The payoff for a natural (a two-card 21) on single-deck games was reduced to 6-5, from the traditional 3-2, 10 or so years ago and has become the rule at most tables ever since, especially single-deck games with low minimums on the Las Vegas Strip.
All things being equal, it’s much better to play at a blackjack table that pays 3-2 than 6-5, same as it’s better to play a 9/6 Jacks or Better video poker machine over an 8/5 JoB or a single-zero roulette wheel over a double-zero.
To get technical, the short-pay on blackjacks amounts to a -1.4% penalty on what’s already a lousy game (since the rules at modern single-deck games are usually: dealer hits soft 17, no double after splits, and sometimes double on 10 only), putting the game in the neighborhood of -1.8% off the top.
But what most players don’t know is that it’s actually worse than that. The casino pays every $5 of a bet at 6-5, but then any portion of the bet remaining that’s less than a $5 increment gets paid at even money. Hence, an $7 bet that gets a natural is paid only $8 (instead of the $8.40 proper payoff), which works out to 5.67-5 blackjack!
It’s true that many players resent the 6-5 payout, but many others simply don’t understand the difference. Indeed, Bally’s — one of the first casinos to deal it — advertised on its marquee that the game was being dealt “by popular demand.”
As always, there can be exceptions, but the rule of thumb is to avoid 6-5 blackjack in favor 
of a game dealing any number of decks, even 6 or 8, that pays naturals at 3-2.
The question arises: At a 6-5 blackjack game, will casinos allow you to double down on a natural? In other words, if you’re dealt and ace and a ten, can you count it as 11, double down, and hope for another ten, so you’re paid 2-1 rather than 6-5
The answer is yes. It’s unorthodox to say the least and it’ll raise eyebrows in the pit, but most casinos allow it.
That said, while deck-composition situations could conceivably make doubling proper, basic strategy dictates that you take the guaranteed win for the natural, whether it’s the 3-2 or 6-5 payoff.
The only time this might not apply is in a tournament, in a spot where you need to win twice your wager to advance. For example, say it’s the last hand of a match, you have $1,200, and you bet the $500 max. The leader has $2,000 and bets the $10 minimum. You get a natural. Now, even at a full 3-2 payoff, your final total will be $1,950 — not enough to catch the leader. Here you should double your blackjack, hoping to win the double and go to $2,200 and advance (not doubling would mean certain defeat).
One more interesting point about the 6-5 game. You probably won’t be allowed, but if a dealer lets you take “even money” when you have a natural and he has an ace up, you should. Unlike the 3-2 game, where taking even money (which is the same as insuring for the full amount) is sub-optimal, in the 6-5 game, being paid even money is preferable. Note that this is not the same as taking full insurance in a 6-5 game, which is something you shouldn’t do. So if you ask for even money in a 6-5 game and the dealer says you have to insure, decline insurance altogether.
Of course, this discussion is provided primarily for its theoretical value, as you should normally not be playing in an even-money or 6-5 game in the first place.
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