Weird Experience With My .308

Well, yesterday Nov. 13th was our opening day for Rifle Season for Deer. Got out to my stand early & was situated at he edge of my back field. Took my Browning A-Bolt in .308 and factory 150 grain Silvertip (the old style) a friend gave me a couple of years ago. I think he bought the ammo back in the late 1990's. Anyway, a nice Buck shows up at the wood's edge trailing a Doe, about 150 yards away from me. He stopped allowing me to get off a shot which I had my crosshairs square on his Lung/Heart area & I fired. The Buck went down, but then got up and sneaked quickly off I fired again but missed. Got down and walked over to where I hit him but saw very little blood, just a few drops, so I started looking for him which I found him in a very tough spot fillled with vines, small pines & Blackberry briers. Got him out, with the help of a neighbor, who was hunting nearby. The Buck traveled about 100 yards into the brush area. When I got him situated to field dress him I noticed the exit bullet hole was only slightly larger than the entrance hole, again in the Lung/heart area. I though this was strange as I've used silvertips in my .270 and the exit hole was much larger than the entrance. Even on a 170 grain .30-30 silvertip bullet. Just wondering if the 150 grain Silvertip bullet Winchester used in this lot of factory ammo was 150 grain bullets intended for the .30-06 or .300 Win Mag. I know for a fact that a .308 will kill a Deer at 150 yards and my Browning A-bolt is very accurate, as the bullet hole was just where I placed it. The 150 grain S.T. seemed to act like a FMJ bullet on that particular Buck. What do you guys think what happened?? This is the longest shot I have taken with a .308 as most of my shots are < 100 yards.