Is that .22? and what diameter "pin" did you use for the tip?
As for cutting the back off, this really isn't necessary unless you plan on using a de-bounce device. The bolt probe handle is still the limiting factor in how far the back you can slide the bolt not the probe itself.
If you shorten the back of the bolt too much, however, then the bolt handle hits the back of the slot before the gun cocks.... and you have to lengthen the slot as well.... Don't ask me how I found that out.... Bob