Honestly... I think my response would be to ignore the old DR system all together when thinking in respect to the new guidelines. Instead the question should be, "What do you want your weapon to do?"
My reasoning for this is thus:
The old system was closer to game mechanics in structure. X weapon deals Y damage to this tier of vehicle. Add in some dice rolls with the SP already in place for Hit Points and you could argue it is a table top system. This lead to a lot of submissions being worked to fit the DR rather than the DR being used to give players and GMs an idea of what the weapon did. For example if you wanted a powerful personnel weapon, your only option was generally to make it PDR5 and hope for the best when someone wrote it IC.
With the new set of guidelines, instead we're asking, "If you shot something with this, what would be the biggest thing that it could theoretically kill in a single shot?" Instead of asking the question, "Where would an SDR2 weapon fit on this scale?" We should instead be looking at the fluff and asking ourselves, "How was this weapon intended to preform in roleplay? Is it supposed to destroy its intended target? Or merely damage it?"
I personally feel that a lot of the DR values were chosen to evoke a certain feeling rather than placed with the effect of the weapon in mind. For example often people considered SDR1 and SDR2 weapons largely useless in starship combat and would opt for the high end of the scale at SDR5, and would only grudgingly use SDR 4 or SDR 3. It gave the impression that unless you were hitting in the top end of each category the weapon wasn't worthwhile. An example of this is the low number of PDR1 weapons that exist to my knowledge. I remember on more than one occasion people commenting that they wouldn't be caught dead with something small like the
Little Killer, this gives the impression that the way the current system is structured lends to players and possibly even GMs disregarding the lower end of the scale as ineffective.
I would instead propose that we look hard at a sample of weapons and units to truly fill out the examples listed to give a better idea of how they should function in character, rather than from the lens of a numerical value. I don't think I personally have enough experience with how the weapons and armors have played out in combat to populate the list myself or I'd get started on that. As such I'd appreciate it if the veterans among us could chip in and start discussing what fits where. The more examples of experiences with the equipment the better we'll be able to place it, which will in turn provide a more accurate example of what to expect.
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A final note, is that I think one of the most important things to emphasize is that an equal tier weapon is supposed to -kill- the target. Rather than thinking that the armor will defend capably against a same tier weapon. I think that learning that a Class 5 weapon will kill a Class 5 target with skilled use is going to be one of the things that people will have a bit of trouble learning. After all the present DR system has it so that an ADR unit can tank an ADR weapon. That is the way the system frames it. So the more clear it is that a same Class weapon will kill a same Class target the better.
Edited to add a bit at the top.