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.

{
    
    if (tx.vin.empty())
        return state.DoS(10, false, REJECT_INVALID, "bad-txns-vin-empty");
    if (tx.vout.empty())
        return state.DoS(10, false, REJECT_INVALID, "bad-txns-vout-empty");
    
    if (::GetSerializeSize(tx, SER_NETWORK, PROTOCOL_VERSION | SERIALIZE_TRANSACTION_NO_WITNESS) > MAX_BLOCK_BASE_SIZE)
        return state.DoS(100, false, REJECT_INVALID, "bad-txns-oversize");
    
    CAmount nValueOut = 0;
    BOOST_FOREACH(const CTxOut& txout, tx.vout)
    {
        if (txout.nValue < 0)
            return state.DoS(100, false, REJECT_INVALID, "bad-txns-vout-negative");
        if (txout.nValue > MAX_MONEY)
            return state.DoS(100, false, REJECT_INVALID, "bad-txns-vout-toolarge");
        nValueOut += txout.nValue;
        if (!MoneyRange(nValueOut))
            return state.DoS(100, false, REJECT_INVALID, "bad-txns-txouttotal-toolarge");
    }
    
    set<COutPoint> vInOutPoints;
    BOOST_FOREACH(const CTxIn& txin, tx.vin)