In one of my recent GURPS campaigns, Saints, Seadogs, and Sachems, I made an extensive document with content focused on gunplay in the flintlock era. I'll post some of that here in the hopes it will be useful to your group.
Flintlock Gun Perks
There are many gun perks that are appropriate for the campaign. These include Armorer’s Gift, Cross-Trained, Quick-Sheathe, Quick-Swap, Cool Under Fire, Deadeye, Gun Schtick (Road Agent Spin), Quick Load, Lightning Fingers, Off-Hand Weapon Training, Standard Operating Procedure (Off-Screen Reload), Supplier (Guns), Tap-Rack-Bang, Weapon Bond.
There are also some new Perks that might prove useful.
Safe Gunner: You have practiced careful use of firelocks, grenadoes, hand mortars, and similar devices. They are generally safe around you. Any time you handle a blackpowder gun, cannon, etc. you increase the weapon’s Malfunction rating by 1 when it comes to issues which might cause a dangerous misfire or malfunction. Typically this will increase the Malf from 14 to 15. Wet powder still bothers you, like anyone else, and broken weapons are no more likely to work for you, but you experience fewer explosions, burst barrels, dangerously short fuses and so forth.
Accurate Loader: You have mastered the art of loading one rifled blackpowder weapon type so that you can eke out a bit more accuracy. With a successful skill roll under normal loading conditions, you get a +1 to Acc. See Careful Loading, below.
Lightning Loader: You are amazingly fast at rapidly loading blackpowder weapons. You get a +2 to skill rolls when you use Hurried Loading (see p. 00). Basically, this means you can reliably load 2 seconds quicker than usual! This counts as a bonus in any Contest of Skills having to do with loading your chosen weapon. Specialize by weapon type.
Good Lock: You take great care of your weapons, especially when it comes to maintenance and environmental conditions. Your weapon is always scoured clean and will almost never misfire in wet conditions if reasonable measures could be taken to prevent a problem. If there is ever a question of whether your weapons would function in the field due to lack of maintenance or proper precautions, yours will. A critical failure still counts however! You cannot avoid fate.
Guns Specialties
To simplify weapon skills and reduce skill bloat, these are the specialties for all flintlock blackpowder weapons.
Guns (Longarm) for any blackpowder musket, rifle, fowler, shotgun, etc. firing solid or shot.
Guns (Pistol) for any gun typically used with one hand – pocket pistol, horse pistol, etc. firing solid or shot.
Guns (Grenade Launcher) for the hand mortar, or if any long arm is firing a rod grenade or cup launcher; each of these is its own familiarity.
Guns Familiarity
Familiarity is very important. Starting characters may specify two familiarities per point spent on a skill. For instance, if you have four points in Guns (Pistol), you can be familiar with up to eight handguns. Starting characters need to specify familiarities, and once play begins those may change.
Barrel: The most important point to note is familiarity with smoothbore or rifled weapon. Pistols and longarms come in both types. Typically soldiers with be familiar with smoothbore longarms; only specialists, such as hunters or target shooters, use rifled longarms. Civilian duelists or target shooters might use rifled pistols, but most military pistols were smoothbore.
Caliber: This is the main point of distinction between blackpowder weapons. Weapons are grouped by caliber: .70+, .60 to .69, .50 to .59, .49 to .40, etc. This captures the distinction between a military musket (.75 caliber) and a .60 caliber fusil de chasse, or between a Navy sea service pistol and French pocket pistol.
Lock: Most weapons in this setting will use one lock type, the flintlock. Variations of the flintlock, such as the English lock, doglock, snaphaunce, and Miquelet lock are a familiarity, at -2 to skill until the shooter has spent 8 hours learning the new weapon. This also counts for multiple-barrel and repeating flintlocks, Nock’s volley guns, Mortimer locks, double-barrel shotguns, etc. All of these have their own strange loading and operating features in comparison to a single-barrel musket. Of course, ancient wheellocks and cannonlocks are -4 to skill.
Load: Variant loads are another issue of familiarity. In reality, every combination of shot and powder that a shooter works up for a muzzleloader is its own “load.” There’s a huge difference in the range and point of aim between solid, buck and ball, and various shot sizes. For purposes of simplification, familiarity with a weapon counts as familiarity with many loads. If the GM thinks the load is sufficiently weird or exotic, the -2 penalty to hit may apply until the PC gets a chance to study it.
Finally, when noting the familiarity on the character sheet, start with caliber, then note the bore, and then any specialty actions and then the lock: Guns (Longarm, .75 smoothbore flintlock), Guns (Longarm, .69 smoothbore snaphaunce), Guns (Pistol, .50 rifled multiple-barrel flintlock), Guns (Longarm, 4-bore smoothbore flintlock), etc.
Techniques
Pistoleros and buccaneers may want to put points in some Gun-Fu techniques, just as for Perks and skills. It's generally best to invest in one technique, as more points are better spent in improving all Guns skill instead of a trick.
Dual Weapon Attack
You can buy DWA (Pistols) which buys off the -4 penalty for an extra attack. This is a Hard technique, and so it costs 5 points to have it at full skill. These attacks may be against two different targets, but both must be within a 30º angle. You cannot DWA and Quick-Shot. You can choose All-Out Attack (Double) and make a Rapid Strike or Dual-Weapon Attack with one of these attacks.
Loading on the Run
Normally, loading while doing anything else besides standing still is penalized. With any Step maneuver, it normally takes +50% longer; anything beyond a Step requires a Gun-3 roll, with a failure taking at least twice as long, and possibly meaning the weapon is dropped, etc. This Hard technique (cannot exceed skill level) lets you buy off the -3 penalty to loading while moving and if the roll is successful the reloading time does not increase to +50% either. Failure means starting over; critical failure means a dropped weapon, spilled powder or shot, etc.
Precision Aiming
This technique can be used in the Flintlock era with the Deadeye perk. One important limitation is that it can only be used with rifled weapons – smoothbore muskets are inherently too inaccurate to make such precision shooting possible.
Quick-Shot
It is a -6 to skill to shoot at two different targets in the same turn whether it is the same weapon (i.e., a double-barrel) or different weapons (a pistol in each hand). You can buy this off as an Average technique, which costs 1 point per -1 penalty you buy off, up to 6 points for no penalty. These attacks may be against two different targets or hit locations, but both must be within a 30º angle. You cannot DWA and Quick-Shot. You can choose All-Out Attack (Double) and make a Rapid Strike or Dual-Weapon Attack with one of these attacks.