This directory for this post was lost in the conversion and re-created here:
This post takes for granted your rounds are loaded properly (OL - 1.180 - 1.230 and proper taper crimp), using a bullet that is a known feeder. On Overall Length, I've found as far as feeding is concerned, shorter is usually better - down to about 1.180.
I've made various posts regarding feeding for the finicky dbl stack .40, but I'll re-summarize a bit.
At the first sign of feeding or stopping on the ramp failures, take out any or all shock buffs and put at least a 14 lb recoil spring in the gun. Then, if the mag springs aren't relatively new, replace them with Wolf .40 caliber springs (of at least .50 - .52 wire diameter). If you're using competition SV followers, check my post (with photos, probably in the Springs forum) on the super-trick mag spring mod (that provides a bit more nose-lift). If this fixes the problem, you can either shoot your blaster like this, or tune the feeding so you can put the shock buff(s) back in.
Next, do a home-tune on the feed ramp. Get some 600 grit wet/dry sandpaper, cut small piece to fit around a empty .40 case and clamp the barrel in a vice so the feed ramp is facing you. Carefully applying even pressure to the entire case, work the sandpaper/case up and down the feed ramp. When you first start polishing, you'll visually notice the difference between the factory cut ramp, and your new polish job. Keep polishing forever, until the entire feedramp is polished from the sandpaper. (You are polishing in the opposite direction a rotating tool cuts or polishes, although it is the correct direction to ensure smooth feeding.) (Before you do this, hold the barrel in your left hand and a loaded round in your right. Push the bullet against the ramp, holding it parallel to the barrel, and note the resistance between the bullet nose and the feed ramp as you try to slide it up the ramp. Then, after polishing in the correct direction, try it again - you'll be amazed.) This can sometimes make a truly miraculous feeding improvement. If you want to really get nutty, repeat the process with 1200 grit!
Then, get a good pair of calipers and a pair of small to medium sized channel lock pliers. Grind the teeth off the channel locks so they are just smooth. You'll probably have to grind a notch in one of the jaws to clear the magazine feed lips when you start torquing on them with the pliers. But before you do any bending on the lips, measure the width of the feed lips at a spot back about an eighth of an inch from the front. This dimension should be approximately .390 - .395. If it's smaller (and it typically is) use the channel locks to open up the feed lips until you get to the specified dimension. Be sure to keep the lips below .400 or you'll create a new problem. When bending, be sure to keep the feed lips pointed straight into the barrel. You can check this two ways. 1) Hold the mag in your hand with the back of the mag toward your face and sight down the feed lips, referencing from the back of the mag. Or 2) With the slide off and the barrel in the frame, insert a mag with a few rounds in it and note how the rounds point up to the feed ramp and chamber. Try to end up with the feed lips at around .395, and pointing straight up the feedramp.
The previous mods will take care of most feeding problems on the .40. If you're still having trouble with a certain mag or two, and you're feeling really brave (and confident) with your Dremel tool, you might try this last super-trick mod. It isn't easy to explain without photos, but I'll take a shot. Basically, you're going to thin the feed lips at the front end of the feed lips to allow the round to point upwards a tad (more than normal) as it heads to the ramp. First, load a few rounds into the mag and let the rounds settle into position against the feed lips. Notice (as a baseline) how the round is (typically) parallel with the top of the feed lips (when viewed from the side). If the rounds are nosed down, that's big trouble. You want them at least parallel, or SLIGHTLY pointed upward. (If you get them pointed too upward, however, you'll create a new problem - the rounds will jam against the hood instead of the ramp.) You'll need a smooth, cylindrical Dremel bit, a little smaller than a .40 round. (I am not at my shop, so I'd guess it's about .250 - .375 in diameter.) Clamp the disassembled mag into your vice with the feed lips pointing toward you. CAREFULLY insert the highly spinning Dremel (with bit) from the front of the mag and equally polish each side of the lips at the exact point where the round pushes up against the lips when under spring pressure. But the tricky part is, you want to polish the front end of the lips slightly more than the back end; this give the rounds the needed upward slope when they're resting against the lips. When you look at the area where you've been polishing on the lips, you want to see a slightly wider polished area at the front of the lips than at the back. If done properly, the lip dimension shouldn't change when measured with the calipers, but the rounds will actually be nosed slightly up in the front when viewed from the side (as previously described). After barely removing any material, load a few rounds in the mags and check the nose-up-angle from the side. Go really slow on this one because you know how much you paid for those babies. And don't even try it if you're not completely confident with your moto-tool, and you're still having trouble after ALL the previous mods. Usually this last mod is not necessary, as one or more of the previous mods will get her running smoothly again.
An abundance of good information. You have touched the basics of a good running gun. Good ammo, the right recoil spring for given ammo, correct feed ramp shape and a mag that holds the ammo in the right position. I may add that the rest of the mag needs to be sized correctly, not just the feed lips. If the mag tube is too narrow, the double stack of ammo will not feed up fast enough to get that top round into that all so important nose up position, no matter how good of a spring you put in it.
I have one question though, on your dremel polishing of the inside of the feed lips. If you think that the gun needs more "nose up" positioning, could you just bend the front of the feed lips to a wider demension then the back? Making sure that the front demension is no wider then the .400 you discribed. At least bending the feed lips can be ajusted back and forth and no material will be removed that can't be put back if you take too much off.