I am working on a pistol build. The upper is a stag left handed with a 16"chf spikes middy barrel cut down to 13.7, adjustable superlative arms gas block and a standard left handed toolcraft bcg. The lower is a standard psa with an h buffer and a strike industries flat wire spring. The ammo is xtac 55 grain.

When not running a can, the bolt will not lock back on the last round but the rifle functions fine. The gas block is open all the way or 4 full counterclockwise turns from the full stop. When I put my can on, it does lock back.

If I'm understanding the problem, I'm not getting enough gas to lock the bolt back without a can.

Does this mean that the gas port is too small and likely needs to be opened up just one size so I get enough gas to fully/properly cycle?

Is there anything else I can try to do to confirm this?

Did you try several mags?

Dump the weights out of your buffer and see if it locks back without the can with an empty buffer.  If it does, you have a decision.  You can either change buffers when suppressed, or open up the hole so you can use the same buffer and then adjust the gas.  

When you chopped the barrel, you reduced the dwell time between when the bullet passes by the gas port and leaves the barrel, so you will have to compensate.

I did. 5 mags. Same pattern. Good suggestion though.

Thanks, this is exactly what I was thinking. Im going to try a carbine buffer and see if that works. If it does, off to adco it will go to open up the gas port.

It's actually pretty easy to DIY the port.  If you know what is there now, just buy a "number" drill bit that is the next size up from there and chuck it up.  Then go shoot it.  If it locks back, done.  If it doesn't, buy the next bit size up from there and repeat as necessary.  The bits cost about $3 at Mac's.

ALWAYS work on the cheapest part.  In your case, that will be the buffer or buffer spring.  The gas port was sized for a full 16" barrel, and since you chopped it down, you have decreased the dwell time, which means less gas pressure, which is leading to your failure to lock issue. Reduce the buffer weight or reduce the buffer spring weight accordingly. Tune the gas block as needed to run with the suppressor.