High extreme spread...

[1stmar]

I have a very short barrel on my mini, 16.5" with the muzzle break so probably 14.5" barrel. Chronograph readings were >100fps spread and high sd. Initially I tried 24.5gr of 4064, lc brass and 55 gr Fimj. I was thinking the es variation was from to slow a burning powder for that length of barrel so I went to a faster burning powder (2230). Same thing, high es.
Not sure in barrel length is a factor or not, or if primer pockets may be leaking?
I use the same process for reloading all rifle calibers, other calibers are single digit sd and low dble digit es.

 

[Metal god]

I have tested how I hold the rifle can change the ES / SD . Next time out try shooting and letting the rifle almost completely freely recoil . Just stay in control of the rifle . My test were with a 13lb 308 with muzzle break so it was easy to control . Your mini should not be to hard to control . Remember this is a velocity test so don't put to much work in group size just get the round over the chrono .

My results were drastic but I did not start with that high of a ES . I was holding my rifle very tight to my shoulder . The lighter my hold got the lower my ES got . I was able to bring down a 36fps ES to 12fps ES with an SD of 5fps just buy how I held the rifle from shot to shot . I don't think you'll drop your ES to 20 or anything but I bet you take off a 1/3 to 1/2 having a consistent hold .

There are other things like inconsistent bullet diameter will cause different internal pressures causing different velocity's .

Changing primers could help

Uniform flash holes

Bullet hold maybe

 

[Road_Clam]

reading into excessive ES can be a frustrating issue to sort out. I too for some reason observe higher ES's when running 4064. Can't explain why. I don't do anything different than when I'm loading W748 , 2000MR , or any of my other medium burn powders. I used to get all worked up when my ES's exceeded 60 fps but now that I've generated a lot of varied velocities and groupings I've realized that I see NO correlation between larger ES's and poor precision. I've shot sub MOA loads and my ES's were up around 70 , and I've shot as poor as 4 moa with ES's in the teens. So in my situation of a recreational shooter I focus on other aspects of precision.

 

[MtnCreek]

There are better powders for 55gr/223 loads. It's a little on the slow side and your load looks to be a little on the mild side. Slowish powder and lowish pressure will often = high SD's.

I wouldn't necessarily trust the chronograph results and high spreads in a 16" mini14 shooting FMJ's is not the end of the world. It's a blaster shooting blaster fodder.