Quick help please check my maths

[NorthernRaider]

They are steel tips machined from mild 7 or 8 mm rod I reckon, I think the tips are threaded and fibreglass has a nasty habit of cracking when you thread it so I'll probably drill out the threads and araldite them into place. Theoretically if I buy 31 inch target arrows i can keep lopping off the head and fitting a new one till I get down to 28 inches???

I've ordered a packet of the lightest steel parallel points i could find, will be an interesting experiment cos they will be far heavier than the original soft alloy tips??????