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.

extern CTxMemPool mempool;
typedef boost::unordered_map<uint256, CBlockIndex*, BlockHasher> BlockMap;
extern BlockMap mapBlockIndex;
extern uint64_t nLastBlockTx;
extern uint64_t nLastBlockSize;
extern uint64_t nLastBlockWeight;
extern const std::string strMessageMagic;
extern CWaitableCriticalSection csBestBlock;
extern CConditionVariable cvBlockChange;
extern bool fImporting;
extern bool fReindex;
extern int nScriptCheckThreads;
extern bool fTxIndex;
extern bool fIsBareMultisigStd;
extern bool fRequireStandard;
extern unsigned int nBytesPerSigOp;
extern bool fCheckBlockIndex;
extern bool fCheckpointsEnabled;
extern size_t nCoinCacheUsage;
extern CFeeRate minRelayTxFee;
extern CAmount maxTxFee;
extern int64_t nMaxTipAge;
extern bool fEnableReplacement;
extern CBlockIndex *pindexBestHeader;
static const uint64_t nMinDiskSpace = 52428800;
extern bool fHavePruned;
extern bool fPruneMode;
extern uint64_t nPruneTarget;
static const unsigned int MIN_BLOCKS_TO_KEEP = 288;
static const signed int DEFAULT_CHECKBLOCKS = MIN_BLOCKS_TO_KEEP;