I think the idea is time dilation.

10,000 years outside the warp has past. Bob the Chaos Space Marine, on the other hand, entered the Eye of Terror last month after the Siege of Terra.

well basically yes but i could prefer a minimum of 100 year...Dave entered the Eye of Terror and go to a carnage party with his brothers which lasted for 100 years for him counting the 13 sort trip to the local booze store for some refreshment. in the down time between the beer runs he fought with some horned guy, crab-girls, crazy bird dudes and piles of slimes, slapped some eldar ass, or just float in the warp in delirium because the warp dust he sniffed...

Either way Bob and Dave has such a grate time they decided go on an invite trip in the friendly neighborhood that is the Imperium...(end they story begins...)

I think the idea is time dilation.

10,000 years outside the warp has past. Bob the Chaos Space Marine, on the other hand, entered the Eye of Terror last month after the Siege of Terra.

Well it may not be that dramatic. Maybe 6000 years have passed for him. Remember, longevity is a common gift for chaos champions.

6000 years is about the time since the erection of Stonehenge.

I find the idea of somebody who is in constant warfare living 6000 years to be absurd beyond absurd, but YMMV.

EDIT: and they sure wouldn't start with 500 XP.

A lot of things in 40K don't really make sense if you think about it too hard. It was after all designed off a fantasy game by a bunch of nerds to be as "cool" as possible with no sense of scale, realism or sustainability.....

Sometimes you've just got to suspend your disbelief and/or re-write the fluff to make it more to how you think it makes sense.

Talos and company in the Night Lords trilogy had been active for some 200-300 years after the Heresy, sure real passage of time was ten thousand years but for them a mere two-hundret something had passed.

And when talking to the Flayer Talos mentions it was some 20 years ago they fought side by side, whereas for the Flayer it had been closer to 80 - or some such... Actual numbers are very fuzzy atm, so just pulling something out of the air that somewhat matches my memory.

Other than that, why not use the rules in Rites of Battle for more specific PAs? If your GM wants to that is... Rules in RPGs are mostly guidelines anyway to keep a game running smoothly, dont like the rule - change it. RPGs are rarely a competitive game like for instance tabletop 40k.

And besides, for a CSM there is nothing sentimental about old stuff unlike their loyalist counterparts. Newer is more often better for CSMs.

And since this is a RPG there is nothing that stops you from getting Storm Bolters, Assault Cannons and other stuff not officially available on a large scale to the Legions at the time of the Heresy, unlike in tabletop where even Renegade Chapters use pre-Heresy gear because of course they dont keep their Storm Bolters and whatnot when going Traitor and the Legions never loot **** of their enemies.

On a sidenote... Most Legions used Mark IV at the onset of the Heresy, Iron Warriors preferring Mark III, then upgraded to the mish-mash armor of Mark V. Mark VI was released during the Heresy and Mark VII right after... If memory serves.