I opened up a new V3 stock I have to get the actual bolt length and stock length as it came from Remington.
The bolt is a hex head 1/4-28 X 6.5 inches long. Not an easy one to find in the hardware store.
The stock length without the buttpad is 12.5 inches long. With the buttpad, 13.25 inches long.
An option you have is to shorten the stock and install a grind to fit pad if your looking for something shorter but still stock appearing.
Another option is to get one of the V3 adapters I printed and add pretty much any 870 stock to that awesome V3 platform.