The request is "four medium-sized cannons w/7 tons of ammunition, IS only, no iXL, 50-75-ton range, moving 65-70kph"?
Here you go:
BLACK WIDOW
Now, word to the wise: this 'Mech is
****. As other posters have told you several times, the reason you can't get what you want is that four autocannons can deal 40 damage. ~2 seconds later, they can deal 40 damage again. ~2 seconds later, they can deal 40 damage
again. So on and so forth, long past the point where any other 'Mech would be melting itself down into slag. Autocannons weigh as much as they do because they're the best compromise of damage, heat efficiency, and refire in MWO by a country mile. Gauss rifles are technically more heat-efficient sources of damage, but their cycle rate sucks and they're terrible in close. An AC/10, meanwhile, can chunk someone for ten points of damage every two to two and a half seconds until it runs out of ammunition, without ever concerning itself with the heat bar.
Laser vomit and other energy-centric builds have much higher initial spikes but cannot
begin to match the DPS/DPH efficiency of autocannon-centric builds. Energy vomit shoots twice, then disengages to spend thirty seconds cooling off before shooting again.
Missiles splatter damage everywhere, distinctly lacking the focused punch of cannons. IS missiles also aren't much
lighter than IS cannons, and in the numbers needed to threaten enemies they're not that much cooler than energy vomit.
If you want to cruise around and hammer away with AC/10s in the heavyweight class at 70kph? Drop two autocannons, then use the speed and agility you gained by not being a Gojira or a Fatnir to find more favorable engagements. You're going to be distinctly worse at this than Clan 'Mechs simply because the cXL exists and weight is the most critical factor for dakka-centric builds, but you can do it.
This CTF-3D, for example, is two UAC/10s with a huge jam chance reduction quirk and nine tons of ammo, moving at 65kph. Get the jam reduction in the tree, go ham on people. Still not a
good build, but miles better than that Warhammer, and two UAC/10s is more-or-less equivalent to four regular AC/10s untril they jam. Which this particular 'Mech is good at avoiding.
Elsewise? You're asking for ponies and plastic rockets here, man. What you want is simply not the way BattleTech
works, and unlike other "Not The Way BattleTech Works" issues, this is an important one Piranha should not **** up.