AR-15 Barrel Length Change

pccraig · January 3, 2020, 09:37 AM

Service rifle competition.

For the past few years I've been shooting SR competition with a standard Colt HBar A2 with a 20" 1:7 barrel and iron sights.

My go to load included a Nosler CC 77g bullet.

I've recently completed a new build. A 16" M4 style rifle with a mid-length gas system and a 1:9 twist with the Hi-lux 1-4.5 optic.

Now I'm wondering with the shorter barrel and "lighter" twist, should I downsize the projectile.

Any insights would be appreciated.

Old_School · January 3, 2020, 09:39 AM

Quote:

Originally Posted by pccraig

Service rifle competition.

For the past few years I've been shooting SR competition with a standard Colt HBar A2 with a 20" 1:7 barrel and iron sights.

My go to load included a Nosler CC 77g bullet.

I've recently completed a new build. A 16" M4 style rifle with a mid-length gas system and a 1:9 twist with the Hi-lux 1-4.5 optic.

Now I'm wondering with the shorter barrel and "lighter" twist, should I downsize the projectile.

Any insights would be appreciated.

May or may not stabilize the 77 gr bullet. Probably won't, but only way to know is to try them. My own 1/9 twist stabilizes 69 gr beautifully.

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ms6852 · January 3, 2020, 10:15 AM

As "Old School" stated the 1/9 twist may be too slow to stabilize the 77gr bullet but you really won't know until you try it out. Even if it does your impact will be lower as your losing approximately 200 fps in velocity. Which can easily be made up by selecting lighter grain bullets. This way it minimizes the changes you have to make in your shooting during competition. Let us know how it goes and the final changes you have made.

Rapidrob · January 3, 2020, 12:57 PM

I run NM MILSURPS,a long range shooting club for surplus military firearms.
Many of my clubs members shoot the AR platform rifles and carbines out to 800 yards.
From what we have found is that in air slower than 10 MPH, the 55 grain spitzer,boat-tail bullet does very well at 800 yards.
In air 10 mph and up to 20 MPH the 62 grain SPBT bullets shoot the best out of a 1:9 twist.
We found the pulled "green tipped" surplus bullets suck. Accuracy is very poor beyond 200 meters. While you may get hits on a man torso sized target at farther distances, on paper they are just all over the place for any decent scores.
The commercial 62 grain are the way to go.
Several of my guys shoot the M4 type Carbine with 16" barrels and they do very well at 800 yards,though bullet drop is as you'd expect due tot he slower bullet.
Here is a target from last winter. 20" HB AR15,open sights,62 grain bullet, from the prone,front bag support only,800 yards.
The air was dead calm and 40 degrees F.

Ifishsum · January 4, 2020, 07:50 PM

I bet you will have to go down in projectile weight. The first year and a half I shot Highpower was with a 1:9 (20") and 69gr SMKs were the heaviest it would shoot well. Challenging at the 600yd line, especially with wind but do-able. At least for my skills at the time. Not to say that it isn't worth trying 77s if you already have some loaded up...nothing really to lose. Apparently a few have been able to get Hornady 75gr HPBTs to work in the 9 twist, I bought some planning to try them in that barrel but ended up getting serious with a new upper with a 7.7 barrel so I never did. Still wore that first barrel out shooting 53gr and 69gr SMKs at reduced distance matches.

January 3, 2020, 09:37 AM	#1
pccraig Member Join Date: April 18, 2007 Location: McHenry County, Illinois Posts: 20	AR-15 Barrel Length Change Service rifle competition. For the past few years I've been shooting SR competition with a standard Colt HBar A2 with a 20" 1:7 barrel and iron sights. My go to load included a Nosler CC 77g bullet. I've recently completed a new build. A 16" M4 style rifle with a mid-length gas system and a 1:9 twist with the Hi-lux 1-4.5 optic. Now I'm wondering with the shorter barrel and "lighter" twist, should I downsize the projectile. Any insights would be appreciated.

January 3, 2020, 10:15 AM	#3
ms6852 Senior Member Join Date: April 3, 2010 Location: Texas Posts: 1,394	As "Old School" stated the 1/9 twist may be too slow to stabilize the 77gr bullet but you really won't know until you try it out. Even if it does your impact will be lower as your losing approximately 200 fps in velocity. Which can easily be made up by selecting lighter grain bullets. This way it minimizes the changes you have to make in your shooting during competition. Let us know how it goes and the final changes you have made. __________________ ONLY TWO DEFINING FORCES HAVE GIVEN UP THEIR LIVES FOR YOU. ONE IS JESUS CHRIST FOR YOUR SOUL AND THE OTHER IS THE AMERICAN SOLDIER FOR YOUR FREEDOM.

January 3, 2020, 12:57 PM	#4
Rapidrob Member Join Date: January 15, 2006 Location: New Mexico Posts: 49	I run NM MILSURPS,a long range shooting club for surplus military firearms. Many of my clubs members shoot the AR platform rifles and carbines out to 800 yards. From what we have found is that in air slower than 10 MPH, the 55 grain spitzer,boat-tail bullet does very well at 800 yards. In air 10 mph and up to 20 MPH the 62 grain SPBT bullets shoot the best out of a 1:9 twist. We found the pulled "green tipped" surplus bullets suck. Accuracy is very poor beyond 200 meters. While you may get hits on a man torso sized target at farther distances, on paper they are just all over the place for any decent scores. The commercial 62 grain are the way to go. Several of my guys shoot the M4 type Carbine with 16" barrels and they do very well at 800 yards,though bullet drop is as you'd expect due tot he slower bullet. Here is a target from last winter. 20" HB AR15,open sights,62 grain bullet, from the prone,front bag support only,800 yards. The air was dead calm and 40 degrees F. __________________ President NM MILSURPS Tonkin Gulf Yacht Club

January 4, 2020, 07:50 PM	#5
Ifishsum Senior Member Join Date: September 15, 2005 Location: Oregon Posts: 1,033	I bet you will have to go down in projectile weight. The first year and a half I shot Highpower was with a 1:9 (20") and 69gr SMKs were the heaviest it would shoot well. Challenging at the 600yd line, especially with wind but do-able. At least for my skills at the time. Not to say that it isn't worth trying 77s if you already have some loaded up...nothing really to lose. Apparently a few have been able to get Hornady 75gr HPBTs to work in the 9 twist, I bought some planning to try them in that barrel but ended up getting serious with a new upper with a 7.7 barrel so I never did. Still wore that first barrel out shooting 53gr and 69gr SMKs at reduced distance matches.