In halo wars, they use an MA5B assault rifle, yet 20 years later they use MA37 then MA5Bs. So why did the assault rifle turn from new, to old, then new again? Because the MA37 is obviously an earlier odel due to the skeletal look. Help!
1. the MA5B was used in the UNSC Marine Crops whilst the MA37 is used in the UNSC Army.
2. the MA5B would have been introduced during Halo Wars and the MA37 was introduced into service years earlier, the MA5B most likely would have been the replacement for a long term assault rifle, when the MA37 would still be in long term use in the Army, but most likely limited use in the Marine Crops.
The Marine Branch and the Army Branch use different equipment. The 37 is apparently still relevant in the Reach army, probably because that was the best weapon they had in their planet. The Marines have better equipment and weaponry because their branch moves around alot from planet to planet or system to system, giving them more flexibility in technology (Earth is believed to have more advanced weaponry than Reach, which is very possible since they are in different systems).
Also, like real-life gun models, the previous versions of weapons can have their own advantages and disadvantages. In this case, the 37 is more accurate than the 5B, while the 5B chose to trade off accuracy with stopping power.