Here is my question:  what is the real life difference between shooting heavier (say, 77 gr Sierra MKs) out of a 1 in 9 twist AR barrel (of any length) and shooting them out of a 1 in 7.  Theoretically/technically, you should shoot the heaver 77 grain bullets out of a 1 in 7 barrel.

But, real world, has anyone shot the heavy bullets in a 1 in 9 tube and what were the results?  Can you really see any difference at 100-200-300 yards?  Do the bullets go unstable at some defined distance or are they unstable and keyhole at 100yds.

What are people's experiences?  I want to build an upper, but most of the barrels, if available, are the slower 1 in 9.  Will that restrict me back to mil spec 62 grain bullets?

Thanks in advance.

That's the great unanswered question, didn't you know that! Either my search skills are severely lacking or nobody has had any problem shooting the heavies out of 1/9 barrels. Every thread I've read about this has a bunch of posts full of theory about it not stabilizing, and a couple of posts by guys that have actually tried it and said it shot fine. I just ordered a bunch of 69 and 75gr Prvi, so I can try it myself.

I just shot 30 shots yesterday out of my 1/9 Doublestar at 100 yards... shot a 280-5x with 69gr match rounds.  I think they are pretty stable.

Today we put 300 rounds of MK262 Mod1 through a 16" Stag with a 1/9 twist at 200yds.  We also shot about 60 rounds of M855 and 120 rounds on M193.  The Mk262 Mod1 shot best at about 3" at 200.  The worst was the M193 at about 4.5" at 200.

Dave

1/9 twist RULZ!

You can cover more ground with the 9in than the 1/7.5, few will debate that.

YES!  I wish I could find the link which includes pics.  Perhaps someone else can point to it.  But I have seen the targets where the heavier bullets when fired from barrels with not enough twist(since I can't find the link it's possible these were shots from barrels with slower than 1 in 9 twists) instead of round holes you can see where the shots keyholed the paper as the bullets were unstable in flight.
As some have pointed out in here however some guns/1 in 9 barrel combination's will indeed shoot heavier bullets successfully.  You have to test fire to see.

Added:  Oh and something else I remember reading...it's not so much the weight of the bullet but the length that affects bullet stability in slower twists barrels.

You need to shoot the heavier grain rounds through YOUR barrel and see how it performs.  I personally know for a fact that the BH 77gr Mk262 5.56 will keyhole out of my Bushy 1 in 9 barrel at 50 yds

I had some 68gr Hornady OTMs I loaded up, and as the loads got hotter, the groups got crummier, until I was keyholing at 25 yards. This was duplicatable with two different 1/9 ARs. I never got near max loadings, for the record. Every barrel is a unique snowflake, you have to figure out what will work with your barrel. :)

Interestingly, the reverse is not always the case either.  I always heard that a 1/7 is poor at shooting 55 grn and lighter bullets compared to a 1/9.  Just for shiggles - I went to the range this past week and ran a bunch of factory loads through my SIG 556 1/7 and my LMT Defender 2000 1/7.  I shot Federal 50 grn SP, Fed 55 grn BTFMJ, Rem 55 grn FMJ, Hornady Steel case training 55 grn FMJ, and some M855 62 grn.  

Of all the factory loads - the 50grn SP gave me the best consistant 10 shot groups at 100 yds off the bench followed closly by the Hornady 55grn training rounds.  The M855 was by far the worst of the lot.  

Disclaimer:  all of the factory stuff was the lower end stuff like the Rem cheapo green box or the Federal Wal-mart value packs.  I'm sure with some quality match ammo or handloads with the right bullets - I might have see different results.  But my point is that you can't say dogmatically that 1/9s always shoot lighter bullets better and 1/7s always shoot heavier bullets better.

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MK262 barely edging out mil-spec fodder tells me that your 1/9 barrel was not to it's liking. It should have cut M855/M193 groups in half.

Alright!  sounds like I'm getting a qualified "maybe!"  I found a buddy who has both 1/7 and 1/9 .223 platforms.  Going to try to get them to the 200 yd range and see what we can make happen.

From what you all are saying, sounds like at moderate ranges, accuracy is OK with heavies in the 1/9, but probably better in 1/7.  I asked the bulletsmiths at Sierra what their opinion was and hope to get an email back.  I'll post it when I get it.

Thanks all,

I just started shooting 68grn BTHP out of my Bushmaster 14.5" bbl with a 1:9 twist. At 100 yards I get perfectly round holes, no key holing. I'm interested in how they shoot at 200-300 yards.

Current arfcom fad will point towards 1:7, but both 1:9 and 1:7 are capable of shooting most rounds. I think 1:7 is definitely more suited towards the heavier bullets and if that was the majority of that upper's diet, I'd go that route.

Most of the Colt and Noveske guys will tell you that 1:9 will not be accurate with anything much heavier than 62gns.  Personally I'm still on the fense b/c I haven't tries heavy ammo at anything loner than 100yds yet.  That said, I think 1:7 will solidify as the new industry standard simply b/c GI's like it.

Not all barrels are exactly 1/9 or 1/7, some may be 1/9.4 and some may be 1/8.7 and so on.

Out of a group of 20 1/9 uppers I owned, some would keyhole 77s, some would keyhole 75s, some shot good groups with both.

Now, I shoot 52s out of my 1/7 uppers and the accuracy is perfectly acceptable. I can also shoot 80s. This is why I stick with 1/7 uppers now.

68/69gr ammunition is perfect for 1/9 twist.

I have no problems with Mk262 77gr Match out of 14.5" and 20" 1:7 bbls (M4 and M16) out to 500 yds with irons.  However, out of my 16" 1:9 Oly the same ammo will keyhole at 50 and 100 yds.  Stopped testing at that point.  


CD

Colt LE6920 1:7 twist at 100 yrds with Hornady Match 75 Grain Boat Tail Hollow Point - 3/4" groups.  62 grain Remington Match ammo at 100 yds - 6" groups.  Go figure.

Good thread! I was thinking on changing barrels until I read this.

trust me, same can be said about 1/7 uppers.

i've owned several 1/7 and several 1/9 guns.

all performed completely differently with lighter or heavier loads.

to be honest? it's kind of a crap shoot, just depends on the barrel you get.

that said? i'll stick with 1/7 any chance i get. just had more luck with them and i prefer heavy bullets for protection. lighter bullets i just use for plinking when not that worried about accuracy.

1/8 best of both worlds will stabilize all but the vld's

That's kinda what I've been thinking.

BTW are there any tools out there to run through a barrel to help determine the twist rate more precisely?

1:7.

From my experience, my RRA 1/9 is extremely accurate using Federal Match 69gr BTHP ammunition at 300m.  Our dept restricts us to either Colt (1/7) and RRA (1/9).  Since I had purchased a full LMT set-up prior to being hired and being aware of  the restrictions, I opted to go the cheaper route and go for a RRA.  I can't compare the two simply because I've upgraded the RRA with a larue 13.0 free float rail system, and a Geiselle 2stage after the factory RRA 2-stage failing on me after about 2k rounds. I have no experience with the heavier 77gr bullets but up the 69gr's seem to the the most accurate for distances outside of 100m.  Unfortunately they are have to be my secondary load due to another dept restriction that limits our duty ammo to Winchester 55gr BTST (Ranger or the more common varmint rounds).

I first wanted a 1/7.
I now wish for a 1/8.
I'll be getting a 1/9.
I hope it doesn't matter 1/10 as much as I'm hearing it does.

Since we seem to be talking barrel makes somewhat, who has experience with Sabre Defence 1:7 vs. 1:9?

While I don't have 20 different uppers with varying twist rates, the above is why I strictly stick to 1/7 now.  The diversity in a 1/7 outweigh that of the 1/9.  I

Mileage varies.  

My Bushmaster 1/9 failed to reliably stabilize 75 gr HTBT, and around 25% of the rounds were key-holing at 100 yards.  

Really it a misnomer about the weight of the bullet -vs- twist. What really is more important is lenght of the bullet. A faster twist favors a longer bullet where a slower twist favors a shorter bullet. What you should do is find the bullet that suits your need and match the twist to it. It does seem that for the most part a longer bullet is heaver it is not always the case. That is why some seem to find that "the heavy bullet shoots poor out of my 1/7 rifle". I know that I'm new to AR style rifle's but I have years of long range shooting, prohorns @ 300/400 yds with a Weatherby .257 Mag. IMHO find a long bullet reguardless of weight and it will fly great outta a 1/7 twist.

If you are not shooting 45 grain or 50 grain ammo there is NO reason to select a 1/9 over a 1/7.  For shorter barrels the faster twist is more important.  Ever had a bullet fail to stabilise on exit and destroy a $1000 silencer?  I have.  It was not caused by a weight/twist problem but instability can be a HUGE problem.  Shooting 55 grain from a 1/7 AT most might casue a tiny decrease in acuracy.  1/9 shooting heavy match ammo can cause a LARGE decrese in accuracy. When a silencer is to be used I would neither shoot heavy ammo from a short 1/9 nor 45 grain ammo from a 1/7 as it might come apart or destablise enough to get a baffle strike.

Ask yourself these questions...

Do I have a barrel under 16"?  If yes, get a 1/7.
Do I shoot ammo under 55 grains? If no, get a 1/7.
Do I shoot heavy match ammo?  If yes, get a 1/7.

If you will NOT shoot heavy match ammo (over 69 grains), you have a barrel of 16" or longer, AND you will shoot ammo under 55 grains then you should get a 1/9 barrel.

The perfect example of where a 1/9 is prefered is a 20" varmint rifle shooting 45-50 grain ballistic tips at rodents.  The perfect example of where you should get a 1/7 is a SBR, using a silencer, that is shooting 75 or 77 grain ammo.

For the average arfcom shooter who will shoot M193 and M855 from a 16" chrome lined barrel, 1/9 vs 1/7 twist wont amount to a hill of beans.  In that case I'd decide if I would shoot more 45-50 grain ammo or heavy match type ammo and make the twist decision based off that.

As for 1/8... Id still pick 1/7 for a SBR where immediate stability for use with a can is preffered although again it wont matter in the real world.  For an 18" SPR barrel it would be a fine substitute for 1/7 but again it wont have any real world difference.

More important is that almost all 1/9 barrels are cheap 4140 barrels and almost all 1/7 chrome lined barrels are slightly better CMV barrel steel.  THe olny 4150/CMV barrel that is 1/9 I know of is Bushmaster.  I dont know of any 4140 chrome lined 1/7 barrels.