Paintball markers like the Ariakon ACP sport a nifty feature called a field strip pin. It's basically a pin that holds the bolt, striker and velocity adjuster in place. Usually the pin itself is held by a kotter pin. A kotter pin can easily get caught on clothing, holsters and anything else which can be a real problem if the pin decides to fall out on the field because of it.
There are a few things you can use to replace the kotter pin for something better:
Any other form of wire
Personally, I recommend a twist tie (like the ones used to tie down toys in the packaging) or the key ring. The twist tie is easier to remove, the the key ring is solid and a for sure method. There's no way a key ring is just going to slide off.
You'll find no cheaper way of modding a marker than this. Go get it done.