Some sources metioned that both versions , Mt-lbm 6mb3 and Mt-lbm 6m1b4, equipt double barrel 23mm gun, Kord 12.7mm HMG and Ags-17/30 30mm AGL. The inly different between them is the 23mm Gsh-23K gun (The 6M1B4 version) and the Gsh-23B or Gsh-23V (The 6M1B3 version). This Gsh-23K gun can explain about the Gsh-23 gun with longer barrels we have seen.
Careful... sometimes they change the designations for guns depending on what platform they are mounted on.
For example the Shturm missile becomes the Shturm V on a helo, but shturm s for the land based model (on the MTLB launch vehicle).
I would suspect the GSh-23V is for the model used on the Hind... but the gun is also shown on the MTLB vehicles you posted above, and is of course used in standard Soviet and Russian gun pods in the form of the fixed UPK-23-250 and the flexible SPPU-22 gun pod.
It would make sense for a K model to have longer barrels as with the larger calibre GSh-30K for use on the Hind.
The gun was also put an this MT-LB.
That might be the GSh-30K...
This Gsh-23 with longer barrels certainly is a rare sight. As already pointed out, the longer barrels probably do not have that big of an impact on the performance of the rounds anyway.
There is not a huge amount of room for propellant in those 23 x 115mm rounds so the extra barrel length would not dramatically increase muzzle performance....
The main selling point of the round is a heavy projectile for its calibre and a potential high rate of fire, but with compact low recoil ammo...
If you compare the types... the single barrel AM-23 gun weighs just 43kgs and fires at 1,250rpm, while the twin barrel GSh-23 weighs 51kgs... 8kgs more but fires at between 3,000 and 3,400rpm... so extra 8kgs for double the rate of fire... but the 6 barrel GSh-6-23 weighs only 76kgs and fires at a staggering 9,000-10.000 rpm.
Note the muzzle velocity is only 715m/s but it uses a rather heavy projectile... the same weight as the 23mm anti aircraft guns of the Shilka and the ZU-23 towed guns. If it used the same light projectile as the US Vulcan cannon its muzzle velocity would be in the 1,100-1,200m/s range.
The Kurganets or T-15 simply are not fully developed yet, and i would prefer to see some stuff showcased that is actually in service and actively being modernized.
Not to mention ready for export...