first off congrats on the buck.......Nutin' better than a warm barrel and fresh backstraps. I'm also a firm believer that a hunter should use the type of ammo and equipment he is most proficient with and has the most confidence in.
That said, I don't buy into the hype of "premium" type bullets for deer sized game at reasonable yardage.....and I don't believe your use of them contributed to your success. IMHO, even on a small, young 6pt. buck, no bullet(unless from a howitzer) from a is gonna kill 'em dead in 10 seconds when shot only in the guts. Altho it wasn't me that shot them, over the years, I've trailed gut shot deer for hundreds and thousands of yards, some that had dropped most of their guts outta the huge holes in their sides...some hit more than once, most were hit with guns bigger than a .270('06, 12 ga, .308). Since the diaphragm is the muscle used to pump the lungs, and since the lungs rest directly on it, by your own description of "i hit is square in the diaphram" tells me, you more'n likely clipped the lungs and/or the liver(which also sits next to the diaphragm) either directly or caused trauma to them from the shock.
it STILL tipped over dead 40 yards, 10 seconds later.
...... a reaction typical to a lung hit deer not hit in the shoulders also.
Don't get me wrong myronman3, I ain't trashin' your ammo, just givin' you credit for a shot better than just "in the guts".