I believe any average person can fully fist themselves up the arsehole, and from a number of posts on this forum I'm pretty sure i know where people are going wrong... If you can get most of your hand inside but seem to hit a dead end, that's because the second sphincter is closed and needs to be open.
The best way to open the second sphincter is by getting on your hands and knees (with your arse in the air) and then insert a toy that's long enough to go past the second sphincter (use the widest toy you can), push the toy in as deep as you can then pull it out quickly and repeat this between 5 and 10 times, you should be hearing the 'pah' sound of air getting inside each time you pull out, the air that's now inside will help to hold the second sphincter open when you try to insert your hand.
With the air still inside and staying on your hands and knees try now to insert four fingers past the knuckles, this time you should be able to feel another opening where there was once a dead end, use your fingers to stretch open this new 'internal' opening, next form the duck shape with your hand and aim your fingers towards the new 'internal' opening as you apply pressure.
You might need to repeat the above process a few times (i always need around 5 times), each time try to stretch the second sphincter open with your fingers.
I go through the process mentioned above and then reach underneath (between my legs), I then insert my hand as much as possible trying to keep pressure on the area so the air does not escape, I then change to a squatting position which I find is the easiest way to fully insert, like this i can go pretty deep and get about half of my forearm inside.
Believe me you'll know when you've made it past the second sphincter as everything will feel completely different to what you've experienced before in a much much better way, and you'll probably need to pull out after only a few seconds as the sensation is completely overwhelming at first.
Hope that helps