Generally speaking, for the new 10-20 Rank player, you will have two machine guns and one shotgun.

• RPK-74
  
• MP-133
  
• HK-243

Of these, the RPK74 is arguably the best all around weapon. Here's why:

• The HK243 has lower penetration and damage than the RPK74. While it's beta-c drum allows it to fire in much longer bursts, in this game, because of how the developers balanced guns, you will consistently die unless you can score one or two headshots in quick succession.
  
• The RPK74 also reloads and moves slightly faster, meaning your chances of being murdered by someone while changing a mag is lower, and you'll get places with this thing faster.
  
• The RPK74 is unlocked earlier and will likely have more attachments available.

So, what about the MP133? Well, first off, if you use it I hate you. But, second off...
Shotguns in this game are extremely strong, as the midrange buckshot headshot returns, but also it can now kill in one shot at close range. Great.

The MP133 is pretty passable all told, but do remember that it has different ammo compared to the machine guns, so your progression may be somewhat diluted if you use them.
The HK243 requires use of AP-OFM to be good. As such, hold off on using it until you have it.
AP-OFM will allow you to have what is essentially a giant assault rifle.
This singlehandedly makes the HK243 go from mediocre to actually pretty good.

The MOR sight has an integrated laser that can be toggled with the laser key. This lets you have the laser very early on and improves your hipfire significantly, removing lots of guesswork.
On the muzzle, the highest recoil reduction is preferable, as is the norm.

The Destroyer has a lot of tools to Not Die.
Let's look at some of these now.

Ravager is trash. I'm serious. It doesn't help you against bullets which are your biggest problem, any more than regular bodyarmour.
Unless the enemy is spamming explosives and incendiary weapons, you can just not bother with Ravager, and autolock Heavy Ceramic instead.
If you want to move somewhat faster and give up what little protection you can get from sniper rifles, then use standard, but I really don't recommend it.

As mentioned before, AP-OFM is your go-to. It boosts your penetration significantly, and will really help you match assault rifle/SMG DPS.
Later down the line with higher caliber higher recoil machine guns, the MG Ammo rounds will become more relevant, as they'll help you stay on target better.

Here's the bit that makes things interesting.
Skills are an important contribution into how long you'll be able to live on the field.
Here's the list:

1. Steady Grip
   At the highest level, this will let you have MG Ammo without the precision malus. It's not great, but it's the best of the early skills. if you want, you can go for Quickness, but...
   
2. Face Guard
   It's the best of them, giving you immunity to smoke coughing, gas, etc, but if you're more worried about explosives then Antifrag can be good.
   
3. Wrangler
   Assuming this one reduces max hipfire bloom, then its the best. Sometimes there really isn't time to aim.
   
4. Medical Kit
   The cooldown is somewhat long, but upgraded this can be a valuable asset in keeping you alive much longer. Combined with the Armour Box, you can survive incredible punishment.
   
5. Ironside
   It's not much, but it's something. Danger Alarm is okay, but you really should be moving out of the way anyways. Besides, this stacks with the armour box armour bonus.
   
6. Fire Stabilisation
   Out of all of them, this is good if you need to walk rounds on target. It also stacks with Wrangler! Physical Training and Stoutness are pretty useless all told.
   
7. Serial Killer
   The other ones just don't help you enough. If you can stack bodies fast, then this is the best of them.
   
8. Healthcare
   Keep you and your friends alive. Simple enough.
   
9. All three are good.
   Use Ironclad a lot? Iron Will. Like to nuke people back to the stone age? EMP. AutoSaiga abuser? Fletchette.

So how do you not constantly die because you can't move fast enough to get places?
Simple. Destroyer makes for an excellent DEFENSIVE class. I'll say that again. DEFENSIVE, class.
[/previewiconThe Destroyer EXCELS at going through five men with ammo to spare in an objective. His strength is being able to shoot and shoot and shoot without stopping. Yes, he can harass enemies at long range, but why bother? He has landmines, he has big gun, he can turn an objective into a deathtrap!
So here's your buy.
Medkit, Landmines, Armor Box, Helmet. Most to least important.
Buy mines as much as possible, and save them if the match is about to end in Target Designation. Deploy the Armour Box on the objective, rebuy all these as necessary.

Secure a chokepoint with a short sightline that is hard to peek with a sniper rifle, and easy to shoot with an MG. This will maximise chances of being able to shoot them dead before they kill you. Solid cover is a MUST. Remember to block off any flank routes with mines, and space out multiple mines to ensure kills.

You can have the Saiga with Ironclad Armour. I'll leave it at that. ;P
Remember to have a medkit with Ironclad, and Armour Box if you can.
All told, this will run you from 375$ to 575$ at minimum. But, it's worth it to keep your armour suit alive.
Otherwise, follow the standard tactics of defense.

And that's about it!