Sounds to me like that GS was spreading it a little thick but to find out what your gun and ammo will do:
1. Test the gun and ammo from sandbag rest to minimize your effect on the results and see what the gun/ammo is capable of.
2. Try different ammo from same sandbag rest to see if that changes the results. Your gun might like one handload, bullet, ammo, etc. better than another.
Good shooting and be safe.