Is it against the rules to use your judgement as to whether to open two or three with a seven card suit? Instance was third seat opening with a seven card suit, two of the top five, and 8 points with distribution - not vulnerable. Playing with players that are very conservative in their bidding so opponents could easily had each held twelve points giving my partner a possible ten points if forth seat didn’t bid after my pre-empt. Help - Opponants said I was going against the rules and had to open three with a seven card suit. I can’t find any answer for this situation - unless it is obvious and you can never open two with a seven card suit.
You are, within certain constraints designed to prevent wholly destructive bidding systems, allowed to make any call that is:
any of a sufficient bid; a valid Double or Redouble; or Pass; and
not otherwise constrained by penalty under the Laws of Rubber Bridge consequent to a prior irregularity.
However your opponents are entitled to a full and complete description of the agreements and understandings between you and partner.
In particular, you are most definitely entitled to make a call based on your understanding of the opponents methods, in the context of the current auction. But if you make a habit of certain actions in common circumstances, such as a third hand opening following two passes as here, this must be added to the partnership agreements made known to the opponents.
Nothing you have stated in your post suggests to me that your opening bid is inappropriate. While not a call I would likely make myself, I see merit in your reasoning when playing against timid opponents.
It would seem that your opponents on this hand have conflated the rules of the announced bidding system (really just guidelines for improved play and partnership harmony) with the Laws of Rubber Bridge.
What happened is that your opponents were citing the "rules" of bidding, as opposed to the laws of bridge.
The latter (laws of bridge) are inviolable. But you are basically allowed to make your own "rules of bidding." Unless you explicitly stated "two means six, three means seven," the "default condition is that "six can mean either two or three, and seven can mean either two or three.
Providing that they are disclosed, the ACBL allows wide latitude for conventions used by players. In your case, the difference between a two or three level opening with seven cards falls within that wide latitude. Also, it should be noted that partner may be as likely to have trouble telling the difference if you only bid two. Put another way, your opponents are not entitled to "full" information, only to "equal opportunity," vis-a-vis your partner.
Here are some examples as to how wide that latitude can be. Technically, a "strong" 2 club bid is supposed to represent s very strong hand. But the ACBL will allow a hand as weak as (s) AKQxxxxx (h) JT9x (d) x (c) ---. The theory is that this hand is good for about 9 tricks on its own, and can make game with "minimal" help from partner, (the Queen of hearts or a side ace). That's true even though the hypothetical hand is decidedly weaker, at least in high card points, than usual.
Also, the ACBL will allow "denial" bids of 1 diamond, following an opening one club, or 2 diamonds over 2 clubs, because the denial bid is only one level higher than the original bid. It would frown on denial bids of 1NT opposite one club or 2NT over 2 clubs, because they take away a lot of bidding space just to say, "I have nothing."