Your average Onix that's "weaker" then a Ponyta would mash a rifleman to a pulp if he tried to battle it one on-one. Rock types, Steel Types, and a select other Pokemon such as ghosts in general would laugh at firearms as their bullets harmlessly glanced off their armor/phased through their body. This likely explains why the Magnemite line is so common among most veteran soldiers such as Lt. Surge and Watson, due to the fact it can take bullets.
Weight also isn't always an issue, as some Pokemon are made out of extremely strong materials for their mass, such as Porygon2, able to withstand explosions and SE attacks from Mewtwo while weighing less then 100 pounds, Metagross, weighing a mere 1018 pounds while effectively having the armor of a main battle tank, and Registeel, which is even lighter for similar durability. Metagross would likely be in a poor position fighting against a Gunship armed with HEAT charges (think Skarmory, but with offensive capabilities), but properly supported, it could easily match the devastation level of an armored fighting vehicle on the battlefield.