it's been years since I've played this game, but here's my 2 cents when it comes to the weapons.
Both the machine gun and the flame thrower shoot at the same speed, but the flame thrower can only hit something in close proximity. I don't have accurate numbers but I believe each gun fires once every 2-3 frames in sustained bursts. The grenade launcher has closer to a 10 frame shooting pause before the grenade is launched, an then you have to take into account travel time before it hits the target (and you can't do anything until the grenade is airborne, so no running, etc etc). Hand grenades are even worse when it comes to shooting. The shooting animation is about the same as the grenade launcher, maybe a few frames more, but the grenade doesn't explode on contact and has to wait for about 2 seconds before going off. Against the bosses it's only good as the last hit since you can drop it an start running for the exit door, or you can use it to hit the boss if it's below you (don't quote me but I believe with the machine gun you have to be running to aim up or down, which is only at an angle).
The only way I can see wasting ammo actually saving time (since you can hit start on the score tally screen to fast forward it I believe) is if you played every single level as you want, counted every single shot you'll need along with how many pickups you get, an then start adding in ammo wasting parts. However you can only run and fire the machine gun.
*Edit*
Loaded up gens an did some frame advance testing, here's what I've found out.
There appears to be a 3 frame rule for nearly everything in the game. Running, shooting, etc, all require that the key is pressed and held for 3 frames before actually affecting gameplay.
The machine gun and flame thrower both take 3 frames before starting, and they fire another shot every 3 frames. The flame thrower counts down 1 ammo every 6 frames while firing (however this might not always be the case as you can sometimes burn up an extra ammo shot every 3rd frame, not sure why). The machine gun counts down 1 ammo for every shot fired (so every 3 frames).
The grenade launcher has a 3 frame start-up time before entering the animation. After that it takes 17 frames before the grenade is actually drawn on the screen, for a combined firing time of 20 frames. For hand grenades, after the 3 frame start-up time, it takes 14 frames before the grenade is drawn on screen. Once the grenade is drawn there's an extra 10 frames where Ripley stands around like an idiot with her arm extended. On the very next frame you gain control over her again. That's a combined total of 27 frames just to shoot. Now, counting the frame where the grenade is initially drawn, it takes 39 total frames before the grenade will explode.
In order to stack commands and take control of Ripley on the first frame available you have to start an action you wish to perform 3 frames in advance. And once an action is started that can be stopped and interrupted (like running) you have to take into account 3 frames after you release the button that you can't do anything. So say you want to run for a bit and fire a grenade, again you have 3 frames before the action will stop you from running and begin firing. Likewise, if you just let go of the direction to stop running there's 3 frames that will go before you actually stop.
The good thing is it appears the game has a constant 60hz polling of the controller so there's only few areas where input is not accepted (I'm assuming the screen fade-outs between levels).
Other frame counts (assumes the 3 frame start-up is already done):
ducking = 6 frames, control gained on 7th frame for duck firing, after animation is done and standing takes place, another 6 frames with control gained on 7th frame
getting on ladders = 7 frames, control gained on 8th frame
getting off ladders = 12 frames, control gained on 13th frame
splattering (too high of a fall) = 160 frames starting on the frame you hit the ground, control gained on frame 161
drop from too high = 80 frames moving downward, falls landing on 79th frame should be the highest possible (tested by walking off a ledge, started on first frame Ripley actually moving downward)
*Note 3 frame start up does NOT affect this* changing weapons = 1 frame (does not interrupt any other animation, can be done during other actions), draws new ammo on 4th frame, weapon picture on 8th. You can then hit fire on the 2nd frame to fire the new weapon, HOWEVER you cannot time it to change weapons while already starting up a fire animation. One thing that's interesting, if you hit A and B on the same frame, then release the A button and continue holding fire, you'll change weapons and fire in a weird way. You'll start firing whatever gun you were holding, yet the next gun will be selected and ready to go. Just stop firing then start up again and you'll fire the gun you previously selected. So if you have the machine gun and you want to fire the flame thrower next, you can hit AB for 1 frame, then let go of A, and start firing the machine gun. Then let go of B to stop and then start firing the flame thrower. I haven't seen how this affects actual ammo counts though, but I will see if I can do some more testing later.