JDJ does make the 600 Nitro single shot T/C Encore handgun but it is not shootable by any measure. Currently manufacured is the .45-70 BFR by Magnum Research Inc. That can top 4000 pound feet of energy; secondly would actually be the .500 Maximum that shoots a .510 bullet and can be pushed slightly hotter then the .500 S&W, though nothing worth even talking about. Hodgdon lists the S&W 370 grain cast bullet as getting as much as 3,700 lbs. Really that's it for repeaters. For the1911 style there is the .460 Rowland that Network Custom Guns has pushed a 200 grain bullet over 1600 in it's Gas Gun. Anything else is really pointless and would hinder you rather then help you.
WSM Magnum, The BMG is the top of the food chain pushing 14,000lbs energy in 36" barrels, but with some special handloading (not for just any handloader) can bump the .700 Nitro over 14,000, though it produces singnificantly more recoil then the BMG for the same energy. This is because it is usuing a 1000 grain bullet as well as the larger bore (bigger bore, more recoil, period). The original 600 and 700 Nitro rounds use Cordite and do not put out like smokeless, also, they handloads mentioned above are only in custom made receivers. JDJ makes an "Ackly Improved" BMG case using a 1170 grain 14.5mm bullet that can get much more energy then the two above. It is a real run that is capable of extreme range and devistation. Accuracy has been phenominal in testing. But if you want to just blow things up. He makes a 950 mangum that uses a 3,600 grain bullet. It is made from a 20mm anti-aircraft round almost straight walled. It can make somewhere between 20,000-30,000 foot-pounds. For the more practical side, Ed Hubel makes the 12 gauge from Hell, posted elsewhere on this website that can do 22,000 pounds at best. All while still being able to shoot factory ammo. Hope this helps.
-Extremist