Most important factors for accuracy - 9MM

[Yondering]

3 hours ago, xdf3 said:

I can't see the first one and the second one is just what I googled before, I didn't find any new answer / explaination about RPM's, barrel twist, etc... No research about bullets losing accuracy by going too fast yet (I'd like to find one)

 

That first link is an article by Scheumann about pistol barrel twist rates. It talks about stabilization and how that affects the bullet, and does mention the two potential problems of too many RPM:

- outward expansion of the bullet from centrifugal force (sometimes a problem in high velocity rifles with thin bullets, not a pistol velocity problem)

- exaggerated imbalance of an unbalanced bullet that is either started poorly in the barrel or has a defect

 

The second of those is the one that applies to us the most - with an imperfect bullet or one where the seating depth and nose runout did not produce a straight start, a slower twist rate minimizes the imbalance. Such a bullet is still less accurate than one started straight with no defects, but a slower twist can be more tolerant of variations in bullet quality. Note that there is no particular twist rate where a bullet is "over stabilized"; it's just a gradual effect as that increases with twist rate.

 

The first of those above (and the second to some degree) is something that has been a problem in some rifle applications, particularly fast twists and high velocity with thin jacketed varmint bullets; a fast twist rate intended for say 77gr 223 bullets could cause bullet blowup with 40-50gr bullets if velocity was high enough. Because of this, many older shooters claim the twist rate has to be matched to the bullet. However, modern bullet construction is much better, and it's common to achieve excellent accuracy with light varmint bullets in fast barrel twists; one of my own examples is a 1:8 twist 5.56 that shoots great with the 75gr BTHP, but shoots even more accurately with the 50gr Blitz despite the fast twist. Keep in mind that with pistol bullets, we still have a large selection of lower quality bullets available; some plated and coated bullets fit this category (although a good coated bullet can be your most accurate option). 

 

The Scheumann article also talks about how the twist rate is important for stabilization, but velocity mostly does not matter. I.e., we're not trying to match a bullet to specific RPM, which would mean a specific velocity, we're just trying to match the bullet profile (length and diameter) to the twist rate. 

 

It mentions the transonic velocity region and how that can upset bullet flight and degrade accuracy; this is why some shooters claim a 9mm shoots best above a certain speed. The truth is simpler - accuracy is better if you can avoid the transonic region. Subsonic muzzle velocity avoids this completely (note my example above about the 100gr RF at 1,000-1,050 fps), which is what we get with most 9mm 147gr bullets and standard loads, but is achievable with any bullet weight. The other option is to keep muzzle velocity high enough that the down range velocity will not drop into the transonic region; this is a major consideration for long range rifle shooters, and results on target often show accuracy going wild after a certain distance. 

 

The article goes on to talk about their experiments in accuracy, using the 1:32 vs 1:16 twist, and eventually concluding the 1:24 twist is best for 9, 40, and 45. The one big omission (at least relevant to this thread) that I saw in the article was that they didn't talk about how much affect the different twist rates have on accuracy, only what it does. It's important to keep in mind that the effect of using a faster twist is less than other significant factors like using good bullets, seating them straight, and shooting in a quality barrel. Most likely, switching to a good match grade barrel of any twist rate (could be a Scheumann or many others) will net you more accuracy gain than changing to an ideal twist rate. Also keep in mind the difference between comparing 2" and 2.5" groups at 50 yards, and 2" to 8" groups at 25 yards; twist rate may be important in the first instance, but is most likely insignificant in the second. 

 

Hope that helps.

Edited August 9, 2019 by Yondering

[xdf3]

On 8/9/2019 at 7:36 PM, Yondering said:

 

That first link is an article by Scheumann about pistol barrel twist rates. It talks about stabilization and how that affects the bullet, and does mention the two potential problems of too many RPM:

- outward expansion of the bullet from centrifugal force (sometimes a problem in high velocity rifles with thin bullets, not a pistol velocity problem)

- exaggerated imbalance of an unbalanced bullet that is either started poorly in the barrel or has a defect

 

The second of those is the one that applies to us the most - with an imperfect bullet or one where the seating depth and nose runout did not produce a straight start, a slower twist rate minimizes the imbalance. Such a bullet is still less accurate than one started straight with no defects, but a slower twist can be more tolerant of variations in bullet quality. Note that there is no particular twist rate where a bullet is "over stabilized"; it's just a gradual effect as that increases with twist rate.

 

The first of those above (and the second to some degree) is something that has been a problem in some rifle applications, particularly fast twists and high velocity with thin jacketed varmint bullets; a fast twist rate intended for say 77gr 223 bullets could cause bullet blowup with 40-50gr bullets if velocity was high enough. Because of this, many older shooters claim the twist rate has to be matched to the bullet. However, modern bullet construction is much better, and it's common to achieve excellent accuracy with light varmint bullets in fast barrel twists; one of my own examples is a 1:8 twist 5.56 that shoots great with the 75gr BTHP, but shoots even more accurately with the 50gr Blitz despite the fast twist. Keep in mind that with pistol bullets, we still have a large selection of lower quality bullets available; some plated and coated bullets fit this category (although a good coated bullet can be your most accurate option). 

 

The Scheumann article also talks about how the twist rate is important for stabilization, but velocity mostly does not matter. I.e., we're not trying to match a bullet to specific RPM, which would mean a specific velocity, we're just trying to match the bullet profile (length and diameter) to the twist rate. 

 

It mentions the transonic velocity region and how that can upset bullet flight and degrade accuracy; this is why some shooters claim a 9mm shoots best above a certain speed. The truth is simpler - accuracy is better if you can avoid the transonic region. Subsonic muzzle velocity avoids this completely (note my example above about the 100gr RF at 1,000-1,050 fps), which is what we get with most 9mm 147gr bullets and standard loads, but is achievable with any bullet weight. The other option is to keep muzzle velocity high enough that the down range velocity will not drop into the transonic region; this is a major consideration for long range rifle shooters, and results on target often show accuracy going wild after a certain distance. 

 

The article goes on to talk about their experiments in accuracy, using the 1:32 vs 1:16 twist, and eventually concluding the 1:24 twist is best for 9, 40, and 45. The one big omission (at least relevant to this thread) that I saw in the article was that they didn't talk about how much affect the different twist rates have on accuracy, only what it does. It's important to keep in mind that the effect of using a faster twist is less than other significant factors like using good bullets, seating them straight, and shooting in a quality barrel. Most likely, switching to a good match grade barrel of any twist rate (could be a Scheumann or many others) will net you more accuracy gain than changing to an ideal twist rate. Also keep in mind the difference between comparing 2" and 2.5" groups at 50 yards, and 2" to 8" groups at 25 yards; twist rate may be important in the first instance, but is most likely insignificant in the second. 

 

Hope that helps.

It does help

 

So for the first part, it might help to consider high quality 115 grains vs good quality 147 grains (just an example). Good quality 115 grains might have issues at longer distances (35+yards) - just a guess -

 

Why do some people say JHP are the best for accuracy and some others pick lead/coated? I can't understand if some information is missing on both sides.

 

I made a research about the transonic region and it seems like, under typical conditions, 1100 - 1300 FPS should be avoided. Even around 1050 at low temperatures.

 

If this has a bigger impact on accuracy, I guess that would make easier to pick 124 - 135 - 147 grains for better accuracy at 35+ yards rather than 115 grains, staying in the typical 125-135 power factor range (with exceptions for 147 grains)

 

I saw that a typical 124 grain bullet can have an SG factor well above 2.0 (I see the optimal range is 1.4 - 2.0). I hope too much is not affecting accuracy more than a 0.5" at 50 yards

 

Actually I'm working on the second part. I'd like to start with a good group at 25 yards, and any group from 2.5" or lower will pass the test for farther distances.

 

I bought some powders and bullets and I'll try them. What I understood is that a 95% of max load will work better for any specific powder, so using a 70% of a fast powder might be a bad idea (better use another powder). Or an 80% of a slow powder. 

[xdf3]

One new thing I've been told: weighting bullets is useless, what counts is the lenght of the bearing surface of the bullet. Having all 124 grains bullets won't be as good as having the same bearing surface with a 2 grains variance. Any confirmation of this?

[RAP]

Yes and no. Weight variance importance is dependent on the shooting you are doing. For Pistol shooting 2 grain weight variance isn’t a big deal and you wouldn’t even notice. However if your shooting 600 or 1000 yard Benchrest 2 grains is a big deal. When I shot 600 yard Benchrest I weighed my Bullets to make sure the lot I purchased didn’t have a big variance. A 1/2 grain variance was pretty typical for precision match Bullets.

As far as bearing surface goes. When shooting for accuracy start with a good quality match bullet. Bearing surface will vary between brands not necessarily bullet to bullet. What variance bullet to bullet (same brand/type bullet) is so small it won’t be a factor.

When loading ammo for accuracy consistency at every step is important.

I remember when I started shooting benchrest an older Gentleman gave me the following advice.

“Find the load that your rifles shoots best. Don’t sacrifice practice by chasing the latest and greatest gadget and don’t get caught up chasing tiny details. Practice Practice Practice!”


Sent from my iPhone using Tapatalk

[DougCarden]

 In conventional pistol bullets the jacket hollowpoint is considered most accurate. If you look at a cross-section of the bullet the heaviest part of the bullet is in the rear and that is the last part that is stabilized as it leaves the barrel. Other bullet styles lend them selves to be in accurate. A 122 truncated cone bullet is very accurate due to a very short pointed bullet front and the rear of the bullet is constructed like a hollow point with all the weight at the back .  There were tests back in the twenties that showed the 120ish grain bullet had great accuracy in 9mm with tc style bullet.  

Quick and dirty take aways are that bigger diameter bullets are more accurate due to engaging the rifling better.  

When I worked up 95 grain loads in 9mm for steel challenge I had sub 1inch groups at 35 yds with 95 jhps.  I had 4inch groups, if you call them groups, with 95 grain fmj.  This demonstrates what a difference having the weight at the rear of the bullet to stabilize it will do for accuracy.

Good Luck,

DougC

[Powder Finger]

2 hours ago, DougCarden said:

 In conventional pistol bullets the jacket hollowpoint is considered most accurate. If you look at a cross-section of the bullet the heaviest part of the bullet is in the rear and that is the last part that is stabilized as it leaves the barrel. Other bullet styles lend them selves to be in accurate. A 122 truncated cone bullet is very accurate due to a very short pointed bullet front and the rear of the bullet is constructed like a hollow point with all the weight at the back .  There were tests back in the twenties that showed the 120ish grain bullet had great accuracy in 9mm with tc style bullet.  

Quick and dirty take aways are that bigger diameter bullets are more accurate due to engaging the rifling better.  

When I worked up 95 grain loads in 9mm for steel challenge I had sub 1inch groups at 35 yds with 95 jhps.  I had 4inch groups, if you call them groups, with 95 grain fmj.  This demonstrates what a difference having the weight at the rear of the bullet to stabilize it will do for accuracy.

Good Luck,

DougC

i found the same with 115's, never could get a 115 fmj to shoot.