the Middle East. Sounds effective to me.

Where did you get this info? I have been searching High and low for a minimun frag velocity/range for the 75/77gr OTM's.  I have never found any data where someone has tested it at extended ranges or reduced velocites.  I don't doubt you but I would shure be interested in reading your sources.

Thanks
Kris

Check the terminal effects section in tacticalforums.com and do a search.  The floor for fragmentation of 75 grain OTM is about 2100-2200 fps.

I was just over there and I read that the floor on the top secret 100 gr round was 2000 fps but I could not find it for the 75/77.

Inkaybee - if you look at the Hornady tests on Bushmaster's LE website, they give maximum effective ranges out of a variety of rifles.

If you break out the ballistics calculator and work backwards, you find that the velocity floor for the 75gr OTM is around 2100-2200 fps based on that information.

If you fish through all of DocGKRs posts at TF on the subject, particularly the ones dealing with short-barreled 5.56mm (like the 9.5" barrel G36K), you'll find pretty much the same thing.

I'm guessing that is the floor for reliable fragmentation. I've seen a picture from the Army of an SS109 round that fragged slightly at 1,932 fps (way below the 2,700 floor often mentioned for the round - [url]http://www.dtic.mil/ndia/2001smallarms/parks2.pdf[/url]) so there is always the chance of the occasional freak occurence.