I don't know exactly what happened with that SU-5
Me too. But AFAIK several SU-5 were used by Red Army (by Soviet mechanized corps of Kiev military district?) in summer 1941 in Ukraine against Germans - probably, they were lost during the hard tank battles. SU-5s from Leningrad and Western military districts also were lost, probably. As for the SU-5s from Far Eastern military district - they participated in Soviet-Japanese military conflict of 1938, and I couldn`t find any data about their losses - those SU-5s could survive the war.
From private talks with Max Kolomiets I heard that 4 SU-6 vehicles existed only on paper. I.e. existed their hulls, but they were not operable.
You know Max Kolomiets, that`s great!
I`ve read his publications about Soviet tanks and SPGs in different journals, on English also.
As for AA SPG SU-6. I also found today almost the same info - one experimental SU-6 was completed for sure (it was developed by professor F.L. Khlystov from designer office of Artillery Academy of RKKA and built by factory No.185). But as I`ve already mentioned - SU-6 was 2 t overweighted in comparison with light tank T-26, aslo 90 hp engine of T-26 was not powerful for such SPG (it had max. speed only 21 km/h). So armored forces rejected the order to serial production of SU-6 after ground tests. Till January 1937 four chassis of SU-6 were completed, ten were under different stages of production (factory No.185). So one SU-6 was completed + 4 chassis + 10 incompleted. Another sources report that 5 were completed, 3 were given to the army. I think that the first info is more correct.
Me seems that all photos of SU-6 represent the single prototype of SU-6 (1935-1936) during the tests. I found the info that SU-6 was tested during September-October 1935 (factory tests), since 13 October 1935 it was tested at NIAP (research artillery training area) near one year. During that year SU-6 was under repair some time because of forced engine and suspension, also it was tested with 37mm automatic AA gun during three months. During the test race (750 km) engine of SU-6 overheated after each 15-25 km with the speed 25 km/h. 416 shots were made from 76.2mm AA gun of SU-6 during the tests (from stationary positions with/without hydraulic cushioning of the suspension, during the move also) - fire accuracy was average, not good.
I heard something about SU-8, but not sure they were more than a single vehicle.
Yes, probably only one experimental AA SPG SU-8, based on medium tank T-28, existed (was built in 1935) - it had better mobility than SU-6 as 400 hp engine gave the max. speed 38 km/h.
As for YaG-10 trucks with AA guns, I'm trying to find their photos, but seems not much of them exists...
Some different info about those trucks YaG-10 with 76.2mm AA guns exists also (12 were produced, 20 were produced or 61?). I found the info that SPG (29K) was developed in 1935 according to the order of the General Artillery Directorate for AA defense of mechanized units. 29K was equipped with the same 76.2mm powerful AA gun mod. 1931 (range of fire - 14 km horizontally, 9.5 km vertically, 15-20 shells/min, 6.61 kg shells) as SU-6 and SU-8. Factory No.8 should produced 20 29K as experimetal series in 1935, but received only 12 YaG-10 chassis. After successfully ground tests in August 1936 army accepted those vehicles. Probably, in addition to 12(20?) experimental 29K several tens were produced as production series. I found a mention that 176th anti-aircraft artillery regiment (Moscow military district) was armed with 50 29K in 1936.
Alex, by the way, do you have any drawing or picture of the Soviet 152-mm SPG of coast defence (engineer A. Tolochkov, 1933), I`ve wrote shortly above. It was a never built project, but of very interesting and progressive design.