I have a Bushmaster HBar 20" barrel w/ 1/9 rifling. I somehow gone my lines crossed and was trying to work up a load w/ heavy bullets (77gr MatchKing HPBT, mimicking the army's MK262 MOD 0 Designated Unit Marksman ammo), of course that just didn't give me the accuracy I was looking for. After more research, I then realized my oops on trying to use heavy bullets. So now on to trying to find somewhere to start. I understand that 1/9 twist is good for approx 52/53 - 69grs or so. I have some 69gr Sierra MatchKing HPBT's, product #1380 for 1/7-10 twist barrels, that I was thinking of trying. I understand that many AR-15 HBar Match shooters use Varget, BLC2 and or Vintavuori 135 or 140, but I don't have those powders on hand. I list below the powders I do have & the loadings in the manuals I have. Lyman 49th - 69gr HPBT IMR4320 - 23.9 - 26.5 IMR4064 - 22.9 - 25.5 Reloader15 - 23.9 - 26.5 W748 - 23.6 - 26.3 Hornady 8th - 68gr BTHP H322 - 21.6 - 23.0 W748 - 23.4 - 25.8 Can any of yall maybe give me some suggestions on where to start with the Sierra 69gr HPBT, or should I go to a different powder or a different bullet. Any help would be greatly appreciated. Thanks for reading & thanks for any help. Hope yall have a happy holiday.
Alliant makes a powder specifically for the AR and heavy bullets its called AR-Comp maybe this will help http://www.alliantpowder.com/products/powder/ar_comp.aspk
The problem the poster has is not the powder he is using for the heavy bullets but the barrel twist rate which is too slow for heavy bullets. For those bullets he would need a 1 in 8 inch or 1 in 7.5 inch twist. The exact twist (or at least a good guide ) is the Greenhill formula where the length of the bullet counts and not the weight, for the same caliber. A one in 9 inch twist is good for the bullet weights (lengths) around 50 grains but you really have to do the Greenhill calculation for the bullet of choice to be sure. LDBennett
Varget is my choice powder for the sierra 69gr HPBT. and If you have a 308 of a 270 or 243 it will be the go to powder for them too. I have been trying to go to the range and try some with IMR 8206XBR I here its better than Varget, But i'll beleve that when I see it. I here some raving about that other powder CFE 223 but you will never go wrong with Varget. My fav load for my Colt 1-9 twist H-bar is Lake city brass 25.3gr varget cci-41 primer.
Of the powders you have listed I would start with R-15. IMO if you can't get R-15 or Varget to shoot the 69gr, move on to a lighter bullet. My Colt 1-9 loves the Sierra 53gr HPMK. It also does very well with other 55gr bullets, even cheap ones. I also crimp my AR ammo with the Lee Factory Crimp die, it helps secure the bullets and improves accuracy. YMMV
