Custom modifications is a total shot in the arm for firearm based decks with guardian access, dramatically increasing the scope of what they can choose to do. While it is unlikely to dethrone melee supremacy, it accomplishes the vital goal of making firearms have an attractive draw compared to swords, knives, and hammers.
The base effect of custom modifications is fairly strong. While being unable to ignore the autofail stings, especially because guardians can easily reach ‘autofail only’ failures, it allows you to play a bit more fast and loose with your expected success rate: if you have a 60% hit rate before custom modifications for example, you actually can expect any given attack to actually hit 84% of the time, which is fairly up there in terms of consistency. This alone is a huge deal, as it means firearms are now inherently much easier to hit with than melee as long as you can get custom modifications out consistently, which reduces the burden of skills and passive skill boosts in your deck overall.
The two upgrades that really push this card though are Extended Stock and Quicksilver Ammunition, which when combined with the base effect allow you to turn any firearm into a viable late campaign weapon, trading play actions and resources for some XP efficiency, and slot efficiency.
These effects are usable on existing ‘premium weapons,’ but the effects of both these upgrades scale better based on how weak the base weapon is, due to breakpoints on damage making 4 damage less of an upgrade on 3 than 3 is for 2, and the re-roll and hit rate increases not mattering as much for big beefy guns. It certainly works with them, but the impact of custom modifications on your lightning gun isn’t showcasing its true power.
Custom modifications really shows its promise when used with more humble weapons, turning them into late game powerhouses. If you run extended stock and quicksilver ammunition upgrades in concert on a ..45 Automatic (2), you get a +4 to hit weapon that does 2 base damage, comes with a re-roll, and can be treated as a +1 to hit, 3 damage weapon with 4 shots on what is effectively a 9 XP package. That is fairly comparable to something like Cyclopean Hammer in terms of its actual table output in many scenarios, as, on most guardians, you are effectively up +1 to hit on the hammer in terms of trying to get the highly desirable 3 damage hit even before getting the re-draw effect. It isn’t actually on par with the hammer of course (this eats up a STTP slot, requires a second play action, and most critically is ammo limited), but the fact you can even make that comparison via squinting really hard with a 1 handed 2 XP weapon is a huge deal.
This is pretty transformative in terms of what decks guardians can get away with now and how you can run a gun focused guardian compared to a melee basher, as currently most guardian 1 handers are locked to 2 damage and tend to have fairly significant limitations to their use. Even the ‘meta’ ones like Survival Knife (2) have limitations that make them non-ideal to depend on as a primary weapon rather than a backup and supportive tool to achieve more reach damage. By being a 1 handed primary, it is now much easier to make guardian flex decks utilizing cross class or neutral tools. These decks were always possible (machete is strong enough to clear a campaign with it alone and bandoleer exists even if it can be clunky) but now you give up way less in order to take a flashlight, magnifying glass, keyring, fingerprint kit or maybe even a saucy multitool if you are looking to try out blue flex, and the actual outcome is quite potent, as spamming 3 damage attacks while consistently finding clues is a very good way to win scenarios.
Other useful guns to use this with are the .32 (You end up slightly ‘behind’ the hammer, but it's good if you already are running combat boosts or are someone like Mark), the .35 Winchester (this makes it very competitive with the Holy Spear in Gun Nun and Holy Warrior Tommy decks, and the redraw effect is especially potent when digging for blesses), the Mauser if you have access to it, and the .45 Thompson, with special mention going to the rogue’s .45 Thompson if you can use it to do some pretty intense AOE damage.
There are some character specific synergies too. Build-A-Becky already was a very good deck, and now Becky is able to 100% emulate a pre-taboo cyclopean hammer regardless of target type or how many swings she needs to make, in addition to sniping, running custom ammo, being enchanted, ect. Tony can use custom modifications with his long colts to make fishing for that bounty refresh easier, though by being a ‘pseudo-dual wielded’ gun there is some anti-synergy both with the concept of running a 1 handed primary and only being able to upgrade one gun with one play, and I suspect you would more run this for its synergy with Mauser and Lucky Cig case than anything else. As a bonus, both of these characters are intellect 3 with strong clue/flex tech in their class, and thus can make good use of the freed hand if they so wish, though Tommy easily wins out more here.
The two biggest winners at the end of the day, and the two decks I had the most success with, are the two classic gumshoes of Joe and Roland, who really were getting tired of running around with glowing hammers and magical Macuahuitl..
Joe has to make a hard choice between the +2 to hit and the +1 damage potential but being able to run detective’s 1911’s as a primary damage dealing tool ends up being fairly potent. As it turns out, seeker has a lot of ways to make up a shortfall of +2 to hit, so you probably want to lean towards quicksilver bullets over the extended stock, but it depends on what your plan is overall, as Joe has access to a lot of damage extension as well via Ice Pick. Michael Leigh, and Ancient Stones, so it really comes down to what your team needs. Combat focused Joe always was possible, but it now feels REALLY good to be a gun focused Joe who uses their wits (and skills and maybe a magic book or two) and their trusty gats to take on the Mythos.
As for our Fed, Roland’s .38 special essentially becomes a better Lightning Gun when on a clue location, which is now very easy to do with the suite of clue dropping cards that Roland has complete access too (and which benefits from deciding to go with 1 handed weapons to utilize research notes) which, combined with his great access to 1 handed tools, makes him a pretty beastly ‘flexy guardian.’ I have found Parallel Front, standard back to be especially effective with this build, as you can utilize consult experts to help support both your economy, ‘reach damage,’ and test taking ability all at once using cards like bugman beatcop and michael to lay down smackdowns and finding far more clues than is reasonable for someone easily able to deal 9 damage in a turn despite only attacking twice, and fairly trivially keeping your guns topped off due to your access to strong card draw even without making extra ammunition stuck to the plan.
As for the other upgrades, they tend to be good, though not the primary reason you are taking custom modifications. Notched sight is a very nice use for throw away XP to save some actions on engages if you are the engaging type, and your friend’s health and (out of game) sanity if you aren’t, and leather grip being a way to offset the action cost of this card into a 1.5 XP cost (assuming you run 2 custom modifications), but there are some considerations to make about counterbalance and extended magazines you will want to consider before buying them, mainly that you need to spend a lot of cards on upgrading or gaining/spending ammo to actually see any reasonable rate of return for XP. Counterweight will definitely be a good upgrade on Build-A-Beast Becky, but we are going to need a few more ‘gunslinger’ events for extended mags to really kick into high gear.