It is impossible to fit more than 6 gauss rifles (even clan) on a battlemech in CBT rules, which MW3 uses for mech design.

Each Gauss Rifle takes up 7 critical slots (6 if clan), not counting ammunition. They also weigh 15 tons (12 if clan).

Each arm on a battlemech has 8 free critical slots, with an additional 3 that can be removed (hand, lower arm, and upper arm actuators), this means you have 11 critical slots available on each arm, hence, only a single gauss rifle can be installed there.

Each side torso has 12 available critical slots, so the max you can fit in there is 2 clan gauss rifles.

The center torso only has 2 available critical locations, and I don't believe compact engines and gyros can free up another 4 slots in the CT to fit a rifle there.

The legs only have 2 slots open, so you can't fit a gauss in there, nor can you fit one in the head with its 1 slot (2 if you use compact cockpit).

So, you can fit 6 clan gauss rifles into a mech, what about the tonnage though. Right off, without ammo, you are looking at 72 tons of weapons. You have a maximum of 17 critical slots available to store ammunition and endo-steel in order to free up weight. XL engine is out of the question as the side torsos are filled with gauss rifles.

Let's assume you pick up clan endo-steel for 7 critical slots for your 100 ton Atlas, costing you 5 tons, you are now at 77 tons. You tank the engine down to a 100 series engine, which weighs all of 3.5 tons. Also, the 10 free heat sinks can't all fit in there, so 6 of them need to be placed inside the mech, eating up 6 more critical locations, leaving you with a total of 4 critical slots left.

1 ton for the gyroscope. You are now at 81.5 tons. The cockpit weighs in at another 3 tons, setting you to 84.5 tons.

Now you need to buy ammunition, but you only have enough room for 4 bins of ammunition weighing in at 4 tons total. This gives you a total of 32 shots for 6 rifles, or ~5 shots per gun, and putting your mech up to 88.5 tons of weight. That leaves 11.5 tons for armor, not bad for a mech with 6 gauss rifles.

So, in conclusion, you can get 6 gauss rifles onto a mech, but that mech is a total piece of shiet. You'd be far better off ripping out 2 of the rifles in the side torso (+24 tons, +12 crits), replacing the engine with a 200 series and the heat sinks with double heat sinks (-5 tons, -1 ton for heavier gyroscope, +2 crits total) so the thing can actually move, and then slotting in extra ammunition (-3 tons, -3 crits), improving the armor with ferro-fibrous armor (-3 tons, -7 crits) and a pair of Clan Large Pulse Lasers (-12 tons, -4 crits).

The final mech has 2 Clan Large Pulse Lasers, 4 Clan Gauss Rifles with 56 shots total (~14 shots each), 14.5 tons of ferro-fibrous armor (~90% maximum armor), and a top speed of ~32 kph. The mech runs a little hot with only 10 double heat sinks, dissipating 20 heat per turn, but only builds up a maximum of 26 heat per turn if it is at maximum stride firing everything.

A popular config might strip out the Clan Large Pulse Lasers in favor of two extra heat sinks and 2 Clan ER Large Lasers to ensure it has the range advantage. The extra heat sinks would necessitate the replacement of the Ferro-Fibrous armor with standard slab, but an additional two tons of the armor keeps it at ~86% maximum armor.