Tonite I found a few queries which reminded me that there are a LOT of people out there who desperately need information. I'll yield to temptation.
The first query: "Can you shoot .38 mm in a super 38"?
This resulted in a hit on the article "XL650: .38 Super", which probably did not provide the answer.
Here's the answer: no.
The .38 Special cartridge (which I assume is the cartridge referred to as ".38 mm", is much longer than the .38 Super cartridge. It would not fit into the chamber. Other apparently similar cartridges which will not fit include the 9x23 and .357 magnum.
There are several cartridges which would fit into the chamber of the .38 Super, but they would not cycle the action because the semi-automatic pistol action which typifies a .38 Super-chambered pistol headspaces on the cartridge case. This is decidedly different from those cartridges (usually revolvers) which slip into the chamber/cylinder until the rim of the cartridge case stops further forward movement. In the case of revolvers, this may leave a small distance between the leading portion of the bullet and the nose-cone, which funnels the bullet into the barrel. This is why you can shoot a .38 Special cartridge in a .357 revolver (the .357 case is longer, preventing you from shooting a .357 Magnum cartridge in a revolver chambered for the .38 Special cartridge.)
The .38 Super case is shorter than a 9x23 cartridge. Although they are very close to the same case length, the 9x23 will not chamber in a pistol chambered for the .38 Super cartridge.
(* ... see correction by Guy Neill in Comments, and the UPDATE below.)
On the other hand, the 9x21 cartridge is very close to the same length, and may actually fit into the .38 Super chamber. However, the 'jump' distance is too long, and other factors may make this an inadvisable experiment. For one thing, there may be a lot of 'flash' when the case does not headstamp correctly (picture lots of fire coming out of the parts of the gun which are not even close to the muzzle), resulting at least in much power being wasted before the bullet enters the barrel. Worse, the 9x21 cartridge when loaded to the Over-All Length of the .38 super, will most likely not seat the bullet firmly into case. In the first case, you may experience a "Squib-Load" malfunction, in which a bullet may be stuck inside the barrel. This is A Bad Thing, because a second round fired through the gun may blow up the gun because the barrel is obstructed. In the second case, chances are than the recoil of the gun may work the bullet out of the cartridge case. This dumps gunpowder into the magazine and the gun will not cycle correctly which might be the least destructive effect of attempting to shoot a short cartridge in a firearm chambered for a longer cartridge. (The .38 Super may be considered a "9x22" cartridge, referring to the diameter and length of the case.)
The 9x19 ("9mm Parabellum" or "9mm Luger") is even shorter than the 9x21. Any problems which you may experience by attempting to fire a 9x19 in a .38 Super would be even worse, but most likely the rimless cartridge would drop fully into the chamber and the firing pin would never reach the primer ... if you are lucky.
Another query is "minimum shotgun shells take down pepper poppers", resulting in a 'hit' at the article "Practical Shotgun".
This question is much less fraught with peril than the first.
But not much less complicated.
First, one assumes that the question was asked by someone who contemplates participating in Practical Shotgun (rare), Multigun (less rare), or 3-gun competition (more common).
The answer, referring to shotgun ammunition, has three parts: caliber ("gauge"), length, and shot size/weight.
For competition, the standard is to shoot a 12-gauge shotgun. Shooting a 16-gauge shotgun may be possible, but ultimately frustrating.
The shotgun is a Hi/Lo Pressure firearm. In part this refers to High Base vs Low Base ammunition, meaning that some configurations of the brass base of the multi-component shell provides more pressure (and we assume higher velocity) than others. These are "high-base" vs "Low Base"; shooting the low-base ammunition presumably results in lesser recoil than high-base. Also it refers to the situation situation where a shotgun shell, when fired, starts out slow (primarily due to the big bore of the gun), and doesn't recoil as much as a rifle which attempts to accelerate a similar weight of metal to a sustainable velocity. As the shot load accelerates down the barrel of the shotgun, the powder continues to burn and continues to accelerate the shot. Because this activity doesn't occur with the suddenness of a pistol or rifle, the recoil is not as intimidating.
What this means is that the shotgun shooter can shot a heavy charge without suffering from the recoil penalties typical of, say a Magnum Rifle shooter. This allows the shotgunner to shoot a heavier bullet/shot weight than a rifle shooter without experiencing unbearable recoil.
In the context of this question, there is not much recoil penalty for choosing to shoot a 12-gauge vs a 16-gauge shotgun, but the downrange effect of the 12 gauge is more violent. That is to say, your chances of shooting a 12 gauge shotgun rather than a smaller gauge (16, 20, 410, etc.) and reliably knocking down a Pepper-Popper is better. There are more pellets, the shot-weight is higher, the pattern is tighter and you'll hit with more, bigger pellets.
How about the length ... of the shotgun shell?
Typically, a longer shotgun shell implies that more powder may be loaded behind a heavier shot-weight. Thus, a 3" shell may be more effective than a 2-3/4" shell. In the actual event, this is not usually a big factor. It is easy to reliably knock down a pepper popper with a 2-3/4" shell.
The ultimate factor: throw-weight and shot size.
In order to knock down a pepper popper with a shotgun, the goal is to hit the target with the most weight at the greatest velocity. Almost everything you have read so far ... high-base vs low-base, 3" vs 2-3/4" shell, 16-gauge vs 12-gauge ... has to do with the velocity.
But what you want to do is to put as many pellets on the face of the steel as possible. There are reasonable arguments to use 1-1/2 ounce loads rather than 1-ounce loads. There are reasons why a #6 shot is better than #7 shot ... and vice-versa.
Shot density puts more pellets on the steel, which is one reason why you should consider using an Improve Choke or a Modified Choke (or other, see the chart) on the shotgun when engaging steel targets at a specified distance. But if you only consider ammunition, it pretty much depends on your ability to hit a 11-5/8" vs 8" target *(the largest diameter of a Pepper Popper vs US Popper) at that specified distance with the bulk of your shot pattern.
Are you confused yet?
Okay, I shoot a Mossberg 590 with an open (un-choked) barrel, and I get along just fine shooting a #6 shot from a low-base 2-3/4" shell at any steel target.
This may seem anti-climactic to you if you have read the whole thing, but you can break your heart trying to figure what is the best shot shell against a pepper popper. The whole point is, center your shot pattern on the target, and it will go down.
It really isn't the ammunition, or the gun, which makes the difference at the distances which we usually engage steel targets. It's whether you can center the shot pattern on the target and put the maximum throw-weight on the target.
What's that? You think I could have said this at the start? Sure I could have, but would you have been convinced that the ammunition is less important than accuracy?
One of the most difficult lessons to learn in shotgunning (or any shooting discipline) is that you have to work on accuracy first. Eventually, you may become so proficient that you can get a hit and defeat the target at extreme ranges by choosing the best combination of gun and ammunition.
But if you have to ask what is the "minimum shotgun shells take down pepper poppers", then you really needed to work your way through the reasoning to reach this conclusion.
I am so pedantic. Sometimes I don't like myself so much.
* UPDATE: 23-JUL-2008
Corrections from Guy Neill in Comments:
Sorry, Jerry, the 38 Super case IS 23mm long, just like the 9x23.Thanks for keeping me honest, Guy.
The 9x23, however, has a larger diameter at the base than the 38 Super ahead of the semi-rim.
(NB: Guy Neill writes the "Neill on Reloading" column in the USPSA House Magazine, Front Sight. In any apparent contradiction between what Guy says and what The Geek says, always go with what Guy says.