I have an unusual situation with my DPMS Panther A4. It has a 16” barrel and is rifled with a 1 in 9” twist. I’ve had it for 3 years, and have fired approximately 2000 rounds through it. Up until recently, I used both 55 grain and 62 grain (green tip) rounds. I never noticed any difference in accuracy between the two. Recently, I visited my range and decided to do some shooting at 100 yards. I was using XM855 62 grain green-tip ammo. The first two rounds were on target. Then, as I continued shooting, I could get no more hits. Finally I tried it at 50 yards – still no hits. At 25 yards, I noticed that all the rounds were keyholing – most hitting the target sideways. I tried some 55 grain rounds, and they still shot normally, with no keyholing, and with acceptable accuracy. Back home, I did some research and found that 1 in 9” twist rifling should stabilize both 55 and 62 grain rounds, which was also my previous experience. I cleaned the gun and used copper remover. I did a thorough job, but didn’t see much copper fouling. Back at the range, the 62 grain keyholing continued, and the 55 grain still performed well. Just for completeness, I tried some 69 grain match bullets, and to my surprise they did not keyhole! I also tried two different brands of 62 grain ammo – they both keyholed. What could have changed to account for this behavior?

Interested to see the outcome of this.

Granted, you say you were previously successful with 62grers out of that rifle, 62 is at the upper limit of what a 1:9 should stabilize.

I was going to say bad lot of ammo, but you also said you tried a few different brands of M855.

M855 is not known for it's accuracy but it's also not known for keyholing.

1:9 is probably the ideal twist for 62gr rounds.  hell, i've shot 77gr SMKs out of my 1:9 AUG and they stabilize perfectly, though not as accurate as 69gr SMKs.  i'm going with barrel issue since the first two rounds shot fine and heavier rounds worked too

Doesn't make sense. I'd expect the 69 gr to have the same issue, because it's length is similar to m855

If you still have some of it left over... pull a bullet and measure the diameter.

If it is undersized... it will not stabilize correctly.

My first experience with Federal American Eagle XM855 62 gr. Green Tip was yesterday at the range as I was breaking in a new carbine (16", 1:9 twist).  The rifle shot Federal American Eagle XM193 as if it was born to group that ammunition.

Then it shot XM855 as if it was a shotgun.  Plus the XM855 grouped below the XM193 at all ranges out to 100 yards.

I thought it was hotter than the XM193 and maybe was shooting lower as a result of being a flatter shooting round.  But when I got back to the house I compared the MV and the XM193 was way hotter.

Still trying to think that one through.  But ordered a chronograph to be sure.

LF

This, a 1/9 should shoot a 62 gr just fine, hell, I've have very good results with 68 grain bullets.

Weight doesnt have much to do with the twist rate. It's the length that matters. More specifically, how much bullet touches the barrel.

 

I'd venture to guess that as your barrel life is decreasing, for whatever reason it's not doing well with the 62's. Either that or you got a bad batch of M855.

I'd probably reserve that barrel for XM193, then once it's shot out, swap out the barrel and return to shooting both.

More specifically, no.

Aerodynamic forces and center of mass of the projectiles is what determines the rotational speed needed for stability. Longer projectiles have more twisting force so need faster rotation to stabilize them in flight.

As for the OP, you might try measuring your twist rate to determine exactly what it is.

I agree.  My 1:9 16" barrel shoots M855 all day long, and has no problem with 69gr. Prvi Match ammo either.



Something is going on with OP's barrel.  Did the barrel nut come loose?



 

Well yeah.  M855 is a rather short bullet.  SMK's are longer.  Mine shoots both just fine.  And it's generally magazine length, not chamber dimensions that limit the COAL.  Unless OP has a lot of M855 that was loaded to a WAY short COAL at the factory, I just don't see the ammo being the problem here.



 

It would be somewhat unusual for bullets so close in weight/length to have performance so radically different .

As the bullet weight/length approached the area that is usually considered unsuitable you might expect to find ammo that wasn't particularly
accurate but to start seeing actual key holing there would usually be a considerable difference in length/weight.

I see the OP tried cleaning the barrel , that is the first thing to occur to me , might revisit that .

Someone asked if twist had actually been measured , that also occurred to me.

The part about the barrel not doing this previously caught my interest , carefully inspect the crown , perhaps there is a nick or some other damage that is destabilizing the bullets on exit ?


Interesting to see the outcome

It is not the length of the cartridge that affects stability , it is the length of the projectile

My first AR was a 1:9 S&W, both XM193 and 855 shot similar groups (3.5 MOA). It was one of least accurate barrels I have owned but it would shoot 2 MOA at 200 yd with 77 gr Noslers. However my dads 1:9 S&W will not shoot the heavy stuff at all.

The only thing that makes any sense here is you've got a bunch of M855 with undersized bullets and even that doesn't make sense.

1:9 twist barrels on 16" carbines in my experience has been a crapshoot on what will stabilize in them or not. Some will shoot 69 and 77 Matchkings well and some won't stabilize a 6) grain Horny hollow point.Then there's the question on if they stay stable beyond 100 or not. I had a M&P 15 carbine that would shoot 69's MOA at 100 but couldn't find a bullet on paper to even see if it was tumbling at 300. This is why all my AR's get a 1:7 or 1:8 barrel on them, it eliminates the stability question, period.

I've shot 69gr SMK (handloads) through my Bushmaster 20" 1:9 AR15 at 500 yards without an issue. My groups weren't all that great....it was my first AR, about 2 years prior to the Marines. With practice, my groups got better.

But I shoot 1:7 barrel now.

As others note, something is wrong.  Your 1/9 BBL should stabilize 62's all day.  Heck, I stabilize 75 gr in mine.  And I shoot 80 gr in a 1/8 all day at 600 yards.  The twist isn't your issue.

My opinion of both the rifle and the ammo in this case is not very high.   DPMS isn't exactly famous for being top tier.  And Federal M855 is pretty infamous for being notoriously inaccurate.  My guess is you probably have a particularly out of balance batch of M855 (the multi-core construction is not good for accuracy), combined with a BBL that's basically shot out.

Heck, I'd check to make sure they didn't forget to insert the steel core into those bullets (a magnet will suffice) - because something is wrong.

That's fine, but there is no system in place for describing bullets by length. Weight is the system in place, so that's what we use.

Usually I can get everything up to a 75 grain Hornady to shoot well in a 20" 1:9 . In a 16" carbine with a 1:9 it's sketchy.

1:7 or 1:8 (24") is the way to go in an AR IMHO. Most of the blown up bullet stories I've seen have been with paper thin copper jacketed bullets designed for .22 Hornet and the difference in accuracy with a light bullet/fast twist is negligible.

I was referring to the fact that, in a AR-15, when a cartridge is chambered from a magazine, and not manually chambered,  the magazine limits the COAL, which in turn limits the projectile length.



 

Try some other ammo to rule out barrel wear or copper fouling, i have a couple 1-9s' and they have no problem with 62;s.