I was really excited to see the 45acp more than anything. I have been looking for a carbine that uses 45acp, something like the Keltec sub2k or the Hi point 995. Both just use either the 9mm or the 40 cal, both of which I dont use anymore. My 40's and 9's have either been sold or swapped and I use just the 45acp now. I know they have a tommy gun that uses the 45acp, but to be honest, once you get past the "cool" factor for 5 minutes, its a cumbersome weapon. I did, like I said in an earlier post, contact Keltec to see the possibility of them doing a 45acp version, preferably with SAXD45 magazines. Another option I have been kicking around is to contact Sons of Guns and see the possibility of custom making a carbine that uses the XD mags, shoots 45acp, and will fold up like the Keltec Sub2K, but honestly, I dont have the money for a custom gun.
Once i saw that the AR had the 45acp upper, I started to think that maybe I could have one lower that does it all- 45acp (to match my handgun), 22lr (for practice and plinking) .223/5.56 (cause ammo will be plentiful) and of course 7.62x39mm (which is my favorite round). The main thinking I had was one lower and multiple uppers, means many weapons at a low weight.
The biggest problem, is money. Like you said, the uppers aint cheap. I did do a little research and found that the AR15 had the following upperscopied from AR15.com forum:
Q: How many calibers can I get uppers that fit my .223 (AR-15) lower? What are they?
A: While the list is nearly endless with all the wildcats possible. Here are the some of the more common and more interesting uppers that are (or have been) produced for the standard AR-15 lower:
223/5.56 and based upon that case
6 x 45
7 x 40
357 Mag Rimless
300 Whisper / .300Fireball / 300-.221
6.5 x 40
.30 Coyote (30 x 45)
.17 Remington
.17 Mach IV
.20 Tactical
.19 Calhoun
5.7 FN (developed by Rhinland Arms - no word yet on who is going to mfg it)
.204 Ruger
6.8 SPC
30 HRT
338 Spectre
10 mm Mag
35/40
219 WASP-C
7.62 x 40
5.45x39
7.62 x 39
.338 x 39
6 PPC
6.5 Grendel / 6.5 AR / 6.5 PPC Improved
22 PPC
7 sCAR ....
30 Carbine
5.45x39 / .21 Ghengis
7 BR
6.5 BR
300 Sabre
8mm (belt fed upper)
.308 (again a belt fed upper - no longer available)
458 SOCOM
44 Mag
440 CorBon
44 AMP
.475 Tremor
50AE
499LW
50 Beowulf
.50 BMG (single shot & side magazine fed)
.50 Blackpowder (1 made)
9mm
45ACP
32ACP
22 LR
17 HV
10mm
7.62x25
224 BoZ
7.62x24
9x23
.357 Sig
.357 Auto
40S&W
223 WSSM
243 WSSM
With thanks to Marty at Teppu-Jitsu for starting this list.