As someone who suffered on a server with 3-5 second pings whenever a 6th person joined (hey, was the first weekend, and I was lucky to get that!) I can recommend two things:
1) grenades work regardless of ping and WILL take down the enemy
2) throwing grenades with a 5-second ping is suicide
3) your teammate will ALWAYS sneak up past you while you're lagging and catch your grenade
the engine lets you walk around freely until the packets go through, then sends the commands in the order you typed them. Leads to some pretty confusing movements, but is fairly easy to compensate for once you get used to it (eg, left two clicks, hold down forward for two seconds, kneel, wait for it to go through... then voila, you're out of the room and hunkered behind the wall!) If you've just got a 200-300ms ping, well, that's actually about par for the course due to the completele lack of east coast servers (their "east coast" are actually midwest)- I was lucky to get below 300ms on my cable modem. Spray and pray at close range, and get the M240 if you can- You can empty clips from the M16 and hit nothing but air, but the M240 will take down an enemy instantly at point-blank range Use frag grenades copriously as they're insanely powerful- flashbangs sure are fun too! People tend to kneel and lie prone a lot- ping doesn't matter one lick when you and your target are standing still. If you can't storm the fortress, just hunker down and snipe slow moving targets.
stat net at the console will show your network info in detail.
But what it boils down to is play smarter and don't rely on perfect aim to take you through. This game is heavy on tactics and camping is a perfectly valid tactic- learn the map and wait for a good opportunity. Position yourself so that the enemy can't evade your fire laterally where ping hurts you. And remember- everyone else is lagging, too, and it's hard to hit a moving target at close range! At long range go prone behind cover and you're almost impossible to hit. But my best advice is: get broadband!! lol