M193 or M855 in 1:7 twist barrel...

T

thedaddycat

New member
I've been reading a lot about different twist rates common to the AR platform. I have one with a 1:7 barrel 20" long. As I understand it the M193 uses a 55 grain bullet and the M855 uses a 62 grain bullet that is long for the weight because of the penetrator core. The article I read talked about the M193 being better for defense due to terminal ballistics at ranges inside of about 150 yards. Accuracy was about equal at those ranges, too.

Now unless I'm missing something, isn't the higher twist rate better for the longer and heavier bullet in the M855? I bought a couple of cases of Winchester Ranger 62 grain and it says M855 on the case. Would I be better off with the M193 ammo? It seems like it costs a bit less which is good, and if it shoots just as well and performs better then maybe I'll get it next time I'm adding to the ammo shelves...
 
The 1:7 will shoot both. With the 1:7 you can go to 80 grns, (some can shoot 90.

But it will also shoot the 55 grn M193. Not the other way around, the 1:12 that used the M193 wont shoot the M855 (or heavier) bullets.

Example from the Army Manual:

Twist%20compairson.jpg
 
Like kragwy said, yours will shoot both. Personally, I get LARGE groups (around 2-3 MOA) with surplus M855 in my 16" 1:7 twist Spikes Tactical upper. This compared to shooting my handloads (55 grain Varmageddon FBHP and 77 grain HPBT) and getting 1/2 MOA or better. Think of your 1:7 twist as a bullet weight limit. You can shoot any bullet weight under 80ish grains (also, like kraigwy said, some 90). Whereas if you had say a 1:12 twist you would be limited to lighter bullets. Just because your rifle has a high twist rate don't restrict yourself to heavy bullets. All rifles are different so just figure out what your rifle likes to eat and feed it a LOT of it.
 
OK, so I'll get some M193 also to run down the pipe and see how it does. That graphic was in one of the articles I read. I think it was the same one that discussed the terminal ballistics of both M193 and M855.

I have a pre-ban Colt Sporter Target Model so here in CT it is a RAW that's listed by name. That's just stupid as far as I'm concerned as I can go out and buy something that is for all practical purposes identical in both form and function.... Oh wait, I get to have the dreaded "Assault Weapon Compensator" on mine!!!

As far as groups go, my eyes are the limiting factor on that account any more. I have a tough time even seeing the steel plate at 200 yards and forget the one at 300... Maybe it's time for a scope for the A2 carry handle.
 
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Just my opinion but I feel the 1 in 7 will stabilize the 55 gr to the point it will drill straight through rather than tumble/breakup as it is purported to do from the slower twist early M16.
This would not be optimum for SD as I see it.
 
Here is a link with loads of m193 vs m855 info as well as other info on 5.56/.223 questions. http://ammo.ar15.com/. However it does seem it was written before 1:7 barrels were widely available to civilians when 1:9 was more common. If you don't want to read all that (it does have good info), in short they conclude that the m193 is better than the m855 out to about 200m (a little less from a 16" barrel) even with fast twist barrels. The also cite evidence (from their own research and FBI research) that the twist of the barrel has near zero effect on bullet tumbling and fragmentation once it hits flesh. The bullet would literally have to be spinning at ONE BILLION rpm(1:7 gives about 350,000rpm). They also make a good case for fragmentation bullets over controlled expansion for self-defense.
 
Just my opinion but I feel the 1 in 7 will stabilize the 55 gr to the point it will drill straight through rather than tumble/breakup as it is purported to do from the slower twist early M16.
Click to expand...

It is impossible for any of commonly used rifle twist to spin-stabilize a spitzer bullet in flesh/water/ballistics gel - you would need rifling that looked like the thread pitch of a screw to even get close to that.

In terms of a spitzer bullet upsetting as it passes through a mostly liquid medium, the difference between a 1:14 twist and a 1:7 twist is totally insignificant. The science behind this is discussed in this link: http://ammo.ar15.com/ammo/project/term_twistduh.html
 
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