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eighteenninetytwo
11-28-2009, 11:53 AM
can a fast barrel twist rate cause a widening of groups if it is not balanced by heavier ammunition. Specifically in this case I have a 1:7 twist 20" heavy barrel and I was using bog standard federal 55gr Xm193. This was the first time with a scope on this AR and I was hoping for much better but the groups were about the same as I was getting previously with iron sights, though last time I was using 62 and 69 grain ammo. I thought that the lighter ammo would just mean that the rifle would group in a different place but that didn't seem to be the case.

Jonathan Doe
11-28-2009, 12:08 PM
I was told that the fast twist barrels "over stabilize" the light bullets. Although, the faster twist barrels will stabilize the heavy bullets with longer bearing surface. I have shot Federal 55 grain soft point ammo in my Wilson 1:8 twist barrel and it showed excellent accuarcy.

818gtiguy
11-28-2009, 2:01 PM
XM193 is about 3MOA so if your group were about 3in and at 100yrds you are doing just fine..69gr, 75gr, and 77gr match ammo will tighten that right up, as long as the shooter does his job..

Bug Splat
11-28-2009, 3:39 PM
Faster twists tend to like heavier bullets. Heavy emphasis on "Tend". Barrels have a life of their own. Some fast twists like lighter bullets and some slow twists like heavy bullets. You just have to try them all and see which your barrel like. My old Savage in 223 with a 1-9 twist loved 40gr bullets. My CMMG AR with a 1-9 twist hates 55gr bullets but loves 40gr, 50gr, 60gr, 62gr and 69gr. There is no reason it should hate 55gr bullets but it does. Think of you barrel as a small child who does not know what he likes to eat. The only way to figure it out is to feed him a little of everything see what works.

eighteenninetytwo
11-28-2009, 4:07 PM
XM193 is about 3MOA so if your group were about 3in and at 100yrds you are doing just fine..69gr, 75gr, and 77gr match ammo will tighten that right up, as long as the shooter does his job..

Don't feel so bad now. All i have to do now is get a 2nd mortage out to buy heavier ammo.

SIGman Freud
11-28-2009, 4:59 PM
Faster twists tend to like heavier bullets. Heavy emphasis on "Tend". Barrels have a life of their own. Some fast twists like lighter bullets and some slow twists like heavy bullets. You just have to try them all and see which your barrel like. My old Savage in 223 with a 1-9 twist loved 40gr bullets. My CMMG AR with a 1-9 twist hates 55gr bullets but loves 40gr, 50gr, 60gr, 62gr and 69gr. There is no reason it should hate 55gr bullets but it does. Think of you barrel as a small child who does not know what he likes to eat. The only way to figure it out is to feed him a little of everything see what works.

Did you only use .223 Remington or alternate with 5.56? If not, then I'm impressed with how well the CMMG took the various weights (with the exception, of course, to the 40 gr). I have been looking at the CMMG standard AR rifle myself. ;)

MicronuT
11-28-2009, 6:12 PM
start with 62 gr and up, test for groups, im pretty sure your rifle will like upwards of high 60's and 70's (for 1/7)

Timmay
11-28-2009, 9:16 PM
From my understanding it's length of bullet, not weight that dictates optimum twist rate.

pontiacpratt
11-28-2009, 9:29 PM
XM193 is about 3MOA so if your group were about 3in and at 100yrds you are doing just fine..69gr, 75gr, and 77gr match ammo will tighten that right up, as long as the shooter does his job..

Today I was able to get about 2" groups at 50 yrds using 55 gr .223 in a 1/9 twist CMMG, looking through a red dot.

akjunkie
11-28-2009, 9:55 PM
can a fast barrel twist rate cause a widening of groups if it is not balanced by heavier ammunition. Specifically in this case I have a 1:7 twist 20" heavy barrel and I was using bog standard federal 55gr Xm193. This was the first time with a scope on this AR and I was hoping for much better but the groups were about the same as I was getting previously with iron sights, though last time I was using 62 and 69 grain ammo. I thought that the lighter ammo would just mean that the rifle would group in a different place but that didn't seem to be the case.


Pretty much everything U need to know is located here.

http://ammo.ar15.com/ammo/

Left column: Performance of 223 and 556 Ammunition.

hope this helps.

aplinker
11-28-2009, 10:25 PM
From my understanding it's length of bullet, not weight that dictates optimum twist rate.

yes, and it's rotational velocity you're trying to mate to it (how fast it twists in time, not distance). Which is to say the same loading from a 10.5" 1/7 will NOT have the same twist rate as a 20" 1/7.

This is why light bullets are an issue - they tend to be higher velocity, which causes faster rotational velocities, which can cause them to shear apart.