I'll start with ammo explosion, as it's quicker and something I've also toyed with over the years. Firstly, because it has always seemed way too dangerous and too much of a penalty for the mech (not to mention the poor pilot also takes damage due to Neurohelmet feedback!). Secondly, because bits of it didn't make sense, both because why would an omnidirectional explosion of some ammo do the same base damage as shaped charges or kinetic impactors, and also why would arm internal damage travel perfectly through the small tunnel of the shoulder joint into the torso?
And thirdly, because of how it played at the table, where every Warhammer pilot (unless piloting the 6K or the 6D) began the game by dumping all their MG ammo since a) with a range of 3 they're never going to use the dang things and b) walking around with a 400 point bomb in your torso is not a good idea.
Back in 2002 when I wrote out a book of houserules, I'd gone with reducing the damage done to 1/3 of the total calculated damage. But that now still seems too high, too much of a penalty. Especially given the automatic damage to the Mechwarrior, and the fact it will generate another roll on the critical hit table (It does/would, right? You're taking internal damage, so that means you would roll, right?), I think a more appropriate level is to mostly maim the area affected, with at best blowing that section off/out and maybe a bit of damage to the adjacent body part (internal when going torso to torso, or to the armour when going between limb and torso).
I'm less of a fan of a cap -- a cutoff feels artificial. I think I'd prefer a simple multiplier of the weapon's damage. Or maybe just straight up weapon damage, or weapon damage +X.
Straight up, an SRM 6 would do 12 points. That blows off the arm of up to a 75 ton mech. (!) Takes out the side torso or leg of up to a 50 ton mech. Even on a 100 tonner, that takes out 70% of an arm, and 57% of a leg or side torso. That still hurts. LRM 20, AC/20, both are limb shearing. But an AC/5 only auto de-arms a 30 tonner, and legs only a 20 tonner. MG explosions are a bit of a sneeze. (Though, again, with the damage to the pilot and chance for additional internal crits, there's still opportunity for more hurt).
Straight up +2 might be interesting. That's 4 points on an MG, 7 points on an AC/5, which auto-dearms a 45 tonner and legs/torsos a 30 tonner. Your SRM 6 now affects 85 and 60 tonners, respectively. And your AC/20 or LRM 20 don't really get that much more scary... though now with cascading damage there's another critical chance roll in the adjacent location.
Going with 2x keeps the MG at 4, but now makes that AC/5 at 10 and so starts auto-dearming 40 tonners. And any weapon damage beyond 11 becomes an auto de-leg or side torso device. AC/10 does that to anything 95 tons and below, SRM 6, LRM 15, AC/20 ammo explosions are super serious.
I think my sweet spot is in that range of "Ammo explosions do 2 points of damage plus the damage value of the waeapon to the internal structure of the ammo's location". (Or maybe 3+X) Not as elegant as 1x or 2x, but easier to calculate than "how much ammo is remaining times the weapon damage" and feels less "artificial" than a cap at 20.
(As a side note, a cap at 20 is interesting as how often will it come up that you'll do less than 20 under the standard rules? 9 shots of MG ammo equals 18, 4 shots of SRM 2, 2 shots of SRM4, 1 shot of SRM 6, 1 shot of LRM 10, 3 shots of LRM 5, 3 shots of AC/5, and etc. In other words, you have so few instances where you would do less than the cap that to me, again, it makes more sense for speed/playability with about the same effect to just set the value to a smallish value rather than worry about those small edge cases.)
More later!
