in v0.12 https://github.com/bitcoin/bitcoin/pull/7081 (by @luke-jr) was merged adding `-bytespersigop` (with default of 20).

this is preventing simple bare multisig transactions from being accepted into mempool when the size is < 400 bytes.  
eg; https://gist.github.com/rubensayshi/ac4f617207f7a50559e85d61c05800be

from 1-of-7 and onwards they are accepted because the ratio of bytes:sigops is high enough.  
also having 2 or more inputs will work because it again tips the ratio of bytes:sigops over.

the reason mainly being that `GetLegacySigOpCount` calls `GetSigOpCount` with `fAccurate=false`.

It looks like changing `GetLegacySigOpCount` to use `fAccurate=true` would cause side effects in the few other places (`ConnectBlock`, `CheckBlock`, `CreateNewBlock`) and simply adding `fAccurate` as arg to `GetLegacySigOpCount` would affect the value stored in `CTxMemPoolEntry.sigOpCount` which in turn affects a bunch of other things.

CalculateDescendants(removeit, stage);
    }
    RemoveStaged(stage, false);
    return stage.size();
}
bool CTxMemPool::addUnchecked(const uint256&hash, const CTxMemPoolEntry &entry, bool fCurrentEstimate)
{
    LOCK(cs);
    setEntries setAncestors;
    uint64_t nNoLimit = std::numeric_limits<uint64_t>::max();
    std::string dummy;
    CalculateMemPoolAncestors(entry, setAncestors, nNoLimit, nNoLimit, nNoLimit, nNoLimit, dummy);
    return addUnchecked(hash, entry, setAncestors, fCurrentEstimate);
}
void CTxMemPool::UpdateChild(txiter entry, txiter child, bool add)
{
    setEntries s;
    if (add && mapLinks[entry].children.insert(child).second) {
        cachedInnerUsage += memusage::IncrementalDynamicUsage(s);
    } else if (!add && mapLinks[entry].children.erase(child)) {
        cachedInnerUsage -= memusage::IncrementalDynamicUsage(s);
    }
}
void CTxMemPool::UpdateParent(txiter entry, txiter parent, bool add)