Well, I got the Spikes Tactical carbine weight buffer installed and took it to the range today. I brought 5 different types of ammo with me:
1. Remington 220 grain subsonic
2. Gunn 220 grain subsonic
3. Hornaday Black 208 grain AMax subsonic
4. Sig Sauer 220 grain subsonic
5. American Eagle 220 grain subsonic

The Remington, Gunn, and Hornaday all failed to cycle. Sometimes they closed on an empty chamber and sometimes the spent case got caught or the new round jammed. But either way, it never properly cycled, not even once.
The Sig and American Eagle on the other hand performed flawlessly. I ran 20 rounds of each through and not one hiccup. I've never had a firearm that was so sensitive to the type of ammo, but at least it's working now.

What Barrel Length and gas system?

The two references I found for the DD M4V5 in 300blk are Midway USA and Impact guns. Both show a specification of 16" bbl and carbine length gas system.

This may be your problem, which you have noticed was semi-solved with different ammo. I believe that carbine length gas systems and 16" barrels are not well suited for all the different subsonic ammo on the market. Dwell time for subsonic ammo is critical with 300blk and the longer gas system and barrel robs the backside of the cycle the pressure needed. With a 16" bbl/Carbine gas sys, you have to fill the barrel to the gas port with pressure, go down the tube, unlock, and then have enough pressure left as it bleeds off the gas tube to still cycle the bolt. With subsonic ammo you have cut the available source of pressure in half.

Example from Hodgdon reloading online
using the same powder, H110:

115gr bullet 20.3gr powder = 2134fps
220gr bullets 9.2gr powder = 1046fps

It gets tricky when you don't know for sure the powder being used and it's burn rate in factory ammo. Best guess for subsonic factory is a lot less powder.

My best suggestion would be to stick with ammo you know cycles it well or reload your own. Might be tricky, but you could try an adjustable gas block. That is, short of replacing with a pistol length gas sys which would also require the bbl to be replaced and I don't sense that is a solution you're looking for.


If I have looked at this the wrong way, someone can surely correct me and I will learn something.

It is a 16" barrel but I don't know the length of the gas system. The variation in ammo must be the type of powder because the bullet weights are all close and they are all moving around the same velocity.

If you have access to another bolt carrier group try that. Built a 8in. blackout that acted the same, drilled gas port, same. Took the bolt carrier from my 12.5 blackout and it worked perfect, but over gassed now, just installed an adj gas block, works perfet now. Sent carrier group back to company and they sent another, never did find out what was wrong with it.

I have had the exact same rifle for 3 years and run a Specwar762 on it both sub and supersonic. I have an occasional FTF with 220 grains but the 208s, and 187s from multiple manufacturers run like a champ. I stick with those. I hunt with the supersonic and silencer and it is a more than tolerable blast without protection. The subs are really quiet but it is more of just fun to shoot than useful for me.

If I were you I would install an adjustable gas block so you can tune the rifle to run well with your ammo and can of preference.

That can must not be producing enough back pressure to properly cycle the bolt with certain subsonic rounds that are lower pressure. So you can try subsonic rounds that generate a bit more pressure, or try a different can that produces more backpressure, or try the adjustable gas block.

If you drill out your existing gas port to the point where it runs with the softer subsonic rounds the rifle will probably be overgassed shooting supersonic rounds.

I think it probably is a gas block issue. I have no experience taking this apart further that regular cleanings so that's not something I want to mess with.
As Meatman said, it's really just a fun toy with subsonic suppressed and I don't see myself using it for any particular purpose. If I was going to hunt with it I'd probably use the supersonic rounds and it's really too long with the suppressor to use for home defense CQB.