I've done as much reading and research as I can stand, and I'm still not confident on bullet weight for my M&P15OR with the 1in9 stock barrel.

Does anyone have some experience with this barrel and heavier loads? As I understand things, it depends a lot on bullet shape, so are there some factory loads that are on the heavier side that produce accurate results?

Try the Federal with the 69 gr Sierra Matchking BTHP. Each barrel is different, but many have said this is very accurate ammo.

Thanks! Now my shopping list has an item!

Much to my surprise, the 1-9 16 inch stabilized the 75 gr Hornaday HPBT-at least to 150 or so yards. The same twist stabilzed in a 26 inch barrel at presumably much higher velocities (2770 fps-which isn't all that much slower than what I see with 55 gr FMJ in a 16 inch barrel.).

Please note this isn't the A-Max bullet, which is much longer. It isn't really the shape, but the length (and velocity) that limits what a particular rifling twist will stabilize.

What he said ^^^^

heavier bullets are longer. They have to be, because the diameter isn't getting any bigger !

Heavier is also slower.

I've only shot 55gr out of my 1/9 barrels.

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Kirk / Spock 2020

This isn't always true... It depends on what the projectile is made of. For example, a 55 gr. Barnes TSX bullet will be longer than a typical jacketed 55 gr. lead bullet. Because the TSX is an all copper bullet, and copper weighs less than lead, there is more copper required to meet the 55 gr. weight, thus a longer projectile.

For comparison:
65gr Sierra Gameking (boat tail) .852" long
55gr Tipped Triple Shock X (boat tail) .898" long

Huh? I think you meant to say that the 55 gr. Barnes bullet is longer than a typical jaceted 55 gr. lead bullet! Which weighs more, a pound of whale or a pound of feathers?

oops, yes, typing too quickly... I'll edit above

You should be good with just about anything from 45gr to 62gr. The 1/9 twist is slow enough not to spin the 45gr apart and fast enough to keep the 62gr stable through about 500 yards, 55gr being optimal. Just my opinion.

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SFC, US Army, Retired