M855 is only 62gr. If you can't keep up, take notes.



 

Thanks for that chart 87GN. That definitely will make me buy a 1x7" twist barrel for my SPR build.

I wonder why Larue went with a 1x8" twist for their LW50 barrels?

You will be fine at least to 600 yards with 75gr BTHP using most 1/8s. If you want to shoot farther than that or you want to use 80s, especially at distances beyond 600 - 1/8 might not be your very best option.

Krieger 1/7.7 match barrels are probably the best thing on the market.

I'm messing around with a Spikes (LW) 1/8 SPR barrel at the moment, it's been killer out to 500 (meters).

I can't say that for some of the cheaper 1/8s that I've owned (DPMS).

Just my $.02 on it but if you are shooting past 600yrds on any sort of regular basis, I'd say it's time to step up to something bigger then .223/5.56

As far as the 1/7 vs 1/8 at longer ranges... my guess is this has more to do with barrel quality rather then twist rate. I would bet a top quality 1/8 twist barrel is going to shoot better at those ranges then a low quality 1/7 twist barrel will?

Shooting 55 grain bullets in a 1-7 twist barrel will give good accy. provided the barrel and chamber is of a good quality, uniformity and correctly done. The freebore diameter of a chamber has a more detrimental effect on accuracy than the length of that freebore. The Wylde chamber in my 1-7.7 Krieger barrels will shoot Sierra 52 grain bullets under an inch at 100 yds with iron sights. The freebore diameter in this chamber is only a couple ten-thousands (.0002) over bullet dia. which keeps the bullet straight as it enters the rifling. I use this same chamber in my .223 varmint rifles with Hornady 50 grain V-Max bullets and they shoot quite well. But, a NATO chamber has a much larger freebore diameter and usually does not give the same high level of accy. Match grade bullets are less effected by a faster than necessary twist than those bullets constructed with less uniformity. Spinning bullets in excess of what is necessary for stabilization will show flaws in bullet uniformity such as non-uniform jacket thickness in the form of larger groups compared to when those same bullets are shot from a barrel of equal quality that has just enough twist to stabilize them. Imagine an out of balance tire. Spinning slowly usually doesn't give any problems, but speed up to 90 mph and you can feel the vibrations. So, in the end it depends on how good your barrel is and how good the bullets are. The only 55 grain fmj bullets I like are the Hornadys. I have a Colt chrome lined H-Bar barrel w/NATO chamber that shoots the 55's into 5" groups at 100, but will shoot under 2" with the Sierra 77. I think the longer 77's help overcome the problems of the large freebore diameter.
EDIT: Cut rifled barrels have twist rates that are pretty much exactly as advertized. Button rifled barrels can vary quite a bit, so your 1-9 twist may actually be 1-9.5 or even slower. A group of our local highpower shooters were having trouble getting Sierra 80 grain bullets to shoot well with a particular batch of Douglas 1-8 twist barrels. They determined that the twist was somewhat slower than advertized.

Provided the twist rate is proper for the bullet, it will make round holes until it slows down to the point it becomes sub-sonic and will destabilize resulting in tumbling.

I own 1/7 and 1/9's both shoot 55 grain fine. However I am thinking of getting the new 1/1 to be extra tactical Given the choice I would go with a 1/7 or 1/8 just more options

So are all these comparisons fair between 1:9 and 1:7 twist? It seems most that run 1:7 are using chrome lined bores, while there are more 1:9 barrels being used without chrome lining. Being that the chrome-lined uses a greater clearance to allow for the chroming, are they needing the advantage of the faster rate to help stabilize the bullet, yet less likely to see the ill effects of the faster twist rate due to the greater clearance? Maybe this is one of the reasons the 1:8 stainless does so well, being the better of both worlds with a faster rate AND tighter clearance? Will the 1:9, when chrome lined be even less inclined to stabilize than a non lined, as well as any of the 1:7, being that too, will be made with less clearance for the chroming?  

Another question besides accuracy, how about impact performance? Any testing done on lighter skinned, police ammo or others designed to open up, as opposed to military, being designed to hold together, between the various twist rates? Would again, a chrome lined 1:7 be less inclined to affect the opening performance than a non lined?

Just thinking out loud here.

If I am understanding your post-
Chrome lined or not has nothing to do with bullet stability. Bullet length and caliber (diameter) dictates what the twist rate needs to be in order to spin the bullet at an rpm that provides stability. Going back to the generous freebore/throat dimensions of the NATO chamber in my previous post, this does not have any effect on bullet stabilization either, but it can have a detrimental effect on accuracy. A NATO chamber is designed for reliability with military ammo which runs at higher pressures than the average Joe-Mart ammo. I suppose the chrome lining takes up some of the slop but still an insignificant amount.  Chrome lining cannot be applied in a perfectly uniform manner and this also normally degrades accuracy. Bullets need stability to be accurate, but chrome lining or no lining in itself does not determine whether a bullet is stable or not. I have no idea if twist rate has anything to do with bullet performance on target.

Just thinking the chrome bore would be creating less drag, so this makes me think 'less contact with bore' so reduced influence of the rifling on the bullet. So maybe the faster rate would benefit here to recover some of the performance?

This thread already has its fair share of misinformation.  Please spend some time studying interior, exterior and terminal ballistics, as well as barrel construction, before introducing any more.

I'm liking my 2 chrome-moly, 1:9, Model 1 Sales Heavy Barrels more and more.

Cheap to buy from the get-go.

Cheap to shoot with the 55 grain Mil-Surps being both cheap and plentiful.

Very accurate.

Best of all, the Tier Snobs and high-strung gurus hate 'em.

My Model 1 CAR 1-9 chrome-moly barrel was a pretty good shooter. I still have a chrome lined GI contour 1-7 20" Model 1 barrel. It shoots my 55 fmj Hornady loads into ~3.5" at 100 yds. and the 77s into roughly 2.5". The Bushmaster heavy 16" CAR 1-7 chrome lined barrel shoots a little tighter than the Mod.1 barrel with fmj 55s but about the same with the 77s. All 10 shot groups, benched with the iron sights. None of these are free floated. I own a variety of .223 stuff including Krieger, Hart, Shilen, Mod.1(Shaw?) and Douglas barrels and have tried different ammo through most of them to see what they like best. I'm a competitive shooter and an accuracy nut, but I still enjoy my less accurate rifles as they have a place and a purpose.
Not much of anything I've said has anything to do with the OP's question. My answer would be nothing much except the math says it will stabilize longer bullets that the 1-8 twist will.

Like I said, very good accuracy.

The very first target shows a three round group of my handload of 19.0 grains of AA1680 under 50 gr. Sierra PSPs, in Remington once fired brass lit by CCI 400 primers. Frankenplinker build #2 really loves those. The second target is Frankenplinker build #1's favorite of 55 grain UMC factory.
The rest are various loads and a few blasting function tests using Lake City '09 stuff.
100 yards, rested, ancient old 16X Bushnell scope.

















The family photo.

If you notice the question marks, I was asking (not stating fact) to help me understand what I HAVE been trying to read and learn about. I thought those questions would fit in the topic of this thread.  If you don't want to answer, don't. If you see some questions you'd like to elaborate on, then pick them out and do so, I'd appreciate it.

He probably gets tired of answering the same questions week after week. He told you what to read up on. The resources are here on this website, check the tacked threads in the technical forums.

You usually see 1:8 on stainless barrels, mostly because stainless isn't hard enough to handle the additional stress of the faster 1:7 spin rate.  A chromed moly steel barrel can handle the higher spin rates from a wear perspective better than stainless if it where cut at the same 1:7 twist rate (if it would work at all, I don't really know).  As many others have stated, if you never shoot past 100 yards, and never shoot anything bigger than a 55 grain bullet, go with the 1:9.  If you do try and shoot heavier rounds any significant distances >200 or 300 yards, you may not hit shit.  It'll shoot, and if you hit what you are shooting at, it will be a happy accident.

55 grain BlitzKings fired from the above stainless steel barrel with a 1:7" twist.

 

I think the Capt may have had too much Capt.....

Damn is this still going on.  this is easy. Ask the High Power shooters what they are using.
Around here that would be 1x7.7 or 1x8.  That should give you an idea of what is what.

Shoot 5.56 USGI Tracer rounds better.

Wow, there's a whole lot of highpower shooters out there with stainless 1/7 barrels who would be most distressed to hear this.  And people (like me) who have shot everything up to 75gr in a 1/9 barrel.

How light of bullets do you plan on shooting?  A 1:7" twist barrel can shoot quality 40 grain bullets quite well.  The 7-shot group of 40 grain V-MAXs pictured below was fired from 100 yards using a Noveske barreled AR-15 with a 1:7" twist

Ignore me, i didn't notice its a ghost thread.

It's interesting to note a 20" barrel with a 1:7" twist just barely stabilizes M856 with a gyroscopic stability factor just above 1.

Late to the party but figured I'd give a to the "stainless barrels aren't hard enough for 1:7" comment.

Generally, SS is harder than chrome-moly.

molon and 87gn, I was just curious what the exterior conditions in the 75/77grn gyroscopic stability charts are.  alt, temp, atm...  I'm particularly interested in how those variables could affect a barrel/projectile combination that is on the edge of stability.

also do either of you have charts for some 69 grn projos?  what software are yall using to make these?

I never understood why so many hate 1:9 in a AR. I bet 90% of the time most of you shoot 55 grain ammo anyway. I know up to 75 grain can be fired from a 1:9 without issue. I have both and never saw any difference between the two.