Welcome to the archery rabbit hole. Do all the learning you can.
Learn about proper spine (it’s not called grains like you said) each company has different numbers for models of arrows they make. With Carbon it is common that the spine which is actually a standard deflection measurement (bend) at a certain weight and length of shaft.
The stiffer the spine the less the bend. So a smaller Spine number is a stiffer arrow. The heavier the point (either field point or broadhead) which IS measured in grains, the stiffer the shaft has to be to maintain arrow tune. - Tune is very important to accuracy and penetration. Tune is simply having the arrow fly dead straight with no wobbling.
FOC (front of center) is also a point of much debate. It is basically weight forward. Like a dart- heavy infront so it flies better. Try to throw a dart backwards and see what happens
Mechanical broadhead tend to fly straighter than larger fixed thus can be more accurate especially if the arrow is not perfectly tuned to the bow and archer. Your Form - or how you shoot the bow- is also very important in accuracy and getting straight arrow flight.
Large fixed broadhead can act like a plane/ wing on the front of your arrow. Imagine a dart with the fins in front, won’t fly as well. So the bigger the blades up front, generally the bigger the vanes (fletching) in the rear have to be to control the arrow.
This mechanical broadheads the blades are tucked in more streamlined and interfere less with arrow flight , like a field point. Not all mechanical broadheads are created equal, nor are the designs. The draw back generally is that it takes more energy to open a mechanical broadhead. If penetration is an issue - like big animals such as buffalo you want all the efficient energy to go into penetration.
Whatever broadheads you use You must test them and see how they actually fly in your set up out to the distance you plan to hunt. You will be surprised than a number of fixed heads (you have to spin test every broadhead on the arrow you plan to shoot) will not shoot to the same point of impact as a field point and groups may be bigger.
Don’t hunt with that same head unless you change the blades or resharpen. Sharpness makes a difference in bleeding and penetrating
A perfectly tuned bow/arrow combo and perfect shooting form archer can shoot most any broadhead effectively
Another consideration is cutting diameter and total cutting surface. Both once again important to penetration. The bigger and more blades the less penetration.
The flip side it increased cutting diameter and surface also tends to cause more trauma and more blood and better blood trails. Drawback is bigger and more blades if they hit bone will stop dead and not penetrate well.
So there is a balance based on the animal you are shooting. A small animal would need less energy to pass through completely so a Larger cutting diameter and more blades would be warranted to kill faster and leave more blood.
An extremely large and tough skinned animals needs a smaller diameter and less blades to penetrate deeper. These tend to be the fixed 2 blade designs. Single bevel believers will trade a bit of sharpness for the argument they can split bone better.
So do your research, lots of opinions and marketing spin out there. But the better you understand the principles you can choose more wisely.
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