EDIT (2018-02-24): On the alphadev branch and the guns branch, we now have V0.9.0b1 with a full (but still beta) implementation of the new gun mechanic. On both branches, you can now also play all game mechanics. There are two orb themes that are designed for the gun mechanic but you can try and theme you like - it just may be extremely challenging. Speaking of challenging: You should use low-difficulty beatmaps and so far, all procedural beatmaps I have tried seemed quite impossible to play successfully in gun mode, so for now I'd recommend only trying this with osu! beatmaps.
If you feel very brave: In Steam, go to properties of Holodance, tab "betas", and select "guns". Wait until the update is downloaded. Start the game. Select either single or dual guns. Default "orb theme" is Drones but there is also one that is similar to the Casual orb theme. Go into one of the environments.
Make sure to pick an easy beatmap. I can full combo 5.5 ones with the regular mechanics - but with the gun mechanic, I feel very comfortable with 2, and quite challenged with 3.0. If you jump right into your usually difficulty, this will only be frustrating.
I said "Early, Raw and Unpolished" for a reason: Most importantly, this version doesn't have proper timing indication. I know that sucks, will fix it tomorrow. So the next release will have the typical osu! style indicator ("shrinking circles"), and I'll also add one or two that are more like "timers". The weird circles that were in the game all along have been in the game since pretty much the first prototypes - time to let them return to the source, like all old programs no one has any use for anymore.
Then, of course, you'll notice immediately that this version still lacks a lot of the oomph we expect from shooting guns. One thing I believe is really important is a little haptic pulse when you lock the laser sight onto the target, and of course also haptic feedback when you blow up the drone or orb. Then, of course, we need proper explosions and sounds.
There are a few more fun things I'll add - but I won't spoil those here ;-)
Finally, sliders and spinners don't work with that mechanic, and in that build, you cannot play any other mechanic (try it - you'll see what the problem is quickly ;-) ).
To be honest, this build is even a little early to gather feedback - but I thought some people might be waiting for the gun mechanic, and I wanted to put it into your hands as early as possible.
Enjoy!