120 SMK Vs 123 SMK....what is the difference?

[LRRPF52]

Thank you. This actually was very helpful.

I've grown to dislike the 123. On-paper, it's perfect bullet. In practice, it's been a nightmare of a bullet for me in the Grendel; 3 BBL's and multiple manufacturers. When I run it mag-length slamming through an AR gas system, is very expensive 3-4 MOA ammo for me.

Thinking it's me, I load it up in my Creedmoor, that I know is a 1 MOA gun. And again, 123's in a Mag-Length semi-auto magazine, give me a nice 3-4 MOA performance, even in the creedmoor. Yet when I tried some 120's this weekend, my group sizes shrank to better than half, and are even crowding 1 MOA (not quite, but close), in my Grendel.

Your post was very helpful in explaining why.

My Bartlein .260 Rem gas gun barrel will shoot any of the 123gr Target bullets into bug holes for sport, with .5 MOA groups at 500 and 1000yds easily, even in the wind.

The only target bullet I didn't get to shoot well through that barrel was the 140gr A-MAX for some reason, but I didn't spend much time with it because the 139gr Scenar, 130gr VLD, 123gr Scenar, 130gr Norma Golden Target, and 142gr SMK shot so well.

I specifically had that chamber throated for the standard/short COL limited by the magazines with VLD/Secant ogives in-mind.

For anyone that is trying to get secant ogive bullets to shoot well in long freebore chambers (that economy barrel-makers like to use instead of the SAAMI chamber) at the lower pressures of the 6.5 Grendel, good luck to you.

Like I said, the 123gr SMK has been one of the most accurate bullets I've shot through several different 6.5 Grendel AR15s, but they all had SAAMI chambers with the compound throat and have all been Bartlein or Lilja barrels.

107gr SMK shoots amazingly well on top of 8208XBR in 6.5 Grendel too using the same formula.

If you have a conventional freebore, I would look at the shortest radius tangent ogive bullet that will get close to the lands and play with them, but chances are you will waste many range trips chasing accuracy that is easily available on your very first trip to the range with a SAAMI chamber and any of the secant ogive, high BC bullets on the market.

For me, getting to the range is no small effort, so I can't afford to waste my time with chambers geared to a profit model that relies on volume of sales vs performance.