Requesting tips for correcting 9mm vertical stringing

[LocoGringo]

I'm developing a load for both a STI Marauder and a Springfield Armory XDm 5.25 in 9mm for 3-gun competition. The STI is my primary pistol with the XDm being my backup. I've learned that if the round passes the plunk test in the XDm, it will pass in the STI. I'm using Silhouette powder and tested 4.8, 5.0 and 5.2 grains with the 5.0 grain load shooting so far left that I couldn't correct with the adjustable sights on the STI. The 5.2 grain load is shooting high, but perfect on left/right. The bullet I'm using is the 135 grain Black Bullets International coated lead bullets with mixed brass and a wolf primer at a COAL of 1.145"-1.150" and CANNOT exceed the 1.150" because the round will not pass the plunk test in the XDm.

At 15 yards I'm getting some vertical stringing and it is more exaggerated at 20 yards, but at 7 yards I'm getting 10 rounds through the same hole right on target with the STI. My technique to shoot is using a rolled up sleeping back to support my forearms while the handgun is free to move as necessary and I'm seated. I'm using the Dillon crimping die in the 4th station, so seating the bullet and crimping are occurring in separate stations on a Dillon 550b. I want the load to be strong enough to hit a MGM spinner target and send it over in 2 or 3 hits, so I'm not trying to "game" it by making minimum power factor. I've chrono'd the 5.2 grain load and the 135 grain bullet is going a very consistent 1106 FPS for a power factor of 149. This load has proven very accurate in both pistols. My questions are these:

1. Would you mess any more with the load if I'm getting such great accuracy at 7 yards but getting some obvious vertical stringing at further distances?

2. Do you think my shooting/testing technique could be contributing to the stringing?

3. Are there things that could be done to fix the vertical stringing?

4. Do you think there is more room for the 135 grain coated lead bullet to be pushed faster or would I be pushing the limits too much? I've called Ramshot for advice on load data since there isn't really any and their best guess was to load in the middle of a 124 and 147 similarly constructed bullet. That's what I've done.

Thanks for any suggestions you can provide.

 

[Old Fuff]

I have no personal experience with the two pistols you mentioned, but as a general rule-of-thumb the most common cause for causing vertical stringing is that the barrel (which tips up and down at the back end) isn't fully returning to the same position between shots. The sights of course are mounted on the slide and don't move in relation to the barrel.

If the heavy bullets you are using are causing an excessive overall cartridge length, the base may rub on the breech face, and cause the issue you see.

I suggest you drop the bullet weight to no more then 124 grains, and benchrest test out to 25 yards.

This is not to say you can't use the heavier bullets, but it may be necessary to freebore the rifling in front of the chamber.

 

[Buck13]

Can you post a picture of the targets? Or just call the lowest hole zero, and measure the distance up to each other hole and list them, e.g.: 0.75", 2.5", 3", 4" (an imaginary five shot group)

 

[gwpercle]

I think you are doing pretty good. The vertical stringing just may be a natural result of the unsupported wrist and recoil etc.
Before changing your load, try shooting , standing , unsupported , no shooting off a bench or bedroll , with two hands...and see what your groups look like.
I know this sounds strange but you may find your groups will round out by trying this method.

 

[LocoGringo]

gwpercle, I thought it might be a possibility that since I was using a rolled up sleeping bag, it might be a bit soft and cause "bounce" which could induce the vertical stringing...I guess. It seems like a good theory to absolve me of any fault. (That's a bit of humor there)

 

[RAT807]

Buy some Starline brass and load the same way. Pay close attention to the pressure it takes to seat the bullet.
Different brass lengths, age of your brass and thickness of brass will vary your neck tension. Starline is very good brass. If this doesn't fix your stringing, you can look at your grip pressure / recoil management. If your grip pressure varies, the shot will also vary. Relaxing your grip can cause the rounds to rise as the gun recoils more upward vs straight back.
You can also adjust your crimp. This really comes back to neck tension. Whatever procedures you use, check for bullet setback. For my gun, I flare just enough to clear the base of the bullet diameter. It is tight but does not shave material from the bullet. A good chamfered mouth helps here. I do not crimp after seating the bullet and the case mouth still measures .376 on a loaded round, right where I want it
After I did all of these, I have shot 4 of 5 rounds into 1 hole at 25 yards from a rest. There was 4 feet per second spread on the crono. The one round out probably me not being smooth as glass on the trigger