Originally Posted by JonM
The purpose of a tracer isn't to hit pin point targets its to direct fire when used in a rifle. Least that's what we used em for.
The phosphor component makes the round unstable in flight not so much the twist rate. As it burns it shifts balance in the bullet and things go nuts down range.
Personally I'll never put a tracer in any rifle I own chromed or not. I like my barrels
My rra nm ar15a2 is 1-8 my krieger precision build is a 1-8.8. I haven't found much they don't like other than 55grn airpulls. But that's got more to do with shtty surplus bullets with visibly off center cores....
Quality of the bullet has more to do with consistant accuracy than anything else.
Supposedly 16" 1-7 won't shoot 55grain well . My bcm is a moa shooter with a 16" barrel at 300 yards using quality 55grain bullets and consistant powder charges.
Each gun is different and need handloads worked up to get the absolute best results. I've seen that differences in ammo when dealing with factory crap is pretty huge even from stuff that's claimed to be "match" grade.
The problem I dont think lies with the twist rate being bad necessarily, but more the quality of the barrel to start with. Pretty much most 1-9 barrels aren't any great shakes and are aimed for the budget market because they perform decent enough with a very wide range of factory ammo. Not what I would consider target grade. More than good enough for the average guy trying to get an ar15 at a blue light special.
My wife had a dpms 1-9 16" it was a consistant 5" at 300yards with 75grain bullets. That's extremely good for a low end budget gun.
My post was not about accuracy at 300 yards. It was about accuracy at 1000. It was not about accuracy out of a 16 inch barrel. It was about accuracy out of a 18-20 inch barrel. It was not about the purpose of tracers either. I mentioned tracers because they are a big part of the reason the military settled on the 1:7 twist barrel for shots in 556 out past 800 meters. I would agree with you that a 1:9 barrel is just fine for shooting inside of 500meters. In fact I'd prefer 1:9 for 300 yards and closer because it won't overspin the lighter rounds so you can pretty much shoot whatever you want through it.
But the OP asked about maximum range on a .556 with an 18-20 inch barrel. Maximum range is not 300 yards. Maximum range is reaching out to just beyond 1000 meters and a big part of the decision the military made to implement faster twists in their barrels involved the longer heavier rounds used to reach out to that range. It takes a heavy grain bullet and ideally a faster twisted barrel to reach out that far with longer bullets and still have good MOA.
I'm not talking about the M193 tracer rounds either.
I'll back up my statements with direct quotes.
The SS-109 bullet uses a "compound" core, with a lead base topped by a steel penetrator, all covered in a gilding-metal (copper alloy) jacket. The L-110 tracer bullet has a copper-plated steel jacket and like all tracer bullets, is hollowed out at the base and filled with tracing compound. Both bullets are much longer in length than the earlier 55gr bullets, especially the L-110 tracer, which was designed to trace out to 800m, verses 450m for the older M196 tracer round. Due to their increased length, these bullets require a faster rifling twist to be properly stabilized. The military settled on a twist rate of 1:7, which is a compromise between the 1:9 twist ideal for SS-109 bullets and the 1:6 twist ideal for L-110 tracers.
For long distance shooting:
1:7 and 1:8 are the best.
Why? Accuracy. For heavier and longer rounds during competition shooting, 1:8 and 1:7 twists are the best for heavy 77-80 grain rounds that I use to shoot competitively at 500-1000 meters. Who needs to shoot tracers anyhow? More importantly, heavier rounds are showing very good results in terminal testing and are proving to be much better defensive rounds.
These are tracer rounds I was talking about
Cartridge, 5.56-mm, Tracer, M856. (Used in the M16A2/3/4 and M4-series weapons.) The M856 tracer cartridge has characteristics similar to the M196 tracer with a slightly longer tracer burnout distance. This cartridge has a 63.7-grain bullet. The M856 does not have a steel penetrator. It has a red tip (orange when linked 4 to 1 for the M249). This ammunition should not be used in the M16A1 except under emergency conditions, and only at targets less than 90 meters in distance. (The twist of the M16A1 rifling is not sufficient to stabilize the projectile of the heavier ammunition).
Note: M16A1 is a 1:12 barrel. But I have fired these tracers out of a 1:9 barrel and watched them begin to veer significantly after 100 yards. The rounds were designed to be stable out to 800meters in spite of the phosphor compound. Not that you would buy a rifle so you can shoot tracers. Just listing the reason that we have faster twist barrels available for .556 in the first place.
Again, inside of 300 yards I like my 1:9 inch twist barrel way better. But the OP did not ask about the best barrel for inside of 300 yards. The OP specifically asked about maximum range with .556 out of a 18-20" barrel. Maximum range is just outside of 1000 yards. It needs heavier grain bullets to reach that far accurately. Heavier grain bullets are better stabilized by higher twist barrels. This is not to say that it cannot be accomplished under any circumstances with a 1:9 twist. But that is not what is normally used in long distance matches.
This is all I have to say about the barrel twist issue on this thread. I am not going to debate it anymore. These are the facts as given by people who are much smarter than I am and much better marksmen than I am.
Eventually I would like to put a 1:8 fluted steel 18" wilson barrel on my RRA LAR-15 and try my luck out to 1000 yards. When I do I will be sure to compare the difference between that barrel and my current 1:9 moly chrome 16" wilson barrel at those ranges and maybe I'll prove myself and the experts wrong and find out that I've wasted my money. But I hope not.