I think I may have found a solution to our problems...
Make DR work like Armour Rating out of Traveller d20.
The first point of DR negates the lowest dice of damage rolled by the attacker, the second point negates the next lowest dice, and so on untill there's only one dice left (the one that rolled highest). After that, each point of DR negates one point of damage.
EXAMPLE 1: a hero fires a shotgun at the bearded devil she is fighting and hits. She rolls the dice for damage (2d10) and gets a 5 and a 6. The barbarzu has DR 5, so the first point negates the lowest dice (the 5), after which the remaining 4 points are subtracted from the remaining dice (6 - 4) for a total damage of 2 points.
EXAMPLE 2: having run out of shotgun shells, out hero pulls out her trusty longsword and attacks the devil with it. She hits. Rolling for damage (1d8 + 2) she gets a 4 + 2 = 6. As the weapon only does one die of damage, all the DR is duducted directly from the damage rolled, for (6 - 5 =) 1 damage.
EXAMPLE 3: after killing the barbarzu, our hero manages to locate a glock and some explosive bullets for it. She is later attacked by a lemure, and she shoots at it, hitting. She rolls damage (2d6 + 1d6 for explosive ammo) getting a 3, a 3 and a 4. The first two points of the lemure's DR negate the two lower die rolls (the two 3s), and the remaining 3 points reduce the 6 to 1.
So thus DR works normally against weapons that only deal one die of damage, and is much more effective against weapons that deal multiple dice. The only modification we would have to make to anything is probably to make the Greatsword deal 1d12, and the Spiked Chain, falchion, guisarme, ranseur and scythe deal 1d8 damage. Obvesously, DR-ignoring guns are very deadly, but would also be very expensive.
I prefer this over TR, because it discriminates against weapons on the basis of their damage die, not their technological advancement. The fluff reson for this is up to the individual DM to work out.
P.S.: AR is covered by the OGL, as it is in the Traveller 20 Lite book that can be downloaded for free from somewhere.
What if the character was born in a world were guns had been part of achient history? Such as a hi-tech world that had fallen from civilised grace, but still had the ability to make AK-47 etc. The legends the PC heard as a child were full of glock-weilding knights slaying bionically-upgraded dragons and demonic machines. How would your explination of TR fit into this setting?
Flip it around the other way - complexity as a floor rather than a ceiling for TR, so that swords and bows don't have the same ideological effects as an MP5.