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-   -   Quick explanation about twist rate please? (http://www.firearmstalk.com/forums/f20/quick-explanation-about-twist-rate-please-34715/)

paul 11-24-2010 02:12 PM

Quick explanation about twist rate please?
 
I have a CMP, 1:7 twist, .223 cal, Colt Match Target rifle. Will it shoot, safely, accurately, 5.56 ammo? How about anything less than 62grains?

Yunus 11-24-2010 03:04 PM

There are others who can provide a more detailed explanation, I can only provide the summary.

.223 and 5.56 are not identical as you know
5.56 rifle can safely shoot either .223 or 5.56
.223 rifle should only use .223 but 5.56 will work however it's not recommended and might prove to be very unsafe.

c3shooter 11-24-2010 03:05 PM

The 1-7 works for 40-77 grain bullets. However, if yours is chambered in .223, it is not reccomended that you shoot 5.56 mm rounds. While they are close, they are not the same. The 5.56 mm chamber WILL shoot .223. There is an intermediate chamber (Wylde) that is AC/DC- goes either way.

stelliott80 11-26-2010 04:58 PM

You got me looking for answers to twist rate questions! I found this, which you may find interesting.

Clint McKee on AR-15 Twists.

I'd never given it much thought. I have two ARs, both uppers made by RRA, both 1:9 twist, and pretty much only shoot 55 gr ball ammo. Accurate enough for me and thousands of rounds through each, still shootin' straight.

Cheers.

CHLChris 11-26-2010 05:28 PM

I, too, am no expert. But 1:9 twist rate does NOT shoot only 55 gr bullets. Out of my 1:9 barrel, I prefer heavier bullets and have gotten good accuracy.

OP, on your .223 chambering, do NOT fire 5.56. It sounds like your rifle was intended as a match rifle for very accurate target shooting.

Just build another rifle. :-)

JonM 11-26-2010 07:11 PM

in general a lighter bullet needs a slower twist rate to stabilize or and not shred the jacket due centrifugal forces. the diff of .223 and 5.56 isnt the bullet itself its the dimension of the case and powder charge. surplus or production ammo marked 5.56 is for 5.56 marked barrels only.

the advantage to slow twist rates is extended barrel life and more effective terminal results for self defense with 55grn bullets.

now opinion section
----------------------

the military went to 62 grain rounds for better armour penetration at longer ranges. this results in poorer performance when it does hit making neat little holes. the 62grn round extends the effective range out further. in my opinion in military terms is of dubious value in a infantry weapon.

our government has decided that it is more important to wound than kill or incapacitate on the theory that our enemies will take time to tend the wounded. the fact is the enemies we face either do not care for their own wounded while the fight is going or do not have the resources to tend the wounded. they also tend to be unarmoured as well.

personally i really like the 55grn round out of a 1in12 barrel.

Jpyle 11-26-2010 09:02 PM

Twist and caliber don't really matter as much as the bullet weight, i.e. grain. General rule of thumb is that a lighter grain round needs a slower twist. Heavier rounds need a faster twist for stability. 1:9 is considered a general duty compromise but will not do as well at the extremes on either end of the weight spectrum.

A rifle chambered for .223 only should not be used for 5.56, the thicker walls and higher pressure of the 5.56 could lead to a failure of the chamber walls, not a good thing. Look for a .223/5.56 or 5.56 marking on the barrel before using those rounds.

A 1:7 should be able to handle grains lower than 62 but as the weight goes down the chance of overspin and bullet disintegration increase, especially with thinly jacketed varmint rounds where the mass of the lead core spins so fast it tears apart the copper jacketing and...poof, vaporizes into a grey cloud.

All that being said, a target match rifle is purpose built for a specific range of bullet weights according to the barrel length and twist...that would be the ammo to use if accuracy is what you are after.

CHLChris 11-27-2010 07:09 AM

JonM, your answer brings up questions in my mind.

In handguns, many people (myself included) prefer as heavy a bullet as is possible for the caliber, even though speed is a little slower.

In the AR-15, does this pattern continue? Wouldn't someone with a 1:9 twist want the largest bullet the twist will handle?


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