someone here posted key holed 75 amax at 50y on a target. and many “say” a 69gr is top for a 1/9
yet both my savage bolt and my 16" ar will shoot 75’s fine, and i even shoot big 80s out of my savage with no issues, even wacked a ground hog with 1 at 200y
Not all 1:9" barrels are truly 1:9". Some measure closer to 1:8" and handle the heavier bullets just fine. Some are closer to 1:10" and don’t handle them well at all. You don’t really know what you’re getting until you measure it yourself.
Also, barrel length plays a role in stabilization. Longer barrels (such as those in most bolt guns) produce higher velocities which help marginal twist rates stabilize the heavier bullets better than the 16" barrels on most AR carbines.
From what I’ve heard is that some manufacturers will make some barrels with more of a 1-8 than a 1-9. Others might make it more on the 1-10 side. I’m sure someone will be along shortly with more information and knowledge then me, but that’s the way I’ve heard it.
Mike
never mind DRich beat me to it.
to mix it real well ,my friend colt upper is 1/7 twist-16’’ and hates all 55 grain ammo/key holes it! -but 62 grain above loves it next feed it 45 grain hp its ok so what the moo?
On the subject of barrel twist… What’s the max bullet weight for a 1/12? I have a friend thinking of cutting a 20" down to 10.5".
Ambient temperature plays a part as well. Those that shoot fine in the 70s and 80s may be an issue when the temp drops. rslivers writes about this pretty often when discussing this topic and has a data model that explains it.
As far as 1 in 12, I would not go any heavier (longer) than M193.
A 1 in 12 is GTG for 55 grain bullets. For longer bullets, a 1 in 10, or 1 in 9 works better. A 1 in 9 will handle the 69 Sierra Matchking, and anything the same BC or lower.
A 1 in 9 may handle the 75’s, but not under all conditions. The key here is reliability. If my 1 in 9 rifle shoots 75’s in Denver, that doesn’t mean it will stabilize them at Camp Perry, which is at sea level.
What the fellows are saying about inconsistent twist rates is true, but it brings a variable into the equation that shouldn’t be there. If I bought a 1 in 9 and it wasn’t, I’d be upset. I know what I want, and I’m paying the money. I get to choose.
Twist rate can also be largely mis-understood. I really don’t know the reason the military specified a 1 in 7. The rumor I heard was the 1 in 7 gave better body armor penetrating capability to their 62 M109’s. This could also be total BS. The 7 twist is really an orphan, it’s too fast for the 80 Sierra Matchkings, and too slow for the 90’s. There must be a good reason I don’t know about.
Mark15
My understanding had always been that the 1-7 was to so they could stablize the longer M-856 tracer rounds. While only a 64 GRN load, the projectile is 1.15" vs. .906" for the M-855. In other words it was less about the weight of the bullet then the length of the bullet and the twist required to stablize it.
The Army commissioned a study back in 1986 that detailed all the reasons they DIDN’T want the M16A2.
They didn’t like the 1:7 twist, instead preferring a 1:9 twist for the then-new M855 round. Their claim was that a 1:9 twist resulted in less stress on the bullet, improved barrel life, and reduced fouling. They also believed that it would result in improved accuracy.
Of course, the fact that the M16A2 was a Marine Corps initiative didn’t help.
Shoot your gun with 75gr ammo out to around 300yds and see what kind of groups you get (use at least 5rds).
C4
The orig. twist rate was 1/14 and they had issues in cold weather. Then went to the 1/12 twist.
Later on, they went to the 1/7 to stabilize the tracer rounds. This is all off the top my head so don’t hold me to it. 
C4
Of course I’m only a little past hip shooting this one. One of the OTHER experts will be along soon to school me. If you don’t first.
Grant, The 1 in 14 won’t reliably stabilize a 55 grain bullet at 223 velocities. I wonder if the cold aggravated that situation?
Before I get jumped on, I know “standard” twist for 22-250, 220 Swift, etc. is 1 in 14. It works there because velocity is high enough, or bullets used are shorter than (lighter than) 55 grain.
I did read that the EX15 (?) had a 14 twist. I’ll bet 12 was a lot better. How far back do you go Grant? I remember the AR15’s as 1 in 12.
Mark15
1:14" worked fine for 55gr M193 until the temp dropped and the air became denser. IIRC, the temperature threshold was in the 20’s, but I’ve lost a lot of brain cells since I researched that topic years ago. In “normal” temps, 1:14" had no problems stabilizing the 55gr FMJ’s. Of course, M193 is quite a bit hotter than standard .223 ammo in the same weight range.
i have, sub MOA, around 2.5" about what i get with my 40gr vmax
Sometimes you wonder wht they specify what they do, as in the following solicitation:
For the M4A1 carbines specified, a 1:9 twist rate is mandatory:
"Rifling: 1 turn in 9 (22.8 cm) R.H. twist, 6 grooves and lands (this is an absolute requirement, no other twist rate is acceptable), weight not to exceed 6.5 lbs, mode of operation semi auto and 3 shot burst. "
Wonder why?
Maybe they will only be using M855 ammunition.
I’ll give you guys a little look inside government requirements documents-
They are, more often than not, written around a specific item that a particular person thinks is the cats a$$. Insertion of seemingly irrelevant or overly specific wording is usually an indication that this is happening. What it does is ensure that only that item can meet the req doc, and therefore is the only itme that can be purchased. This can be a good thing or a very bad thing, depending on who is doing the writing and how much they know.
Well said.
This may have something to do with it though:
The U.S. Department of State, INL Section, U.S. Embassy Bogota, Colombia , has a requirement for the purchase of a six hundred (600) M4A1 type assault rifles, to be donated to the government of Colombia for INL program purposes.
Might have to deal with more M-193 then M-856. Don’t know though, I’ve never been any further south then Panama.
Friend of mine and I bought identicle URs with 1:9 barrels a number of years ago.
Both URs shot XM193 and Hornady TAP 75gr interchangeably, and grouped well.
Being my only 1:9 AR, I consider mine to be my “beater”; meaning that if I’m going to be doing lots of drills/rapid fire at moderate distances, that’s the gun I use. As such, I have many rounds through it, and it continues to shoot heavy bullets well.
Friend’s UR, a “backup” piece (only shot occasionally) with far fewer rounds, just started “patterning” (uses most of an IPSC target at 100 yards) one cold (mid-30’s) day, and has never been able handle heavy bullets since then, under any conditions.
My beater continues to rack up the rounds and shoot the 75gr well.
Friend’s lightly used/identicle UR started patterning with 75gr, and it was not a gradual thing…like someone threw a switch…one day it was “grouping”, the next it was “spraying”…yet it still shoots XM193 with the same accuracy!
Interesting…
-Rainman