I remember my service M16A2s and M4s having a 1 in 7 barrel twist, and FM 23-9 telling us that the 63gr M855 was the round to shoot, because the 55gr M193 would have a looser pattern among other things (mistaken?).
My S&W M&P 15 carbine today is marked 1 in 9 in twist. Would this be to make it better to handle a balance of rounds rather than the faster 1 in 7 (M855) or the old slow 1 in 14 (M193)?
sputster
Posts: 760 | Location: Kansas | Registered: 18 December 2003
The theory, and I use the term lightly, with the 1:9 versus the 1:7 is that a 1:9 produces less pressure in the same loads. Supposedly you can, in theory, run hotter loads with less pressure in the 1:9. Unfortunately the 1:9 will not stabilize the tipped or monolithic bullets in the 70-75 grain range.
Perry
Posts: 2252 | Location: South Texas | Registered: 01 November 2005
My S&W M&P 15 carbine today is marked 1 in 9 in twist. Would this be to make it better to handle a balance of rounds rather than the faster 1 in 7 (M855) or the old slow 1 in 14 (M193)?
No. A 1:7 is more versatile because it will stabilize every bullet made for the .224.
Perry
Posts: 2252 | Location: South Texas | Registered: 01 November 2005
Perry, varmint type bullets in a 1:7 at high velocity may not be very useful... 50 grain speers tend to blow up regularly out out of my 1:7 ARA2 chromed 20 incher. Velocity is upwards of 3600fs
For general use, the 1:9 is good, as long as very heavy & long bullets are not intended, especially out of short barrel lengths.
Most 50-69 grain bullets do fine in a 1:9 16-24 inch barrelled 223.
My 1:9 CAR does not stabilize 75 Amaxes, but the same load in a AR15A2 Service rifle (1:9 twist) is a MOA load out to 600 yards (HP slow fire)-velocity between the two is the key- only 2435 from the 16 incher, 2750 from the 20 incher, both are wilson barrels-they do not fit an AR magazine, however.
Originally posted by Andrew cempa: Perry, varmint type bullets in a 1:7 at high velocity may not be very useful... 50 grain speers tend to blow up regularly out out of my 1:7 ARA2 chromed 20 incher. Velocity is upwards of 3600fs
For general use, the 1:9 is good, as long as very heavy & long bullets are not intended, especially out of short barrel lengths.
Most 50-69 grain bullets do fine in a 1:9 16-24 inch barrelled 223.
My 1:9 CAR does not stabilize 75 Amaxes, but the same load in a AR15A2 Service rifle (1:9 twist) is a MOA load out to 600 yards (HP slow fire)-velocity between the two is the key- only 2435 from the 16 incher, 2750 from the 20 incher, both are wilson barrels-they do not fit an AR magazine, however.
Best
How in the world are you getting 3600fps out of a 20" bbl? What is your load? 1:7 IS the most versatile when pressures are kept with in SAMMI specs.
Perry
Posts: 2252 | Location: South Texas | Registered: 01 November 2005
Originally posted by sputster: I remember my service M16A2s and M4s having a 1 in 7 barrel twist, and FM 23-9 telling us that the 63gr M855 was the round to shoot, because the 55gr M193 would have a looser pattern among other things (mistaken?).
My S&W M&P 15 carbine today is marked 1 in 9 in twist. Would this be to make it better to handle a balance of rounds rather than the faster 1 in 7 (M855) or the old slow 1 in 14 (M193)?
The military chose the 1:7 twist not so much because of the M855 round (which would stabilize with a 1:9 or 1:8 twist).... but because of the long bullet of the M856 Tracer.
When in doubt it's better to error on the side of a faster twist than a slower twist.
Originally posted by Andrew cempa: Perry, varmint type bullets in a 1:7 at high velocity may not be very useful... 50 grain speers tend to blow up regularly out out of my 1:7 ARA2 chromed 20 incher.
I've had 52 gr. Sierra MKs blow up on me out of my 1:7" Colt HBAR..... especially when the barrel heated up.
SUPER GROUPS....... until one shreds the jacket .... and then it won't even hit the paper!
Thank you all. I reload for other calibers, but this rifle will probably stay with 55gr. FMJs most of the time.
I appreciate the explanation, DaMan - I carried and shot the M855 and M856 in my magazines in theater and didn't realize the tracer was a major factor in the twist selection.
sputster
Posts: 760 | Location: Kansas | Registered: 18 December 2003
