1:9 16-inch AR with 77 grain, bullet stability

[Cump]

I recently zeroed a Vortex Strikefire at 50 yards on my AR. Colt 6721. 16 inch heavy barrel, chome-lined, 1:9 twist.

I tried my first MK 262 77 grainers in it, knowing the twist rate is not ideal. At 50 yards they were significantly more accurate than m193 or m855. Right at moa, which is saying something considering I was shooting.

I know that this doesn't prove 1:9 barrels are as good as 1:7 with heavy bullets. Obviously, I was shooting at close range, before significant velocity loss. But it might be good to note that instability wasn't immediate. (Some posters, not here of course, seem to suggest heavy bullets do somersaults out of a 1:9.) When I get out again with the AR (might be a while because of work), I'll try 100 yards.

If you've shot 77 gr with a 1:9 barrel, what was the max range you kept typical groups without keyholing?

 

[HOOfan_1]

Most mk12 SPR clones use a 1 in 8 twist and the 77 grain SMK mk262 ammo was designed for them.

1 in 7 was designed for tracers which are probably longer than the 77 grain bullets

 

[Jackal]

1:9 is just as good as 1/7. If the military switched back to 1:9, all the hardcore "milspec" fanboys would switch back to it. I personally find the ability to use lightweight bullets at screaming velocities more useful than the heaviest bullets at a plodding pace. Kind of sad nowadays that most people look down on the 1:9...

 

[Eb1]

It will not stabilize in a 1:9. The bullet is two long.
1:9 is good to about 69 grain SMK or 68 grain Hornady Match. You can count the 75 A-Max out as well.

You might get lucky and have a 1:8.5 twist or unlucky and have a 1:9.5 twist. My old Colt HBAR was a 1:9.5 and would not shoot over 64 grain bullets worth a flop. Sure you could keep them on a man size target at 100 yards, but after that 'zing' who knew where they went. Kinda like shooting M193 at 600 yards. It will not make it.

Get a cleaning rod and put a patch on it. Make a mark on the rod, and pull the tight patch until the rod makes a complete circle. Then make another mark, and measure the distance between marks. That will tell you close to what the actual twist of your barrel is.

I was a fanboy and had to have a 1:7 twist, but now I'd give anything for my old Post Ban 20" Colt HBAR with a 1:9.5 twist (marked 1:9) that would shoot 55 grain pills into a dime at 100 yards, and out to 300 yards was way under an MOA.
But I started shooting HP comp, and needed the 77 grainers to make it to 600 yards.. In my 16" mid-length with rifle hand guards I can shoot a M193 to 540 yards accurately. After that they go 'zing'. I can shoot 62 grain M855 to 600 yards, but they are not near as accurate as the 77 grain SMK.

 

[Bexar]

Cump...What muzzle velocity were you shooting and did you shoot anything past 100 yards yet? Higher velocity will sometimes provide stability but as velocity degrades so does stability.

I'm not entirely familiar with the 77gr MK but if they are, and I suspect they are, hollowpoints with a hollow cavity towards the front. This sets back the center of gravity of the bullet reducing its ballistic length. Length of bullet not weight is what determines "spin" stability.

Plus...MKs have a thinner jacket than GameKings. This further aids to stability by increasing the axial alignment in the bore. A (theoretical) 77 gr. GK may not demonstrate the same stability as a 77gr MK.

Everything is relative to the results in your rifle on almost any given day.

All that said...it sounds like you had fun and that's the bottom line.

 

[Cump]

They are brown box mk 262 mod 1. HPBT. Rated for 2750 fps, but I did not use a chrony to find out velocity out of my barrel.

 

[moxie]

Jackal,

The very early M16s and variants had a 1:14 twist, but this was changed to 1:12 where it remained until the mid-80s, when the A2 were issued with the 1:7 twist to go with the longer/heavier M855 and tracer round. Don't recall any issue 1:9 versions.

 

[ClemY]

I tried my 77 gr. Sierra handloads in my 9" twist rebarreled Mini-14. They shot very well, but when I looked carefully at the target, I could see that the bullet holes in the paper showed "scuff" marks on the holes indicating that the bullets were in a bit of a yaw as they went through the paper. This was at 50 yds. I haven't tried them at longer range.

These loads shoot very well in my 8" twist ARs.

 

[Hardtarget]

Always glad to see this kind of info. I'm still tinkering with bullets and loads in my AR. Trying to find what shoots best and stick to that. I'm not afraid to learn from others work. I understand that each rifle is different,but, if results are really bad for one gun it doesn't make sense for me to expect much better from my rifle. This "testing" part of reloading and shooting is the most fun of the hobby.

Mark

 

[Bexar]

I tried my 77 gr. Sierra handloads in my 9" twist rebarreled Mini-14. They shot very well, but when I looked carefully at the target, I could see that the bullet holes in the paper showed "scuff" marks on the holes indicating that the bullets were in a bit of a yaw as they went through the paper. This was at 50 yds. I haven't tried them at longer range.

These loads shoot very well in my 8" twist ARs.

Gen. Hatcher in his book Hatcher's Notebook discusses bullet yaw. Bullets in properly twisted barrels and even some incorrectly matched twist quit yawing over longer distances. He shows a 30 caliber bullets path through a wood block shot at close range and one shot through a similar block at longer ranges. The close range bullet yaws and tumbles through the block but the long range bullet "drills" a straight line into the block.

Unfortunately I had loaned my two Hatcher authored books and they were never returned so I can't quote chapter verse. Rearward bullet yaw is a sign of twist and velocity issues...front bullet yaw is caused by washed out bores and damaged crowns.

I doubt yours is the latter case. That said...Ruger Mini-14s did have a run sometime back of slightly oversized bores.

Anyway...I'll post some bona-fides references on yaw in a little while.

https://www.youtube.com/watch?v=Z3h0pMQAFUU

Dry lecture but informative.

 

[carbine85]

As previously stated the 1:7 is used for tracer rounds. I have a 1:9 that is an awesome shooter. My 1:8 is the perfect twist and shoots sub MOA with 69 and 77 bullets. My 1:7 is a good shooter but I bought because it was the only thing available at the time.
It's not the length of the bullet that's the biggest factor, not the weight.

 

[Lord Teapot]

the bullet spin really comes into play at longer ranges, try shooting further and see if you still see these results.

 

[Cump]

I ran my numbers through the berger twist rate stability calculator. I used the bullet length provided by the JBM list: http://www.jbmballistics.com/ballistics/lengths/lengths.shtml

The JBM length (.994) appears to account for the hollow tip, but I am not sure. If it doesn't it would seem to improve the numbers. Correct me if I'm wrong.

1:9 appears to provide "comfortable stability" at that bullet length and velocity at my likely shooting altitudes (4,200 ft and up). At the lower altitude range, I should start seeing "marginal stability" at a little under freezing. At the higher altitudes, I should be able to expect stability until I can no longer feel the trigger finger.

My last range day was at 6,000 feet. If I were shooting near sea level, stability would be marginal on all but the hottest days. Didn't realize the effect this would have. (If you are at sea level, with a 1:9 you can send your mk 262 to me

I'll try it again at 4,200 ft at 100 and 200 yards on a cooler summer day.

 

[AABEN]

Jackal,

The very early M16s and variants had a 1:14 twist, but this was changed to 1:12 where it remained until the mid-80s, when the A2 were issued with the 1:7 twist to go with the longer/heavier M855 and tracer round. Don't recall any issue 1:9 versions.

There was thousands of 1/9 made then came some 1/7 now some 1/8 are getting on the market now. I like the 1/8 and the 1/9 my Arma Lite is a 1/9 it shoots the 69 gran very good 3 shots at a 100 yd one hole. I jest got my knew Windhan VEX it is 1/8 I do not have it set up to my liking yet need a Timney trigger for it. I like a 3 to 31/2 pound pull.

 

[Grumulkin]

Nosler 77 gr. BTHP bullets give me about 0.6 MOA at 100 yards out of a 1:10 twist barrel.

 

[MistWolf]

Sierra testing shows that the 1:9 barrel does not stabilize the 69 gr SMK bullets as well as the tighter twists do. Tests show that the 69 gr SMK fired from tighter twists (resulting in higher rpms) have better ballistic coefficients, which means the bullet is more stable

 

[Bexar]

I ran my numbers through the berger twist rate stability calculator. I used the bullet length provided by the JBM list: http://www.jbmballistics.com/ballistics/lengths/lengths.shtml

The JBM length (.994) appears to account for the hollow tip, but I am not sure. If it doesn't it would seem to improve the numbers. Correct me if I'm wrong.

1:9 appears to provide "comfortable stability" at that bullet length and velocity at my likely shooting altitudes (4,200 ft and up). At the lower altitude range, I should start seeing "marginal stability" at a little under freezing. At the higher altitudes, I should be able to expect stability until I can no longer feel the trigger finger.

My last range day was at 6,000 feet. If I were shooting near sea level, stability would be marginal on all but the hottest days. Didn't realize the effect this would have. (If you are at sea level, with a 1:9 you can send your mk 262 to me

I'll try it again at 4,200 ft at 100 and 200 yards on a cooler summer day.

Cump...I'd love to give you a specific answer that nails your question specifically. Unfortunately, as you're discovering, there are tremendous amount of variables from bullet design to atmospheric conditions to individual rifles and how you have your toes crossed.

You are pushing the envelope with the 77 gr bullet but as others have mentioned some bullet rifle combos haven't read the book on how they are supposed to behave. I've shot premium 30-06 out of a known precise rifle (more of an instrument than a rifle) that wouldn't group 4 inches at a 100 yards. Thinking it was a bad box...I shot them in another 30-06 and it produced a one inch group. Thinking again the precise bragging rifle may have something gone astray I fired a different commercial brand and it produced an excellent group.

Anyway...keep us posted...enjoy your shooting. Thanks to you and others for sharing y'all's experiences.

 

[moxie]

AABEN,

Note that I was referring to military issue M16s and variants, not civilian ARs.