Guns don't make the man, the man makes the gun.
You can buy the big weapons all you want, it doesn't mean anything if you don't have the skill to use it and it's not like you can't kill someone with your weak gun just because that person has the biggest gun, I've sniper headshotted loads of people at the maximum range of the SV98 when they were using the top sniper, no problem.
Let them waste their money if they want, it doesn't make them any better at the game than you just because they have the biggest gun