Why am I getting vertical strings in my grouping?

Today I was doing some load development for my 9mm Glock (Storm Lake Barrel) and 9mm M&P CORE (Storm Lake Barrel). Both groupings where shot with 5.2gr (Glock) and 5.4gr (M&P) of BE-86 behind an Extreme 124gr RN . I was using a pistol rest and distance was 15 yards. I have a Trijicon RED dot on both so sight picture issues should be eliminated.

Also, I shot at least 2-3 groups with 5 different loads for each pistol so my technique shouldn't be the issue. I have read that M&P 9mm's sometimes unlock early. However, I got a similar string with the Glock as well. I know BE-86 works great in 9mms because I get 7/8" groups with the Glock at 15 yards with a 115gr ZERO FMJ-HP behind 5.2 gr of BE-86.

What is causing these strings from both pistols?

I'm using this rest:

http://www.ctkprecision.com/compact-shooting-rest.aspx

it has thick padding.

I would normally agree with with Jim and JohnSa that the pistol is bouncing. However, these were the only 2 out of 17 groups with vertical strings. I shot under 1" groups with the same pistols on the same day using the same rest. That's why these 2 groups are so perplexing? Why are they in almost a perfect vertical line with no windage variation. without the elevation difference, these 2 groupings would likely be one ragged hole. Unfortunate, neither the M&P nor Glock are capable of that type of accuracy.

Ruger 15151 said:

Also, I shot at least 2-3 groups with 5 different loads for each pistol so my technique shouldn't be the issue.

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Here is a great article in in 6mmbr that discuss reasons/cure for vertical and horizontal strings.

http://www.6mmbr.com/verticaltips.html

They aren't always random groupings. Unfortunalty, most of the issues mentioned don't apply to pistols.

macgrumpy said:

Is that an 8 MOA red dot?

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I have a 3 MOA on the M&P and 6 MOA on the Glock. I usually turn the brightness as low as possible and still see the dot. I use a orange dot on the target to help center the red dot.

ATLDave said:

If you flip a coin 5 times and record the results, and then do that 5-flip-group 17 times, the odds are strong that one or more of those groups will come up all heads or all tails. Doesn't mean the coin is broken.

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I hear what you are saying. Its difficult to shoot 5 different groups with the same load and get the same pattern more than twice on some days. However, what confuses me is that these 2 groups out of different guns but with similar loads (5.4gr & 5.2gr of BE-86) with the same bullet ( Extreme 124gr RN .3556 ) produced almost perfectly vertical strings.

ATLDave said:

Did you shoot other groups with those same exact loads?

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I only had enough rounds at 5.2gr to shoot a second group with the Glock. I saw very similar results but the vertical string was a little more varied. ( 1 shot was pulled low )

Hoser said:

Varying grip pressure.

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That makes sense.

The pistol rest that I am using has a V-block to support the barrel/frame and an adjustable pad to support the bottom of the grip. I can't get a normal grip with my weak hand and still rest the bottom of the grip on the pad. Therefore, I have adjusted my grip slightly because I thought resting the bottom of the grip on the pad would eliminate any elevation movement.

However, I have made more rounds using the same load data. This time, I will use my normal grip and grip pressure and make sure the bottom of my hands are resting on the support pad instead of adjusting everything so the bottom of the grip rests on the support pad.

Minus the elevation variations, both these would produce some pretty tight groups.

I'll share my results once I am able to get to the range.

Thanks everyone for your input.