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.IsCoinBase())
        return true; 
    for (unsigned int i = 0; i < tx.vin.size(); i++)
    {
        const CTxOut& prev = mapInputs.GetOutputFor(tx.vin[i]);
        std::vector<std::vector<unsigned char> > vSolutions;
        txnouttype whichType;
        
        const CScript& prevScript = prev.scriptPubKey;
        if (!Solver(prevScript, whichType, vSolutions))
            return false;
        if (whichType == TX_SCRIPTHASH)
        {
            std::vector<std::vector<unsigned char> > stack;
            
            if (!EvalScript(stack, tx.vin[i].scriptSig, SCRIPT_VERIFY_NONE, BaseSignatureChecker(), SIGVERSION_BASE))
                return false;
            if (stack.empty())
                return false;
            CScript subscript(stack.back().begin(), stack.back().end());
            if (subscript.GetSigOpCount(true) > MAX_P2SH_SIGOPS) {
                return false;
            }
        }
    }
    return true;
}
int64_t GetVirtualTransactionSize(int64_t nWeight)