Rock River Arms AR LAR-PDS Pistols? With a choice of .223/5.56mm or .300 Blackout
http://www.rockriverarms.com/index.cfm?fuseaction=category.display&category_id=415 I would suggest getting the railed model and not the cheaper rubber weird grip.
No buffer tube at all. The internals are different, so the rear of the receiver has a rail like the Sig MPX and KAK makes an adapter if you need a brace. Then if you latter choose to SBR it, you don't have a buffer going in the extension and/or hinge, so you can fire it with the stock folded. Sig comparisons can be seen at
http://www.thefirearmblog.com/blog/2016/07/07/review-sb-tacticals-new-collapsing-sig-mpx-brace/ I know you are looking for something that isn't a collapsible and/or folding stock, just wanted to throw future options out there to consider.
That is the shortest option. Flat back of the receivers with a quick disconnect hole with a rail section.
After that, there are the collapsible systems that use special buffers and springs, and/or some with springs in captured spring designs.
A 4 inch model
http://www.gunsmokeenterprises.net/product.sc?productId=5&categoryId=2
found from this thread
http://www.ar15.com/forums/t_8_39/503097_An_AR_15_andamp_quot_Michigan_Pistolandamp_quot_.html
I know it doesn't seem to fit but the advice in there crosses over. The Link above is updated from that thread for the part source. There are now multiple generations/versions.
MVB MPK (Micro Pistol Kit) - Complete Buffer Tube System is 4 7/8" inches
While the upper end comes around
Phase 5's AR Pistol buffer and tube at 7 3/8 inches.
Unless you want to count the length of the A1 & A2 Stock Receiver Extensions to make the size reduction look even better.
There are a couple of collapsible stocks that you could just use the buffer tube and remove the stock pieces but with the prices around $300.00 -$500.00, that cost might not be worth the savings in length.
Staying closer to mil spec with just special Buffer Tube, BCG, and Spring cuts down about 3 inches. While buying the other non mil spec design drops 7 inches. Which may lead to problems down the road for parts that wear out. No one thinks about springs and other spare replacement parts needs 15+ years down the road for the special designs. Staying away from RRA & Sig special designs allow the going back to regular longer carbine buffer & receiver extensions systems, if you can't get parts.
edited from above The Dead Foot Arms AR pistol kit option is a 2.55" MCS Buffer tube , so that gets you down( or is it up?) to a 5 inch reduction in length.
similar thread http://www.ar15.com/forums/t_3_122/702847_Looking_for_a_short_buffer_tube_.html