The rapid fire progression in GURPS Campaigns is reasonable. It is also...
My personal preference is as follows:
| Shots | Bonus to Hit |
| 2-3 | +1 |
| 4-7 | +2 |
| 8-15 | +3 |
| 16-31 | +4 |
| 32-63 | +5 |
| 64-127 | +6 |
| etc. | etc. |
The above makes a difference at the low ranges (where most rates of fire and PCs will be) and flattens out a lot better at the very high ranges.
These are for ugly, gritty, and more realistic gun bunnies.
When under fire, there are tiny, invisible wounds flying through the air. Dodging is an interesting idea in theory, but in practice tends to be less effective a defense than the opponent's tendency to miss. Still, we naturally duck, flinch and otherwise react when we hear the explosive sound of invisible death coming.
There are two options available to characters:
Note that any character with a Will of 16 or higher will take no penalty. They are cool, bad-ass, etc. Combat Reflexes should add its bonus to Will for purposes of reducing the penalty; and the Gunslinger advantage should negate the penalty flat out.
On the Will roll to ignore bullets, you may wish to add modifiers for how "safe" the character feels. As a rough guide, +5 if you have near-total cover, +3 if you have 1/2 cover, +1 if you have slight cover; +2 if you know you aren't the actual target. If bullets can't hurt you (DR 20 or so) and you know this, you succeed automatically.
It is harder to dodge a muscle-powered missile attack than a thrown dagger and it is significantly harder to dodge a firearm attack. You dodge a muscle-powered missile attack at -1 and a firearm attack at -2 when being attacked from the front. If you have Peripheral Vision this penalty also applies to attacks from the side. For 360° Vision it applies to attacks from the front, side and back.
These penalties do not normally apply to attacks from the side or back, not because it’s easier to dodge when the attack comes from the side or back, but due to existing penalties. There is already a -2 penalty to dodge when attacked from the side and you simply cannot defend at all when attacked from the back. These side and back penalties already take into account the difficulty you are having in sighting the attack. The same as the difficulty you would have in trying to guess where not to be and weaving and ducking appropriately trying to avoid being hit.
Everyone loves when a critical hit is rolled, except the recipient. According to the rules the victim of a crit gets no defense. Not in my games! As an option we decided that anyone who is a victim of a critical hit gets a chance to make a critical defense. If he rolls a critical defense then the blow has been negated. Anything other than a critical and its time to roll on the critical charts. Defenders rarely make this roll, but when a blow is coming that could lay out your character, it is a very exciting roll (and a good time to use Luck). The victim of a critical defense does not get a roll to try and negate it because he already failed to get a critical on his attack roll.
I also feel that getting an extreme critical should result in a more extreme outcome. On any attack/defense roll of "3" or "18" I feel the critical should, on average, have less of a chance to come out a boring "does normal damage." In this case I have the player roll 4d6 and take the most extreme combination of three of the dice, and that is the result. For example, the player rolls 4, 5 and 2. That's 11, which is no extra effects. Now, rolling that extra die, it comes up "1." Taking the most extreme of the three (the result closest to the lower or upper end of the crit chart) the result is "7." Yay- extra damage!!! This does not apply to any other critical rolls, even at high or low skill levels. Only a natural "3" or "18" invokes this effect.
I have used these rules for years and found that while they do not change the overall results that much, it does ramp up the "fun" factor in my players.