We have been working with this weapon some time now and I think it is a vastly under-rated system. To recap. I bought a 10.5" LMT Upper unit from one of our staff instructors. It came as pictured less the Micro Aimpoint. I had a number of M4 lowers in stock so I grabbed one and transferred to myself it as a pistol. This would eliminate any possible concerns over the "constructive possession" issues. I added a pistol buffer tube and began working with it. More on the pistol buffer tube matter later.
We have had "pistol" discussions before, mostly relating to the AK series of stcokless SBR rifles. These have zero utility and should be avoided. That is what I have said from the beginning. The M4 however, due to the design need of having a receiver extension (aka "Buffer Tube"), allows it to be used exactly as a rifle...almost.
We used two different systems to use the M4 pistol (I almost want to call it a "stockless rifle" since that is in effect what we have). One we obtained from our friends in the Czech Special Forces. They have a way to shoot the diminutive Skorpion VZ-61 Machine Pistol using three points of contact. They place the buttstock on their cheek and ignore the shoulder. Being a 32ACP, only Justin Beiber would find the recoil objectionable. This is best used for CQB, or close range problems where a center of mass (or center of face) shot is required. The rifle...err, ah..pistol, is brought up to the face, and fired as soon as you have alignment, seeing what you need to see. Quite fast actually and reminiscent of the old SAS MP5 techniques.
Invariably someone will try to use some sort of sling to "enhance" this. I advise against it as it is simply unnecessary and return nothing for the effort. Just bring it up, get that visual index, and press.
But this system is not solely for CQB. We have shot this out to 300 yards and hit IPSC steels repeatedly. When greater accuracy is needed for longer shots, or for extremely precise close shots, it is a simple matter to obtain that fourth point of contact by bringing the end of the buffer tube back into the shoulder.
It is also a simple matter to reach out with the weapon (we have taken it to 300 yards, but 200 is probably more reasonable ballistically) in any of the usual supported rifle firing positions. You may need to modify them slightly, as you would when working with a short stocked rifle, but the results speak for themselves.
Several students in the recent rifle course noted that while an SBR may not be legal in all states, this M4 Pistol will not only be legal anyplace a handgun is legal, but also legally carried with any CCW permit. Interesting option for those living in left-leaning regions of the country.
We have also found the M4 Pistol to be an excellent vehicle weapon, being extremely light and handy and capable of maneuvering in the close confines of a Tahoe, Jeep, BMW, or similar sized automobile.
Invariably there will be those on the internet that will tell you that you could put on a rifle buffer tube and bring up documents and letter from the ATF. Look guys...a pistol buffer tube is about fifty bucks. Do you really want to kick over the ATF anthill for fifty bucks? I do not. I have friends that work for that agency and I called them to ask. The unanimous reply was to not be a cheapskate and try to find some loophole to avoid spending fifty bucks. I agree.
Try the concept out, I think you will be amazed at what you can do with the M4 Pistol. Oh, by the way...since i live in AZ, I did submit my Form 1 and send in $200 to make it a bonafide SBR, but for the guys that cannot do that, this is a very viable option.