Comments about Nikon BL-3 Battery Chamber Cover for MB-D10 and MB-40:

Simple add-on that increases your flexibility for your power consumption. During your shooting to be able to use the same batteries you may already have from a D2X or D2XS with your D300 and D700.

Comments about Nikon BL-3 Battery Chamber Cover for MB-D10 and MB-40:

Boosts D300 frame rate from 5 to 8-frames a second.

Comments about Nikon BL-3 Battery Chamber Cover for MB-D10 and MB-40:

required for use with ENL4 battery and MBD10 battery pack for D300.

Comments about Nikon BL-3 Battery Chamber Cover for MB-D10 and MB-40:

The BL-3 Battery door was originally made for use with the MB-40 Battery Pack for the awesome Nikon F6 Film camera to boost the shutter frame rate on that camera. It also works with the MB-D10 Battery Pack which is used for the Nikon D300 and D700 Digital Cameras. This door cover is used with the 11.1V /1900mAh EN-EL4 or tthe 11.1V 2500mAh EN-EL4a. This higher Voltage gives you the ability to use the higher available frame rate of 7 frames per second in the D300 and 8 frames per second in the D700. The higher capacity gives you highly extended shooting time over the default 7.4 V 1500mAh En-EL3a. You still keep one EN-EL3a in the camera which will only be used when any battery in the MB-D10 runs low on powe, if you set up the battery usage menu properly on the camera.
The upside is faster shutter frame rates and extended shooting time.
The downside is only for those people that do not already own a D2H, D2HS, D2X, D2XS or D3 and are using a D300 or D700 for a backup or second format camera.
For those without any of the previous mentioned Pro DSLRs, you will have additional expenses for the 11.1V EN-EL4 series batteries and you need a MH-21 or MH-22 Battery Charger to accompany the BL-3.
My suggestion is to buy the MH-21 as it works perfect for any capacity Nikon or after market EN-EL4 series 11.1V Lithium battery and is much less expensive and easier to find on the used market than the MH-22 that comes with the Nikon D3.
The shutters in the D300 and D700 are just as durable as the older D2 series pro cameras. Using high speed frame rates will decrease the shutter life of ANY camera. However the extended battery life during NORMAL use is worth it for sports, large events, and wedding photography.