Abrahms would be about AV 13-14, and ignoring missles seems possible because of the explosives on the outside. Also possibly ignore any other kind of solid-shot projectile? (i.e. Railgun, Autocannon, etc.)
Again, this supposes that there have been no advances in metals/armor in 38,000 years. That's unlikely, given that in just five centuries we overcame the problem of firearms rendering body armor obsolete. As such, I seriously doubt Abrams armor comes close to approaching that of the tanks of the far future.
The one thing that we have an advantage over the tanks of the future is speed. Imperial Armour has most tanks going 40km/h or slower, which is pathetic compared to today's tanks. I shake my head in consternation when I read such stats. I guess it comes down to the fact that, wargamers or not, civilians sometimes miss some things that strike people in the military as obvious.
Then we have our Air Force. Yes Space marines are good but put them up to a Raptor or Eagle with Tiger missles and womp.
Again, good luck having an Air Force at all, when your opponent is engaging you from low orbit.
Navy would play a big part, too. Even in WW2 we had 15 in cannons on some battleships, and now I think we go up to like 12 on Destroyers.
The Navy would play the smallest part of all. Moving at snail-like speed compared to orbital ships, the average surface warship would be merely a target.
But even if you don't take this into consideration, the fact of the matter is that modern technology--in terms of tanks, artillery, etc.--just wouldn't compete with what the Astartes would bring. It would be a shoddy far future, indeed, if we were basically using the same-old, same-old as we do today.