Greeting and welcome,
Namjaigai
You never have to apologize for any question you ask here, my friend!
As to your question about the difference between a sub12 foot pound and a 20 joule rifle, the short answer is that the difference is minute.
However, if you really want to know, the first thing is to quantify each so that we're comparing apples to apples.
12 Foot-Pounds = 16.27 Joules
20 Joules = 14.75 foot-pounds
As you have seen, the difference is only 1.52 foot-pounds of energy (FPE) or 3.73 Joules.
The next thing we need to know is the ballistic coefficient of the round and the distance at which your target is.
Of course, we're really splitting hairs at this level and since I don't have that info, it's not possible to calculate. One thing is for sure, though. It's extremely small and the squirrel/rabbit/etc isn't going to know the difference! LOL
.
More important than comparing energy levels is knowing your ability. To make an ethical kill (Don't be mean; kill them clean) you need to be able to put every shot into a 1" circle. Every time! That is the determining factor on how far you can take quarry. Don't be overly impressed with those amazing long distance shots you see on YouTube. Realize that videos are edited and they're only showing you their best shots, not showing you how often they miss!
.
Here's my "rule of thumb":
5-8 FPE for mice and small rodents
12-20 FPE for small game (8 FPE on target)
20-30 FPE for ground squirrels, rock chuck, etc
30+ FPE for coyote, pig, etc.
100 FPE for deer
Note two things: These are very general recommendations and I have yet to take a 'yote or larger. But, I've recently purchased some heavier caliber guns and have plans in the works for doing so next spring.
~ Mondo