[archy]

Not enough fire power.

Fit the 25mm gun power turret from an M1025 Bushmaster Humvee. Next problem.

[R. Scott]

Way back when we had the opportunity to compare the 20 mm and .50. We preferred the higher rate of fire, ease of loading, ease of maintenance and lighter weight of the .50. The chain guns and other light cannons are good against vehicles, but not much better than a machine gun against people – and people are the problem, not vehicles.
A good old fashioned low tech 5 ton fitted with armor plate and a few machine guns would do nicely.