According to the recollections of Soviet fighter pilots, they very rarely exhausted all of their ammunition. They considered the ammunition of a Soviets fighter (Yak, LagG/La) to be quite sufficient. I wrote about it earlier, but you repeat the erroneous thesis about insufficiency of the ammunition of Soviet fighters again.
And I will keep repeating the this "erroneous thesis".
Because it is true that the Soviet aircraft, in general carried less ammo. That is true, it is not erroneous unless 120 is somehow greater than 140 or 150.
The flip side is if the soviet fighters could perform their missions, for the most part, with the existing ammo under the conditions/missions they were flying.
Which is a different question/problem.
When we are comparing aircraft from different nations that flew under different conditions it helps to understand the limitations.
For instance US fighters flew with anywhere from 200-201 rounds per .50 cal gun to 425-430 rounds per gun. Now were they flying with too much ammo and hurting performance (30 seconds of firing time roughly) and they should have all used 200-240rpg or were the planes with 200-240rpg running out an times and not able to
complete missions?
Might depend on who (which pilots) you talk to, when (what year or even month) and in which theater and/or base (land or carrier).
A Soviet fighter with
a 20mm ShVAK cannon often carried 120rpg, Which at it's nominal rate of fire of 800rpm is 13.333rps. Using 13rps because I am being lazy that means a firing time of 9.3 seconds. Which may very well have lasted most soviet pilots through the combats they were engaged in. It is certainly better than the firing time of the Spitfire Vb, The 109E and the early Zeros and all of the French fighters of 1940 by around 50%.
Now a lot of militaries often pushed pushed things to extremes, just in case. And some times the extremes actually hurt the performance in the field. P-40 is a classic example, went from 370lbs of guns/ammo to 600lbs and on the P-40E to 901lbs without changing the engine much. Some field modifications, and some P-40Ls and first P-40Ns dropped the armament weight back to about 600lbs to try to get performance back. Firing time of these planes were about 15.4 seconds. Vast bulk of the P-40s went back to six guns with more ammo. What the Soviets did with them I don't know. I also strongly suspect that when carrying large bombs under the wings (2 or 3 total) the P-40s were not carrying full ammo.
Getting back to the Soviet fighters, the 20mm ammo ran out after about already mentioned 9.3 seconds (or perhaps a few tenths more) but the 12.7mm UB machineguns lasted longer. Synchronized 12.7mm UB machine guns are often credited with 900rpm or 15rps. It seems in western books the UB guns most often had around 220rpg for a firing time of 14.7-15 seconds. A few planes had as little as 170rpg (11.4-12 seconds) and some has 250rpg and The Mig-3 had 300rpg for 20 seconds?
This mismatch between cannon firing time and machine gun firing time was very common.
Now for the Soviets, are the pilots say if they didn't run out of cannon ammo or just ammo in general? For most Soviet pilots they had about a 5-6 warning time between the cannon running out and the 12.7mm guns running out.
I will note that the LA-5 went to 200rpg and since the gun fired slower ( I don't know by how much) the LA-5 pilots had
at least 60% more 20mm firing time than the Lagg-3 and Yak pilots had. Late La-7s got three guns and 170rpg.
The Soviets may have been happy with the ammo carried and the firing times they got. Other nations may not have been. Wither that was based on combat results or theory is subject to question.
US P-39 was another example of over specifying. They took two .30 cal guns with 500rpg out of the nose and then stuck four .30 cal guns with 1000rpg in the wings. Increase in ammo weight alone was over 180lbs and the need to keep the .30 cal guns running for 50 seconds after the cannon and cowl .50s had run out of ammo around 30 seconds earlier? In service the loading chart often stated just 300rpg (15 seconds of firing time).
I don't know what the 'magic' time was. US Navy wanted a lot of firing time. Doesn't mean they were right for land based fighters flying 1-2 hour missions.
And the flip of that Planes with the firing duration of Soviet fighters might have been a handicap in carrier battles, as shown by the Japanese at Midway. Granted the Japanese Zeros had a shorter firing time than the Soviet planes but the Japanese were working to increase the firing time substantially either during the battle or just after. And then they increase the firing time about 25% over the new longer time.