It's worth noting that there were not exactly massive improvements in this time period in gunnery changes. The early F-86 Sabre aircraft, (up to the F86A-5) did use what was essentially the same gunsight as the P-51, which was gyroscopically aimed, and not radar aimed. This gunsight (The K-14/Mk 18) required input of target wingspan and distance to target in order to be accurate at computing lead. Outside of those parameters, it got more and more inaccurate at computing a target lead solution which was a negative. The MiG-15 and MiG-15bis also used a WW2 era gyro gunsight, specifically the ASP-1N. This had the same limitations of needing fairly accurate parameters to be 100% accurate.

This "equality of sights" changed however during the production of the last few F86A-5 Sabres and F-86E Sabres in early 1951. The US introduced the A-1CM gunsight-AN/APG-30 rader combo at this time. This new gunsight was revolutionary because it allowed instant and changing calculation of lead using radar as long as the enemy aircraft was in the radar's "cone of effect". The MiG-15 series aircraft never got an upgrade to this type of radar guided system, so they never managed to get as accurate of a targeting solution.

Another piece of info worth noting is the F-86A/E/F's armament. The 6 AN/M3 .50 Browning machine guns were essentially the same as the P-51s AN/M2s, they just had a higher cyclic rate of 1200 rpm instead of ~800. The USAF looked for an upgrade to this gun system for higher lethality in the M-39 (T-160 at the time) 20mm revolver-style autocannon. This armament was theoretically much more deadly, due to the fact that it had a higher rate of fire (1500 rpm) and also carried much more damage potential due to the ability to use 20x102mm HE (high explosive) ammunition. This gun system was tested in the F-86F-2 variant as a quad gun mount, of which there were 10 built (according to North American tech rep John L. Henderson as referenced in MiG Alley by Thompson and Mclauren) and some (4? not entirely sure on the number) were sent to Kimpo airfield for testing for project "GunVal" (read report here: http://www.dtic.mil/dtic/tr/fulltext/u2/031528.pdf). The problem as outlined in the report was that large amount of exhaust gasses from these guns were and did cause compressor stalls on engines that they were not able to correct. This caused the military to just continue with the six AN/M3 armament for the rest of the war for the reasons that it was essentially "good enough" and was logistically much easier to continue use of.

One thing worth mentioning was that the Mig-15/MiG-15bis had a 37mm and two 23mm cannons that were originally designed to intercept bombers like the B-29. This meant that the aircraft was designed to pack as much firepower as possible into the small frame. This necessitated the use of the small N-37 and NR-23 guns. The problem was the compactness of the guns meant somewhat low muzzle velocities. The N-37 and NR-23 only had muzzle velocities of ~690 m/s each compared to the AN/M3s ~900m/s. It wasn't exactly "slow" ammo in the Russian guns, but it was slow enough that in very fast jet combat it could cause some difficulty aiming during maneuvering and trying to lead a target doing so.