Very close, but not entirely correct. The difference is a bolt is intended to be inserted through a hole(s) and tightened or released by torquing a nut, a screw is intended to either be inserted into a preformed thread or by creating it's own thread and then be tighened or released by torquing it's head. There are fasteners that only qualify as bolts (round heads, plow bolts, track bolts) and ones that only qualify as screws (wood screws, self tapping screws). There are also some standards which fasters must meet to qualify for certain designations, ie there are actual diameter differences between ASME bolts and ASME hex cap screws. The length of the shoulder is actually a function of the specific size/length combination and the type of fastener. All that being said, as far as the hardware store is concerned, as long as you have the right grade and size they are basically interchangable. The Gr8 are almost for sure technically going to be cap screws.
As far as Gr8 shearing before Gr5, that's an interesting theory. The shear strength for a material is generally accepted as 1/2 tensile strength. That would make Gr8 about 25% stronger in shear than Gr5. (roughly 75ksi vs 60ksi) The only possible explanation at that point would be based on Gr8 being brittle ve Gr5 being ductile and in an high load instantaneous impact the Gr8 fracturing while the Gr5 yielded. At a quick glance, the problem I see with this theory is that the ultimate tensile strength for a Gr5 is less than the yield strength of a Gr8. ie the Gr5 fails at before a Gr8 would start to permanently deform. Ok, enough engineerspeak: In practice, I suppose that if just the right joint configuration existed, just the right load were applied, and you had a Gr5 on the high end of it's strength and a Gr8 at the low end of it's strength that it might be possible to achieve a condition where the Gr8 sheared and the Gr5 permanently deformed INTO the joint (ie jammed between the two halves thus locking itself in). Personally, if it were even possible, I'll take the failed Gr8 'cause that Gr5 is going to take a lot of blood, sweat, and tears to get out.
