Better performance with under stabilized 75-77gr?

[Molon]

How (in)accurate would a 75gr bullet be at home defense ranges ( less than 25yds, any more than that and you will probably be looking at murder charges) out of a bore with a twist rate of 1:9 or 1:12? The only time I shot a 75gr in a 1:9 I was getting less than 1.5" @ 100yds. Even if my rifle was shooting 4X better than average that still means a 1.5 or less at HD ranges.

Here's what happens when you fire the Hornady 75 grain A-MAX from a barrel with a 1:9" twist. The group was fired from a bench-rest and accuracy was still horrendous. As you can see from the target, several of the bullets were striking at nearly 90 degrees of yaw. It's very likely that a bullet striking at such a large angle of yaw will have a very shallow penetration depth and fail to penetrate to the FBI's minimum depth of 12". As already stated by others, results would be completely unpredictable.

[williamN]

Thanks for the pic.

I thought 1:9 was border line for accurately shooting 75+gr but that looks well beyond border line. I must have been very lucky.

[matthewdanger]

Some 1 in 9" barrels have an actual twist rate closer to 1 in 10" some are actually closer to 1 in 8". The individual barrel should be measured, or better yet, actually tested with the ammo you intend to use. Some barrels that are marked 1 in 9" will stabilize 75 grain projectiles.

The other thing to keep in mind that is saying the a x twist rate will stabilize y weight projectiles is actually kind of a short hand of sorts. The actual issue is not so much the weight of the projectile but the overall length of the projectile. Heavier projectiles tend to be longer. Faster twist rates stabilize longer projectiles more effectively than slower twist rates. It is just easier to think and speak in terms of weight than to get down to the level of thousandths of an inch in length.

The short answer is not all 1 in 9" twists are true 1 in 9" twists and some 75 grain projectiles are shorter than others. Some 1 in 9" marked barrels will work with some 75 grain ammo, so it is imperative that you test your specific ammo in your specific barrel.

[williamN]

Some 1 in 9" barrels have an actual twist rate closer to 1 in 10" some are actually closer to 1 in 8". The individual barrel should be measured, or better yet, actually tested with the ammo you intend to use. Some barrels that are marked 1 in 9" will stabilize 75 grain projectiles.

The other thing to keep in mind that is saying the a x twist rate will stabilize y weight projectiles is actually kind of a short hand of sorts. The actual issue is not so much the weight of the projectile but the overall length of the projectile. Heavier projectiles tend to be longer. Faster twist rates stabilize longer projectiles more effectively than slower twist rates. It is just easier to think and speak in terms of weight than to get down to the level of thousandths of an inch in length.

The short answer is not all 1 in 9" twists are true 1 in 9" twists and some 75 grain projectiles are shorter than others. Some 1 in 9" marked barrels will work with some 75 grain ammo, so it is imperative that you test your specific ammo in your specific barrel.

That makes sense. Thanks for the reply.

[5pins]

The 75gr A-max is longer then the 75 or 77gr BTHP's and will not stabilize at all in a 1/9 barrel.

[williamN]

The 75gr A-max is longer then the 75 or 77gr BTHP's and will not stabilize at all in a 1/9 barrel.

Very good to know. Thanks for the info.

It was the 75gr BTHP that shot so well in my 1:9 S&W barrel. I have a new MP15X and am hoping it shoots the 75gr as well as my 15T did. Now that I know all 75gr bullets are not created equal I will do some research before buying and testing.

Thanks again for all the replies.

[Tactical Rancher]

Add one more data point to your set:

My S&W M&P15A with 1:9 shoots SMK 77-gr and PRVI OTM 75-gr accurately at all ranges. The elevation where I shoot is between 6k and 9k ft and temperatures have varied from 30 to 90 deg F during sessions.

I want to try the A-Max under these conditions to see what happens too.

[Zhukov]

Thanks for the pic.

I thought 1:9 was border line for accurately shooting 75+gr but that looks well beyond border line. I must have been very lucky.

It's not the weight of the bullet but the LENGTH. Not all 75gr bullets are the same length, and the AMAX is a very long bullet. Same for the 70gr TSX.

We generally use weight as a guideline when it comes to specifying what bullet is stabilized in what twist rate barrel since weight it easier to quantify than the length of the bullet. This works most of the time, but the limitations are shown when you're on the borderline already.

BTW - Molon's tests have clearly shown that a 1:7 barrel is more accurate than a 1:9 (assuming that the 1:9 properly stabilizes the bullet). The difference was noticeable. It won't make a difference at CQB distances or out to 100 yards (unless you're going for precision), but I'd be hesitant to try for much beyond that range.

[SGT D USMC]

I shot serria 69gr match king hp In a 1 in 12" at 50 yards. The target board was aprox. 24" by 36" about half didn't hit the paper, some were a 90 degree key hole, others were less , but no group at all. I would put the effective range at less than a shotgun.

[Tactical Rancher]

BTW - Molon's tests have clearly shown that a 1:7 barrel is more accurate than a 1:9 (assuming that the 1:9 properly stabilizes the bullet).

I think you meant to say, "assuming that the 1:9 DOESN'T properly stabilize." Benchrest guns routinely have the slowest twist rate possible for their preferred bullet to retain more velocity.

It may be a moot point here, because we all know that 1:7 barrels MORE RELIABLY stabilize 75+ grain projectiles under the widest range of conditions, but I just wanted to clarify the point.

My experience has shown that when they DO stabilize a given bullet, 1:9 barrels perform the same or better than 1:7 barrels, all else being equal (benchrest, bullet, powder load, chamber, barrel profile, trigger, shooter, optics, etc.). Just my $.02.