I got a hold on how to get the heartbeat,without hacking.
If you have used the messenger glitch,you know u have tried to glitch the heartbeat.Well I have found out how to get it.
1.Double click on M16A1 AP Mine.(make sure its highlighted)
2.Do the messenger Glitch to try to glitch the heartbeat.As you see it redirects to another explosive.
3.Just keep on doing the messenger glitch on the heartbeat even though it directs you to another explosive,eventually you will be able to buy it.

p.s I did this with a nub character i made.It may just work for nub charcters.
and Yes,it worked for me........ :P