Fast twist and lower velocity question

tpcollins · March 30, 2013, 08:19 PM

My Colt AR has a 1:7" twist and it shoots 55gr factory ammo fairly well. I've only reloaded 60gr bullets using IMR-8208-XBR. Although my powder range is 21.5 - 23.6 grains, my most accurate loads for (3) different brands of 60gr bullets was 22.4 grains of powder.

I know the 1:7" twist will accommodate heavier bullets but do you think my 16" barrel just likes 22.4 grains or does the 1:7" twist prefer a slower velocity (basically the same thing almost)? Thanks.

Marco Califo · March 30, 2013, 08:26 PM

See this thread: http://thefiringline.com/forums/showthread.php?t=520997
for a calculator of bullet stability. Velocity, bullet weight and length have various impacts on stability. Plug your accurate load into the spreadsheet tool.

tpcollins · March 31, 2013, 05:05 PM

I've used this stability formula in the past but this is the first time for my .223 Colt. On another website I found supposedly the "ideal" bullet length for various twist rates and for my .224 1:7" it's 1.004" - 1.157". My 60gr Hornadys and 60gr Noslers are .758" and .867" respectively. These give me a SG of 5.+ and I believe the ideal range is 1.4 - 2.0. My fps is close to 2700 and I can get closer to the 1.4 SG if I increase the length input to 1.150".

I realize that heavier bullets will stabilize fine because they are longer but I can't believe I'm relegated to just using heavier bullets . . .

Marco Califo · March 31, 2013, 05:56 PM

Yes, I think you are looking at using 62-75 + gr bullets for the sweet spot of 1:7" twist. My 1:9" is more versatile. See:
http://www.accurateshooter.com/cartridge-guides/223rem/
"We’d only select a 1:7″ or faster twist barrel if we had a need to shoot the 90gr VLDs."
I would paste the section on twist, but I am not supposed to here.

balderclev · March 31, 2013, 06:31 PM

+1 Marco.

I have a Tikka with a 1:9 twist. I shoot Hornady 37gr varmint grenades up to Hornady 62 gr rounds with less than MOA accuracy. Never tried any bullets over 62 gr. Really thought the 37 gr would overspin but they did just fine.

tpcollins · March 31, 2013, 06:51 PM

Nice link Marco, I've always seen the maximum weights for 1:7" twists, I wish there was a chart for "minimum" weight as well.

Marco Califo · March 31, 2013, 07:26 PM

1:9" will handle Hornady 75 HPBT
In fact I can hit the 400 yd steel plate (almost) everytime.

March 30, 2013, 08:19 PM	#1
tpcollins Senior Member Join Date: February 18, 2009 Location: SE Michigan Posts: 558	Fast twist and lower velocity question NOT WORKING !! My Colt AR has a 1:7" twist and it shoots 55gr factory ammo fairly well. I've only reloaded 60gr bullets using IMR-8208-XBR. Although my powder range is 21.5 - 23.6 grains, my most accurate loads for (3) different brands of 60gr bullets was 22.4 grains of powder. I know the 1:7" twist will accommodate heavier bullets but do you think my 16" barrel just likes 22.4 grains or does the 1:7" twist prefer a slower velocity (basically the same thing almost)? Thanks. __________________ What direction did that last shot at Kennedy come from? Last edited by tpcollins; March 31, 2013 at 04:56 PM.

March 30, 2013, 08:26 PM	#2
Marco Califo Senior Member Join Date: April 4, 2011 Location: LA (Greater Los Angeles Area) Posts: 2,598	See this thread: See this thread: http://thefiringline.com/forums/showthread.php?t=520997 for a calculator of bullet stability. Velocity, bullet weight and length have various impacts on stability. Plug your accurate load into the spreadsheet tool. __________________ ............

March 31, 2013, 05:05 PM	#3
tpcollins Senior Member Join Date: February 18, 2009 Location: SE Michigan Posts: 558	I've used this stability formula in the past but this is the first time for my .223 Colt. On another website I found supposedly the "ideal" bullet length for various twist rates and for my .224 1:7" it's 1.004" - 1.157". My 60gr Hornadys and 60gr Noslers are .758" and .867" respectively. These give me a SG of 5.+ and I believe the ideal range is 1.4 - 2.0. My fps is close to 2700 and I can get closer to the 1.4 SG if I increase the length input to 1.150". I realize that heavier bullets will stabilize fine because they are longer but I can't believe I'm relegated to just using heavier bullets . . . __________________ What direction did that last shot at Kennedy come from?

March 31, 2013, 05:56 PM	#4
Marco Califo Senior Member Join Date: April 4, 2011 Location: LA (Greater Los Angeles Area) Posts: 2,598	Yes, I think you are looking at using 62-75 + gr bullets for the sweet spot of 1:7" twist. My 1:9" is more versatile. See: http://www.accurateshooter.com/cartridge-guides/223rem/ "We’d only select a 1:7″ or faster twist barrel if we had a need to shoot the 90gr VLDs." I would paste the section on twist, but I am not supposed to here. __________________ ............

March 31, 2013, 06:51 PM	#6
tpcollins Senior Member Join Date: February 18, 2009 Location: SE Michigan Posts: 558	Nice link Marco, I've always seen the maximum weights for 1:7" twists, I wish there was a chart for "minimum" weight as well. __________________ What direction did that last shot at Kennedy come from?

March 31, 2013, 06:31 PM	#5
balderclev Senior Member Join Date: March 20, 2013 Location: Benton, Arkansas Posts: 120	+1 Marco. I have a Tikka with a 1:9 twist. I shoot Hornady 37gr varmint grenades up to Hornady 62 gr rounds with less than MOA accuracy. Never tried any bullets over 62 gr. Really thought the 37 gr would overspin but they did just fine.

March 31, 2013, 07:26 PM	#7
Marco Califo Senior Member Join Date: April 4, 2011 Location: LA (Greater Los Angeles Area) Posts: 2,598	1:9" will handle Hornady 75 HPBT 1:9" will handle Hornady 75 HPBT In fact I can hit the 400 yd steel plate (almost) everytime. __________________ ............