Well, this isn't quite accurate. It all depends on where the initial contact with the catcher is -- if the initial contact is in front of the plate, it doesn't matter if R3 slides through.

On the other hand, under OBR . . .
 

PONY pony game. PONY is a youth league that uses OBR with few additions, not including a FPSR or other sliding contact provisions. Its pony division is 13-14yo's.

R1, one out. Batter grounds to F6, who fires to F4 crossing second for the force. As F4 pivots behind second to throw to first, R1 slides feet-first hard into him. The slide took R1 within reach of the base, but his legs were well past the base. His right leg in the air but not above the fielder's knee, and his left leg got some air as the slide got into F4's legs. F4 began to fall forward towards first as he began his throw. R1 then jerks his legs sideways toward third, taking F4's legs with him and causing F4 to fall sideways to his left. F4 interrupts his throw to break his fall. R1 ends up with second base in his right armpit and F4 at his feet.

Under Fed, BR is out two or three different ways. But what result under OBR? I was okay until the runner slashed the fielder's legs out from under him, then called interference, BR out. OC thought it was just robust baseball.

Good question. One option is malicious contact, since you have a player basically kicking and tripping another player. I'd have to see the play to know whether we'd have an EJ here.

Another option is to do what you did: rule interference. I'd lean this way as well: you had not one act by the runner, but two. First, he slid within a reach of the base and contacted the fielder (legal). Second, he intentionally kicked the fielder's legs out (interference). I think that's how I'd explain it to the coach.

The general problem here is that without the FPSR, you have to determine what counts as interference on a slide play. In pro ball, this runner would probably not be called out for interference, but he'd get drilled the next time he came up to bat. So the pros have a remedy for this behavior that you would not want to allow on an amateur field.

And that's the sticky spot for interpreting interference on slide plays: the OC wants you to use a pro interp in a context where the remedy is not available to the defense. That's an argument for having a somewhat more restrictive approach to interference on slides.

As much as people bash FED for their approach to safety, it's worth recognizing that some 80% of baseball injuries happen during slide plays. Baseball is a contact sport, but I think that at least some of these injuries are preventable with a proper application of the rules.

While PONY uses Official Rules of Baseball, every league and tournament I have ever worked or read about has adopted rules prohiting crashing into fielders in a malicious manner. It usually reads "slide or avoid collision." This is always discussed at the ground rules, for the very reason that there is no provision in OBR for such enforcement. But you cannot barrel over the catcher in PONY baseball, despite not being stated in the national, official PONY rules.

Here is an example from one Pony League's local rules:

MALICIOUS CONTACT RULE
A player / runner not only can be called out but also ejected from the game for making contact in a malicious manner. Example: A runner knocks over a defensive player in an attempt to dislodge the ball to avoid being called out. Malicious contact will be in the sole judgment of the umpire. The penalty for malicious contact is also the sole judgment of the umpire.

Here is another example from a different Pony League:

Any runner is out when he/she:
4.1.1. Fails to slide or to take other evasive action(s) to avoid contact with the defensive player. A defensive player may not interfere with the advancement of the runner to any base. A defensive player may not position himself in the base path unless he is making a play on the ball. Such a position, which results, in the opinion of the umpire, in impeding the progress of the runner to a subsequent base, shall necessitate the umpire awarding an additional base to the runner. A catcher may not block the plate in violation of Official Baseball Rule 7.06(b)(note). For example, a runner will be awarded the base to which he or she is advancing if, in the determination of the umpire, the runner used evasive techniques that included going outside of the base path to avoid a collision or contact with the defensive player.

This rule is intended at a minimum to end intentional decisions by a runner to collide with a defensive player in order to dislodge the ball or cause sufficient injury or pain to cause the defensive player to release the ball. The umpire will determine violation of this rule. Should the umpire determine mat the runner has violated the rule, the runner will be called out, however, and the ball will remain live. In the event the failure to slide or to take evasive action(s) causes the defensive player, in the determination of the umpire, to be unable to complete a double play, the umpire may determine that both runners involved in the double play are out. All the decisions of the umpire, with regard to this rule, shall be final and cannot be appealed. PENALTY: The umpire shall eject the player determined to have violated the rule for the remainder of the game in which the violation of the rule took place and shall warn the Manager and the bench and shall report the violation to the NSBA 1st VP within 24 hours of the ejection.

I was ok with the slide up until R1 jerks his legs sideways. I would call that willful and intentional interference under OBR and get two outs.

On the subject of willful and intentional interference the MLBUM has this to say: "Runner has willfully and deliberately interfered with a fielder with the obvious intent to deprive the defense of the opportunity to make a double play. Batter-runner is declared out for the runner's interference." It goes on to say that R1 is out also even if he had not been put out on the play. It also says that "If, in the judgment of the umpire, a runner willfully and deliberately interferes..."

So if you judge to be, then it is, and 2 outs, any other runners return to their bases.