There is a difference between asking the GM to create a house rule for you and something being ambiguous by the rules. This case is not ambiguous. A reading of the rules shows no ambiguity. If something says that it can use Flechette ammunition only, and there is a type of ammunition called Flechette, that is the ammunition it can use.
Knowing a slight bit about guns, this isn't exactly a out of line either. Trying firing a high power round out of a gun not rated for it and your gun goes kaploowie.
As far as most restrictive, no, I take what the rules
clearly state and propose that. If I tell someone about a house rules, I tell them it's a house rule or common house rule. I don't tell them to go and tell their GM, "Well this is ambiguous so can I do it," when the rules are quite clear. That's just effectively trying to sleaze something by the GM without being wholly honest.
However, a more moderate interpretation
This is the stem of the problem you seem to have with me. I don't give people advice based on interpreting the rules to a point where I think they were intended. I tell them based on what the rules
actually say.