What are the differences between the chest fit and the rig fit option?

I have both the Rig fit in white and the Vest fit in black, but I have modified both my bags a bit. These are the biggest differences between the stock versions: Rig Fit: - All four straps have Side Release Buckles, are 1.5" wide, non-elastic, are adjustable with a slider - Back piece is breathable mesh - Main bag is 9.75" wide, 6.25" tall, 1" deep - The front pouch is on the right while wearing the bag, 5.5" wide, 3.75" tall, 1.25" deep and has a 5.5"x2" hook-and-loop patch - There are three, Molle straps on the front left side Vest Fit: - Top two straps have Side Release Buckles, are 1.5" wide, elastic, adjustable with a slider - Side two straps are 2" wide, elastic, adjustable with a slider. These two DO NOT have buckles. - The part that rests on your back is perforated neoprene and has a zippered triangular mesh pouch that is 5" tall, 4.5" wide at the top, and 2.25" wide at the bottom. There is a reflective 2x2" patch at the bottom. - Main bag is 10" wide, 6.25" tall, 1" deep - The larger front pouch is on the left while wearing the bag, 5.25" wide, 3.75" tall, 0.75" deep - My version (the old one) has two Molle straps across the front pouch - It looks like the current version has two 1" hook-and-loop straps in the same location - The smaller front pouch is on the right while wearing the bag, 3.5" wide, 3.5" tall, 1" deep, has a 1"x1.5" hook-and-loop patch and a side-release-buckle to keep it closed

What is the Difference between rig and vest fit

The rig bag has only one rectangular pouch in the front and has a point point belt system. The vest comes with a rectangular pouch and small buckle pouch in the front and the axillary straps are spandex with the shoulder straps being a buckle system.

What are the internal dimensions of the small pouch on the front of the bag?

Roughly 5” x 4” and expands almost to a inch when trying to pack stuff in.

Can a S&W 40 cal fit in it with the holster

Yes I believe it will, I have sig P365 with the 12 round clip and it fits with room to spare. It should fit just fine. I think a full frame pistol would fit in this thing. I wear mine to mountain bike and it's very comfortable. Just adds a little more weight to training days, but I'll take that in exchange for safety in the woods any day! Hope this helps.