Well, i would recommend training with any of the default classes, without using its special perks (perk 1).
Technically, the most powerful/decent weapons are from the default classes.

If you plan to play a mid-long range, but dont plan to snipe, take the M16 and train with it. After using such a weapon, you'd generally be more cautious not to engage close range targets.

The MP5 is also a reliable weapon, however, once you use it, it may be difficult to change to any other SMG weapon. (It has one of the highest accuracy and stability any SMG can offer)

Snipers should stick with the M403e or R700 as they are best to improve skill.

Shotguns are basically not the best option to train with, generally because the maps tend to not favor shotguns. (unless you play shipment or in closed corridors)

All in all, practice makes perfect, keep training a weapon and range you favor before venturing into any other skill.

Perks wise, keeping your perks as stopping power + deep impact will generally improve you aim after a while. Obviously, you need to improve for that to happen

for one of my ways to train in cod4 is

Use all the class, Assault, SMG, LMG, Demo, Sniper. Then combine all ur plays in each class u use n use a single kit just like u hav all 5 class.

Omg i cant understand myself. XD