I'm of the opinion that deer are thin-skinned and really don't require coc broadheads for complete pass-thrus especially at average bow ranges under 30 yards, Muzzys,Thunderheads, and Wasps have killed a lot of deer. I've shot a wide variety of expandables and fixed blade heads and am currently shooting a 100 grn low profile head with a sharpened chisel point, the blades are solid and .030 so no give on contact with ribs,etc. With 9 grn/inch carbons they really provide great pass throughs. I usually only draw between 57-63#, so not a lot of poundage is required. At 10 yards shooting the set-up you describe, you should have had trouble finding the arrow on the ground on the other side or it will be buried a few inches in the ground.
I would say the 2nd shot hit bone. I usually watch FOC balance on my arrows and paper tune the whole set-up. Erratic arrow flight such as a strong right or left kick will really rob your arrows of energy and you can imagine what happens if they enter the deer slightly sideways.
Another great link for info on your arrows,
http://home.att.net/~sajackson/archery.htmlAbsolutely what I was talking about. Now I will not knock the Thunderheads as a whole as I know several folks who shoot them religiously, but that was the head my friend was shooting when he lost his buck. Yes in retrospect, we have figured that he hit the beef bone of the shoulder, in the area it tee's out. We did find the deer later in the season, only he was chasing does. I caught him out in the open once last year and so far once again this year. He is an awesome buck and if we meet again, I believe my friend will get his broad head back.
As for the arrow flying true, your spot on. I was haveing that issue with my previous set up. I ended up changing to a shorter head which helped out dramatically. I really don't like to get into brands as they are like underwear, everybody uses what fits them the best. My choices over the last two years have been varied, I have most of the Magnus tips, along with several other brands. I have for the most part hung with the COC concept with a lean towards the thicker blades. Granted, a whitetail shot through the ribs really doesn't pose much in resistance, but a hit to the shoulder blade, or have a big river bottom hog out in front and you want all you can get hitting them. If you are a little off, and do hit the shoulder blade the heavier blades will help hold things straight and hopefully break the shoulder and go on through. For the most part the heads I hit the woods with the most are 100gr Razor Tricks, Hellrazors, and 125gr Slick Trick Magnums which bumps me up a tad more but due to it being a slightly more angular blade and not a true COC head I still come out about even.
Bringing up the heavier arrows again. IF your shooting an already quiet bow, adding some weight to the arrows will quieten it down even more. There is a balance you will have to reach for yourself with regard to speed-vs-weight. When I first got the FMJ's, I had the cut to 29" even. More so I could easily remember and make sure they were always cut the same. My Tomkat was only pushing them out at around 240fps, which still was more than adequate for deer and medium hogs, as long as they were hit in the ribs. With the new Admiral, and getting 270fps, that really jumps the penetration ability of the arrows to around 76#. (I posted the wrong data above.) Coupled smaller diameter shafts, and with the COC heads, for the most part I am wondering how far they will be stuck in the dirt the other side of the animal. However, should I screw the pooch so to speak on my part of the shot, I have a little added insurance that they will punch on through leaving two open holes rather than one plugged with part of the arrow.
As was mentioned the flight of the arrow will rob you of energy even if your pushing 400fps, if your arrow isn't flying true, you will still not have the energy of a slower lighter arrow that is flying straight.
I hope the info helps you out. The bottom line is use what you have confidence in, and shoot it well.
Good luck.