Originally Posted by canebrake
asking whether you're not still that 15 yo mall ninja with mad Google skills.
I prefer the term, Middle aged Mutant Ninja Mechanic, and as far as my wife is concerned, I have no Google or computer skills to speak of.
If it were not for spell check, you might think I was a 12 year old apprentice Mall Ninja.
I still have to ask those 15 year old mall ninja's for computer help now and again.
I do believe they get some sort of satisfaction helping someone more than 3 times their age with the "simple things".
Either way, It was not my intent of focusing the discussion towards the .50-GI, but on the interchangeability of the Glock upper and lower assemblies.
I feel safe with the .45-ACP rounds weight and speed to do a good job in stopping the bad guy without "overkill penetration", so if I can have that "not too fast" speed with a round that is a little bigger and heavier, and considering the expansion of the .50-GI, I'm interested, especially if I can have my same .45-ACP gun also shoot those for fun as well.
In regards of the info below, I feel safe in feeling that the Glock 21 is a .45 win, .50 win.
And as a last resort, (and just for the fun of it) I can fall back on the old "my gun is bigger than your gun" excuse.
Stolen .50-GI info from Wikipedia
The .50GI operates at pressures comparable to the .45ACP, around 15,000 psi (100 MPa). Felt recoil is not unlike that of the .45ACP. The .50GI has developed a reputation for accuracy, though this may be due to the high precision of the semi-custom and very expensive Guncrafter pistols themselves. In one test, the 300 grain (19 g) JFP (jacketed flatpoint) gave a 25-yard group of 2.24 inches, and the 300-grain JHP (jacketed hollowpoint) and 275-grain JHP gave a 25-yard group of 2.14 inches.
The penetration in gelatin (but not necessarily the kinetic energy) of the .50GI is significantly different than the .45ACP. While it is one of the few examples of the largest caliber projectile (.50) in a semiauto handgun (or any firearm not considered a Destructive Device by the BATF, for that matter) it was purpose built to have a recoil impulse and kinetic energy substantially less than the magnum .50 caliber rounds such as the 50 Action Express (semiautomatic) or .500 S&W Magnum (revolver). Factory loaded ammunition has a kinetic energy of around 500 ft·lb. The Guncrafter Industries' website has suggested loads that push the cartridge into the realm of the .44 Magnum.
In one evaluation the following performance difference was noted between the .45ACP and the .50GI: "It actually pounded my steel target with so much force that it knocked the entire 100-pound plate and stand combo hard enough to make it furrow the ground it stood upon.[...]Folks, these .50 calibers really do hit that hard. [The 300 grain TMJ] caused dings in steel targets that normally fracture .40 and .45 cal rounds into so much dust." 
The cartridge is rarely used in law enforcement or for personal defense due to limited availability of ammunition and guns chambered for the cartridge. Currently, the only commercial handguns available in this caliber are Guncrafter Industries' own Colt 1911 handgun variants and its Glock 21 / Glock 20 conversion upper. However, at least one gunsmith has produced a custom revolver for the .50GI.
* 185 gr (12 g) JHP, 1200 ft/s, 591 ft-lb
* 275 gr (18 g) JHP, 900 ft/s, 495 ft-lb
* 300 gr (19 g) JFP, 700 ft/s, 350 ft-lb
* 300 gr (19 g) JHP, 860 ft/s, 493 ft-lb
Stolen .45-ACP info from Wikipedia
The result is one of the world's most effective combat pistol cartridges, one that combines very good accuracy and stopping power for use against human targets. The cartridge also has relatively low muzzle blast and flash, as well as moderate recoil. The .45 ACP also operates at a relatively low maximum chamber pressure rating of 21,000 psi (145 MPa) (compared to 35,000 psi/240 MPa for 9mmP and .40 S&W, 37,500 psi/260 MPa for 10mm Auto, 40,000 psi/280 MPa for .357 SIG), which helps extend service life of weapons it is fired in.
Like many pistol cartridges, it is a low-velocity round, and thus not particularly effective against body armor. Another drawback for large scale military operations is the cartridge's large size, weight, the increased material cost of manufacture compared to the smaller 9x19mm Parabellum cartridge, and lack of compliance with Standardization Agreements pertaining to handgun ammunition currently enacted between the US and many of its allies.
Even in its non-expanding full metal jacket (FMJ) version, the .45 ACP cartridge has a reputation for effectiveness against human targets because its large diameter creates a deep and substantial permanent wound channel which lowers blood pressure more rapidly. However, some writers, such as the published work of Marshall and Sanow, have cast the reputation of .45 ACP as being the "best" at this task into debate. Marshall and Sanow's work, while receiving criticism from Dr. Fackler and others, show the .45 ACP, loaded with the best hollowpoint bullets and fired from a 5 in (13 cm) barrel to be a good "one shot stopper", somewhat better than the 9x19mm, equal with the .40 S&W, and only a few percentage points behind the "King" of the Marshall and Sanow study — the .357 Magnum fired from a 4 in (10 cm) barrel.
The wounding potential of bullets is often characterized in terms of a bullet's expanded diameter, penetration depth, and energy. Bullet energy for .45 ACP loads varies from roughly 350 to 500 ft·lbf (470 to 680 J). It has been shown that bullets transferring over 500 ft·lbf (680 J) of energy in 12 inches of penetration can produce remote wounding effects sometimes called hydrostatic shock. However, at these levels of energy, these remote wounding effects and enhanced incapacitation do not always occur, in which case bullet performance depends on directly crushing tissue by means of expansion and penetration. The table below shows common performance parameters for several .45 ACP loads. Bullet weights from 185 to 230 grains are common. Penetration depths from 11 inches to over 27 inches are available for various applications and risk assessments. The Marshall and Sanow "one-shot stop" rating varies from 65% for the non-expanding FMJ which produces a ballistic pressure wave of 252 psi to over 90% for the some JHPs. The average incapacitation times (estimated for a 170 lb male shot in the center of the chest) vary from 7.2 to 13.8 seconds.
Being a moderate-powered cartridge, the wide diameter of the .45 ACP bullets produces a decreased tendency to overpenetrate, which reduces the projectile's possibility of passing through the intended target with enough velocity to injure another person. The combination of stopping power and controlled penetration makes the .45 ACP practical for police use, although numerous issues, including the resulting decrease in magazine capacity and the larger size and weight of pistols chambered in this caliber, have led more police departments in the USA to adopt sidearms in 9x19mm, .40 S&W, and .357 SIG. Modern versions of .45 ACP handguns have magazines capable of holding as many as 14 cartridges, such as the Springfield Armory XD or Glock model 21.