Huge difference in group size due to FPS, why?

[TerryYu]

Can anyone explain this phenomenon that I saw today?

I have a M&P Pro 40 that I loaded 140gr coated bullets for. Went to the range with 2 loads. One was a ~850fps minor load (4.3gr N320) and another was a 1275fps major hypervelocity (8.6gr HS-6) load.

The 850fps load shot terribly, group size was the C zone at 25 yards while the 1275fps load was able to put all its rounds on top of each other. I love the accuracy of the 1275fps load, but it hurts to shoot an entire match with it.

Trying to figure out why the slow load shot so terribly.

Could it be that it's not stabilized enough going 850fps? I mean I load 180gr major and it's around 850-900fps and it does fine.

Insufficient bearing surface vs a 180gr?

Spends too much time in flight? Almost 50% less time required to go 25 yards at 1300fps vs 800fps.

N320 is less accurate than HS-6? It's perfectly accurate in other loads I shoot.

 

Any clues to lead me in the right direction? The end goal is a nice minor load for 40 that maintains decent accuracy.

 

 

Edited December 11, 2016 by TerryYu

[Hi-Power Jack]

It's possible that PF 119 is a bit slow for the 140 gr bullet.

Shouldn't be the powder - N320 is a great powder.

I would definitely work up the Minor load to PF 135 - 140 and see what happens.     

[TerryYu]

Sounds good. I'll add more power and see what happens!

[jwhittin]

Agree.  Your gun doesn't like the sub minor load. 

Add 2 to 3 times your measured STD to the velocity needed to meet PF. That's you desired velocity to make PF. Adjust for accuracy as needed.  Don't go below 2*STD.  

[rustybayonet]

Back to OP.

This phenomenon is controlled by:

Barrel

Gun

Bullet

Speed of bullet due to powder charge.

This video will break it down to the basics.

You need to get your fps to match your other factors, to fast or to slow, way off target.

[Yeti]

14 hours ago, Hi-Power Jack said:

It's possible that PF 119 is a bit slow for the 140 gr bullet.

Shouldn't be the powder - N320 is a great powder.

I would definitely work up the Minor load to PF 135 - 140 and see what happens.     

+1 agreed, too slow.    I load 140's to make ~140PF/1000fps.  

More speed!

[TerryYu]

3 hours ago, rustybayonet said:

Back to OP.

This phenomenon is controlled by:

Barrel

Gun

Bullet

Speed of bullet due to powder charge.

This video will break it down to the basics.

You need to get your fps to match your other factors, to fast or to slow, way off target.

Not sure what the video is showing me. Not a poa/poi issue but a shotgun sized group problem. 

Really digging the call of duty style hit markers in the video. Where can I get that mod for my AR?

Edited December 12, 2016 by TerryYu

[cowboy85306]

Howdy- AGREE with speed too slow to be accurate. Have experienced this repeatedly in CAS where many shooters attempt to use truly minimal powder charges ( on the theory that "Big and Close" steel targets only have to be heard, so grouping does not matter ) and the ratio of miss vs hit increases ( along with flintlock style hang fires ). In my PPC revolver, I've NEVER gotten my best grouping with the "classic" load of 2.5gr Bullseye behind a 148 gr wadcutter; the stupid gun, which has likely never read about what is supposed to be best, LOVES mid range speeds with 158 SWCs , but  NOT with coated bullets- it also likes lead and lube!

[rustybayonet]

2 hours ago, TerryYu said:

Not sure what the video is showing me. Not a poa/poi issue but a shotgun sized group problem. 

Really digging the call of duty style hit markers in the video. Where can I get that mod for my AR?

That video just got it to the very basic theory. Because the airsoft gun adjustment can go far below and far above safe velocities of a lead thrower, it can show major differences in your powder charge. I don't chrono yet, but I load for accuracy. Every firearm will shoot a little different, even with the same exact ammo. that is why law enforcement can use a bullet like a finger print of a gun. If your major load is right on, you may be stuck with the hard recoil. You can work up the powder for your minor +2 gr at a time, and see if that tightens up your group. If not you may have to change your bullet. Here in the great white north, we have powder, but not much choice, so I have found I can get any powder to shoot accurate, just takes some test round groups before I get there.

[TerryYu]

So I went and loaded some more test loads bumping up the power by 10% each time and it was a direct correlation between power and accuracy. 

As the velocities rose the groups shrunk and I got to a happy medium at about 1000 FPS and a 2 inch group at 25 yards. Not the best showing for the gun but that is good enough for me until I burn up the rest of the bullets and can switch back to 180s. 

So in short, more power = more happy 

[MemphisMechanic]

That's usually the way it works. It's one of the reasons many of us load our 9mm Minor PF to 135ish PF and not to 128 - or whatever number gives you confidence in hovering just about the minimum.

Reliability and accuracy generally improve with velocity.

[fastluck13]

Play with powder and speed. Some powders may be better suited to minor. For example, a slower powder may not burn completely at lower velocities where a faster powder does. Complete burn should be cleaner and more consistent.

[dauntedfuture]

Most people shooting uspsa with .40 shoot 165-180g bullets.  Heavy bullets and fast powder produce less perceived recoil.   Given two loads with the same power factor and a light and a heavy bullet, most people find the light bullet load to be a little more snappy.  With two loads, same bullet, and the same power factor with a fast and a slow powder, most people will find the fast powder feels like there is less recoil.  

  While I'm not certain why the one load shoots poor, that's insignificant as it would not make power factor so that load would not be a feasible combination even if it shot .0001 at 100 yes.  

   Bump up the charges and get the velocity close, I suspect they will both shoot almost the same.