I am looking at reloading 5.56 mm (getting back into reloading after a decade not doing it). Most 5.56 mm ammo I buy is 55 gr. I have two AR's, one a 16" barrel (1-9" twist) and a 20" (unknown twist rate, built before 1994).
I read the 20" barrel will produce a greater muzzle velocity and a flatter trajectory than the 16" barrel and that the 20" barrel was designed for the 55 gr.
Then I read in Lyman's Reloading Handbook that shooters are using 80 gr 5.56 rounds to compete effectively with shooters of 30-caliber M14/M1A rifles on the 600 yard stage of the National Match course. However, the 80 gr bullet requires a 1-7" or 1-8" twist barrel for proper stabilization.
I am looking at getting a 55, 62, or 75 gr bullets and am wondering which will be most accurate for my AR's "way out there" without having feeding issues. Thoughts?
There are a lot of factors involved but, to make a long story short the 69 grain has worked best for me. It will fire out of your 1:9.
My Bushmaster HBar is 1:9 and it shoots 69's really well. It will also shoot 75's (HPBT) ok, but that has proven it's limit.
Back in 2009, Boltgun71 taught a Designated Marksman class (the prototype Milseed) to a unit from the SCNG using a combination of Army Marksmanship Unit and Appleseed methods.
At the end of that class the students shot a qual course and also worked on shots out to 700 yards using 62gr ammo. Their first round hit hate with M4's and acogs was 70% out to 700 yards.
In a 1:8 or 1:7 upper an 80gr bullet (which must be loaded for single feed) will have advantages at 600+ yards, but that doesn't mean that a 69 or 62 can't shoot well at that distance.
With your rifles I wouldn't consider anything heavier than 75 and would probably stick to nothing heavier than 69. Spend you money learning you position at 25 and 100 yards and then worry about heavy bullets and medium range shooting.
I would certainly agree with the suggestion that 69 grain bullets are likely to be your best choice with that barrel. I would also consider the Berger Match Target in 72 grain. Pretty sure they will stabilize and they should equal or exceed what the 69's can do.
When your barrels are worn out, replacing them with fast twist barrels will let you shoot some 75 grain bullets, by then your rifle skills will be improved too.
1X9 twist seems to me to be the most versatile. It will shoot from 55gr (inexpensive M193) to 69 (commercial match) well. 1x7 was designed to stabilize the M856 military tracer round. Just my experience.
I use the 69gr HPBT Nosler, and in a pinch I go with the 69gr Sierra Match King in my 1:7 twist barrel (my CMP/NRA Across the Course Service Rifle and my AR-15 for Varmit Shooting.
I reload either 55gr or 62gr Winchester FMJ for my M4 Carbine with a 1:9 twist, though I am sure the 69gr loads would work fine...I just reserve those loads for competition and long range shooting.
Depends on your definition of "way out there". I shot a 1-8 twist RRA with the 52 grain Hor. BTHP on the 300m International course quite effectively. I have since switched to the Hornady 75 BTHP as I got a good deal on them and enjoy simplicity... one load, all shooting. You might have issues with this bullet in your twist rate. I would think the 69 gr would be OK but probably not optimal. Look at the 60 gr Vmax. This can easily be loaded to mag length, will work in your twist rate and has a pretty impressive BC. If the 75 or 69 hornady won't work, that might be your best bet.
If you really want to shoot in excess of 400 yards with an AR, a new barrel with a 1-7 or 1-8 may be necessary to optimize this caliber way out. Think of it as a mini-.308 with heavy bullets. On the 600 yard line, I do fine with the 75 Hornady loaded to mag length. If you want to deal with single loading an 80 gr SMK or Nos CC will give a slight advantage over this bullet. My shooting isn't quite there yet to justify the greater expense so I'm sticking with the 75 for now.