Sam1911: I found your post on actual trials of this concept the most useful Post for me. Thank you thank you for giving your experience.
I have considered building one of these pistols. Your post gave me pause. Can you try to explain to us exactly why you ended up being more effective with a pistol? Can you explain exactly what about it made you less effective with the A.R. pistol? Can you narrow it down to particular aspects?
Sure! Though I should preface by again saying I am not talking about building a variant that can/will be used as an SBR-like carbine, such as with the SIG brace, or even shouldering a long buffer tube. What I've tested is shooting these (and AK pistols, KelTec PLRs, etc.)
AS HANDGUNS.
I practice, train, and compete with service style handguns very regularly. Most primarily that's either a 1911, an xDM, or a S&W 629. I use handguns in the role of defensive or "practical" shooting, which means dynamic courses of fire shot at high speed and at ranges of 0 to 30 yards, with multiple targets, moving targets, use of cover, and so forth.
When using the rifle-pistols this way, against a timer, they are not competitive. That means (to me) that I can't get hits on target as fast, as accurately, with them as I can with a handgun. Reasons for this?
1) Bulk, weight, balance -- they don't sit in the hand like a handgun. They're bulky and slow to get on target and to move through transitions between targets. They don't have that svelte liveliness in the hand that lets you really react smoothly and quickly to the threats.
2) Sight configuration -- They generally have rifle sights, designed to be used with the stock mounted against the face (or even optics used the same way) and breaking that situation to try to use them at arms' length as a handguns means peering at and trying to align sights that are now too far away, too small, and bouncing around as you try to hold up this most-of-a-rifle.
3) Reloads -- Holding 85% of an AR-15 at arms' length and trying to perform a fast reload is pretty clumsy compared to reloading almost any common service sidearm.
4) Carrying/draw/presentation -- Can't holster these, can't really carry them like a normal handgun so you don't have that smooth, fast 4-count draw-stroke some of us have gotten down to 1 second or faster. There are a variety of carry styles, mostly based on the problematic one-point sling concept, but that's a pretty bad compromise compared to a strong-side belt holster.
5) Pistols are faster than rifles anyway -- This one is controversial among some of my pals but it is definitely what I see. And I understand that it doesn't really speak to your question. I have run tests many times and I can say with complete confidence that I am faster at putting multiple aimed shots on multiple targets with a handgun than with a (
shoulder-mounted) long gun, rifle or shotgun, on targets within about 20 yards. So, while no-one ever has suggested that an AR-15 "pistol" is as effective to use as an AR-15 carbine, I am still faster/better with a handgun than with the carbine. So it is only logical that the difference in performance would be even greater when comparing a service sidearm to an AR "pistol."