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.

return state.Invalid(false, REJECT_DUPLICATE, "bad-txns-inputs-spent");
        
        view.GetBestBlock();
        nValueIn = view.GetValueIn(tx);
        
        view.SetBackend(dummy);
        
        
        
        
        
        if (!CheckSequenceLocks(tx, STANDARD_LOCKTIME_VERIFY_FLAGS, &lp))
            return state.DoS(0, false, REJECT_NONSTANDARD, "non-BIP68-final");
        }
        
        if (fRequireStandard && !AreInputsStandard(tx, view))
            return state.Invalid(false, REJECT_NONSTANDARD, "bad-txns-nonstandard-inputs");
        int64_t nSigOpsCost = GetTransactionSigOpCost(tx, view, STANDARD_SCRIPT_VERIFY_FLAGS);
        CAmount nValueOut = tx.GetValueOut();
        CAmount nFees = nValueIn-nValueOut;
        
        CAmount nModifiedFees = nFees;
        double nPriorityDummy = 0;
        pool.ApplyDeltas(hash, nPriorityDummy, nModifiedFees);
        CAmount inChainInputValue;
        double dPriority = view.GetPriority(tx, chainActive.Height(), inChainInputValue);
        
        
        bool fSpendsCoinbase = false;
        BOOST_FOREACH(const CTxIn &txin, tx.vin) {