I built a 300 blackout SBR with a Syrac ordnance piston and wanted to use a MVB arc stock but I could not get it to run without short stroking on subs with a can and without a can it would not cycle supers. I even went down to less than 3.0 oz on the buffer with no success. Was wondering if anyone else tried this combo and was successful? I had to go back to an adjustable DI in order to get working and I will keep the piston for another build in the future. Here is an after pic. She works great now.

OK I am going to state right up front I don't have any experience with that stock.  I do have experience with the BO and pistons so here goes...

Thinking you STILL need to go lower on the reciprocating mass.  I built my BO with the AA XLP system.  Through conversations with Aro there he recommended their lightened carrier and a pistol-length system with a standard buffer.  Right after the first trigger pull it has been exceedingly reliable with supers and subs, without a can.  

Now I will say that your setup must require a short, stiff spring.  Because you are already at a disadvantage due to case capacity of the BO, a standard-mass carrier and a stiff recoil spring seem like a bad combo for the blackout.  If you can get a low-mass carrier I'd start there.  If you can get a softer spring that will likely help as well.  

Ultimately it's all about reducing resistence and that is friction and mass.  Grease it up, break it in real well (work the action over and over while watching the evening news), then move to the more serious solutions if that doesn't help.

Thank for the advise. I tried everything in order to get the Syrac Ordnance piston to work with the MVB stock (grease greased taken apart over and over, talked with Jason from Syrac Ordnance and the guys over at MVB industries, etc etc). (I forgot to mention that it worked fine with a standard buffer tube) MVB even sent me a new set of buffer springs (the MVB takes two springs, which I think is the problem) and it still would not function correctly. I have a Syrac Ordnance piston YHM 300 blackout build with the same length barrel etc and it works fine but it has a standard buffer tube. Here is a pic with the my YHM 300 piston build, the BAD problem MVB/ Piston build (before I went back to DI), and my MP5. Also below is a pic of the MVB ACR springs so you can see the setup. Right now I'm happy with it as a DI gun but I was wondering if anyone else had the issues between the MVB stock and piston combo

This certainly sounds like a potential source of your cycling problems. You're over sprung.
If possible, have you tried running it with just a single spring?

The springs sit on top of one another within the MVB buffer tube (see pic below). I thought about cutting a few coils on the larger spring but I think that would have been a bad idea, ever MVB discouraged it when i talked with them. They wanted to replace the springs and take the buffer down in weight. Did both with no success

I was going to suggest cutting a coil or two off, but I see later in your reply MVB said not too.

Be that as it may, for academic purposes, you may want to cut a coil off and see what happens.

Worse comes to worse your gun cycles too fast, or,  if you cut too many doesn't have enough length to send the BCG home and brace it while in battery) and you're out $10 for a new replacement spring.

My thoughts are that if you have to cut the springs then you are really screwing with the engineering and something bad might happen because it should work without cutting a coil.
However I think cutting the spring would be the only way you could get it run now, that along with a less then carbine weight buffer 3.0> oz. I am not sure how much the springs for the MVB cost, they are different in size from a normal buffer springs.

I gave up on getting the gun to run with a piston a while back and converted the upper to direct impingement. The gun functions fine with the MVB with a DI upper. I wanted to see if anyone else tried this stock and piston configuration and had the problem I did. I'm happy with SBR in DI right now but don't get me wrong I love my piston guns too. My YHM/syrac 300 BO piston SBR is awesome

With a 10.5" 5.56 and Adams Arms XLP my ARC stock works just fine. I went with H2 weight in the buffer.

300blk and Pistons are a different animal. 300blk uses pistol powder. There are a bunch of factors that could be affecting it. That being said my 8.5 DI 300blk works fine with the stock and I don't have any motivation to go to piston.

Dunno if your piston is adjustable for gas flow but the XLP is. The other factor to look at is the powder in your 300blk. If you load your own you can try other powders.