No, there've not been any huge changes recently that I recall -
Check Smurfy's for the latest data. Most likely it's just people are using different tactics and builds. PSR may have you fighting against a different set of opponents than you did six months ago as well.
One thing you should be aware of is that if you are using those machineguns on a location that doesn't have anything in it, you're not getting the extra damage - ditto if all the internals have been destroyed. This is because the machine gun's structure damage is based on damage done to "internals" - the actual equipment inside the "component," e.g. right arm, left torso, etc - via critical hits. It does
not do extra damage simply by attacking exposed structure.
I shall explain: When you deal damage with any weapon to structure, the game first rolls dice to see if a "critical hit" is generated - possible results are none, one, two, or three. Next, the game randomly assigns the critical hit to one of the eligible pieces of equipment, and deals that weapon's damage (multiplied by 1,2, or 3) to the "critical hit points" of that item - this is
not structure/armor damage; it applies only to the "crit hp" of the item. If the item's crit hp is depleted, the item is destroyed; if not, it remains fully functional. Finally, if any critical hits are successfully applied to a piece of equipment, the game deals 15% of that critical damage to the structure of the 'mech in that location. And MGs get
huge bonuses to crit damage. This is where all that extra structure damage you're used to is coming from.
That's all really complexicated, so let me give you some examples. I'm blundering along, staring downrange, when your MG Nova plunks down behind me and opens up my rear armor with a laser blast. For simplicity, we shall pretend that I am a potato, and do not notice that this has occurred, so you proceed to light me up with machine guns to explain the concept of situational awareness in a language I am sure to understand. Now, for
each tic of MG damage (or laser damage, whatever,) that is dealt to my structure, you're rolling to generate crits. So say you roll a triple crit with a tic of MG damage (that's 0.08;) for normal weapons, this would deal 0.24 damage to a random item inside that component - but you're a machine gun, so that crit will actually do a hugely magnified to
2.16 damage, yielding a total of 0.404 damage dealt - and that's just one crit roll from one MG from one tenth of a second firing. So if you're blasting away at a torso stuffed full of heat sinks or ammo, those MGs are going to devastate your opponent.
However, consider if that torso is empty - say it's only got Endo-steel taking up those slots. Well, Endo-steel can't be critted, so you're out of luck - you're only doing 0.8 dps for each MG. Similarly, if you're using a big gun (like a Gauss Rifle) and score crit on a heat sink (which has 10 crit hp, like most things,) you're only going to get 1.5 extra structure damage, because the bonus damage is calculated on the crit hp damage done, and does not take the weapon's base damage into account. A Gauss Rifle, with only 3 crit hp, will yield a depressingly small amount of bonus damage - but don't feel too bad; there's a 90% chance a destroyed Gauss will explode for 10 points of internal damage. =)
PS: Smurfy's leaves me with some confusion about how exactly crits are rolled: I had believed that a "triple crit" rolled three separate crits against randomly assigned equipment items, but the verbiage from the Machine Gun popup in Smurfy's site (mouse over the target icon) lists one bloc of damage for each crit tier. Smurfy may be aggregating the total damage for clarity, or I may have made a faulty assumption based on the old tabletop crit rolling system. I'll ask him, but it's really unimportant for your question. I wrote the above explanations on the assumption that a single hit can only result in a single critical roll - simply be aware that this assumption may be wrong.