I do it this way:
1) copy the data1.1.0.pk3 file, put the copy anywhere you can get to it.
2) rename it to data1.1.0.zip and extract(if you're running windows. If you're running linux, like me, you can probably just extract it as is.)
3) you will probably end up with the .zip as well as the extracted folders, so you can delete the .zip.
4) for this case, go into models/weapons/blaster and put your model there. Make sure that the model is in md3 format.
5) open the weapon.cfg, at the very top it should give a file path ending in blaster.md3. Change the model file name to (your_model_name_here).md3. Again, make sure the model is in .md3 format.
6) now go back out so you're looking at the folders that you extracted from the .zip. select the folders and and zip them up into a zip archive.
7) change the extension from .zip to .pk3. For recognition purposes, I change the name of the zip also (I usually go with data1.2.0.pk3 just for kicks

)
8-switch out the new pk3 for the original and run the game. If done right, the game should start normally.
I usually use /devmap to start a game when switching out models. This allows the use of cheats, this makes it easy to test models when using models for weapons that require credits. Let me know if you have any better luck. Also, if it works for you like it works for me, the folders that you extracted in the beginning will remain there after you create the archive. I recommend saving these, it makes it easier to make additional changes.