2011 open - light recoil springs and accuracy

[racer-x]

Thought I would share an interesting troubleshooting exercise with a very definitive solution.

 

First the baseline. Using my standard 9 major loads in a brand new gun (9 major open from Atlas), I put 5 rounds in a ragged hole under 1" diameter while sighting in at 50 yards. Nice!

 

Fast forward 1500 rounds or so and I'm having problems with my 8" steel at 50 yards in practice. I set up a target and shoot some groups at 25 yards... 5"+ groups are the best I can do now?

Cutting to the chase, I tried a bunch of things over the next few days:
- removed & remounted optic mount & optic - no change
- swapped optic to different DPP - no change
- tried some factory 9MM minor loads - much worse
- tried ammo from same batch as baseline test - no change
- tried shorter OAL, same PF - no change

 

Since this is a stroked gun, I needed to be careful with recoil springs and ensure they didn't coil bind.  Gun felt great with the spring it came with, so I hadn't experimented with it yet.

 

Wolff 8# variable did not bind and went in the gun for testing. Great groups again!

Tried a Wolff 9# variable as well and groups were still great. Swapped stock spring back in and groups were poor again.

 

Did some spring testing after that to quantify my range results.

* stock spring in gun was a Wolff 9# fixed with 5 coils removed (33 coils remaining) so that it doesn't bind.

spring (coils)        3.700"   1.370"
--------------------  -----    ------
Wolff 8#F (32)        4.63#    8.20#
Wolff 8#V (33)        4.01#    8.05#
-------------------------------------
Wolff 9#F (38)        4.66#    9.07#
Wolff 9#F (33)*stock* 3.26#    8.22#
Wolff 9#V (32)        4.29#    9.17#
-------------------------------------
Wolff 10#V (33)       5.23#    10.14#
-------------------------------------

3.700" = in battery spring height
1.370" = fully compressed spring height (stroked)

My conclusion is that the stock spring (after 1500 rounds) was not maintaining enough pressure with slide in battery for optimal accuracy out of this gun. I didn't test this spring when it was new, but assume it was a little stronger than 3.26# at 3.700 when new, and that was just enough more to get best accuracy.

 

I have tried several other springs since  and settled on the Wolff 8# variable. Gun tracks great with minimal dot movement and quick return, while maintaining best accuracy out of this gun.

 

 

 

[Hi-Power Jack]

Thanks for the info - I never would have guessed (would have actually bet the farm against )

springs would have affected accuracy.

 

What is a "stroked gun"   BTW ?

[HoMiE]

1 hour ago, Hi-Power Jack said:

What is a "stroked gun"   BTW ?

When shorty or mid length guns have commander length slides, stroked is just building it to run regular 5” length guide rods. 

[racer-x]

Here is a good description:

 

Atlas Gunworks Video