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 (req.indexes.empty()) {
                    
                    BlockTransactions txn;
                    txn.blockhash = cmpctblock.header.GetHash();
                    CDataStream blockTxnMsg(SER_NETWORK, PROTOCOL_VERSION);
                    blockTxnMsg << txn;
                    return ProcessMessage(pfrom, NetMsgType::BLOCKTXN, blockTxnMsg, nTimeReceived, chainparams);
                } else {
                    req.blockhash = pindex->GetBlockHash();
                    pfrom->PushMessage(NetMsgType::GETBLOCKTXN, req);
                }
            }
        } else {
            if (fAlreadyInFlight) {
                
                
                std::vector<CInv> vInv(1);
                vInv[0] = CInv(MSG_BLOCK, cmpctblock.header.GetHash());
                pfrom->PushMessage(NetMsgType::GETDATA, vInv);
                return true;
            } else {
                
                
                std::vector<CBlock> headers;
                headers.push_back(cmpctblock.header);
                CDataStream vHeadersMsg(SER_NETWORK, PROTOCOL_VERSION);
                vHeadersMsg << headers;
                return ProcessMessage(pfrom, NetMsgType::HEADERS, vHeadersMsg, nTimeReceived, chainparams);
            }
        }